btrfs: qgroup rescan races (part 1)

PATCH In review [email protected] 4 participants 18 messages May 2, 2018 – Jan 16, 2020 Btrfs file system

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[email protected]

author

2018-05-02 21:12 UTC #0 lore raw

v3 cover btrfs: qgroup rescan races (part 1)

From: Jeff Mahoney <[email protected]>

Hi Dave -

Here's the updated patchset for the rescan races. This fixes the issue where we'd try to start multiple workers. It introduces a new "ready" bool that we set during initialization and clear while queuing the worker. The queuer is also now responsible for most of the initialization.

I have a separate patch set start that gets rid of the racy mess surrounding the rescan worker startup. We can handle it in btrfs_run_qgroups and just set a flag to start it everywhere else.

-Jeff

---

Jeff Mahoney (3):
btrfs: qgroups, fix rescan worker running races
btrfs: qgroups, remove unnecessary memset before btrfs_init_work
btrfs: qgroup, don't try to insert status item after ENOMEM in rescan
worker

 fs/btrfs/async-thread.c |   1 +
 fs/btrfs/ctree.h        |   2 +
 fs/btrfs/qgroup.c       | 100 +++++++++++++++++++++++++++---------------------
 3 files changed, 60 insertions(+), 43 deletions(-)

Signature · 1 line

2.12.3

[email protected]

author

2018-05-02 21:12 UTC #1 lore raw

PATCH v3 1/3 btrfs: qgroups, fix rescan worker running races

From: Jeff Mahoney <[email protected]>

Commit 8d9eddad194 (Btrfs: fix qgroup rescan worker initialization) fixed the issue with BTRFS_IOC_QUOTA_RESCAN_WAIT being racy, but ended up reintroducing the hang-on-unmount bug that the commit it intended to fix addressed.

The race this time is between qgroup_rescan_init setting ->qgroup_rescan_running = true and the worker starting. There are many scenarios where we initialize the worker and never start it. The completion btrfs_ioctl_quota_rescan_wait waits for will never come. This can happen even without involving error handling, since mounting the file system read-only returns between initializing the worker and queueing it.

The right place to do it is when we're queuing the worker. The flag really just means that btrfs_ioctl_quota_rescan_wait should wait for a completion.

Since the BTRFS_QGROUP_STATUS_FLAG_RESCAN flag is overloaded to refer to both runtime behavior and on-disk state, we introduce a new fs_info->qgroup_rescan_ready to indicate that we're initialized and waiting to start.

This patch introduces a new helper, queue_rescan_worker, that handles most of the initialization, the two flags, and queuing the worker, including races with unmount.

While we're at it, ->qgroup_rescan_running is protected only by the ->qgroup_rescan_mutex. btrfs_ioctl_quota_rescan_wait doesn't need to take the spinlock too.

Fixes 8d9eddad194 (Btrfs: fix qgroup rescan worker initialization)

Signed-off-by Jeff Mahoney <[email protected]>

---
 fs/btrfs/ctree.h  |  2 ++
 fs/btrfs/qgroup.c | 94 +++++++++++++++++++++++++++++++++----------------------
 2 files changed, 58 insertions(+), 38 deletions(-)

fs/btrfs/ctree.h +2 -0

index da308774b8a4..4003498bb714 100644
@@ -1045,6 +1045,8 @@ struct btrfs_fs_info {
 	struct btrfs_workqueue *qgroup_rescan_workers;
 	struct completion qgroup_rescan_completion;
 	struct btrfs_work qgroup_rescan_work;
+	/* qgroup rescan worker is running or queued to run */
+	bool qgroup_rescan_ready;
 	bool qgroup_rescan_running;	/* protected by qgroup_rescan_lock */
 
 	/* filesystem state */

fs/btrfs/qgroup.c +56 ~~-38~~

159 lines not on this page · raw message

Signature · 1 line

2.12.3

Nikolay Borisov

2018-05-03 07:24 UTC #2 lore raw

Re: [PATCH 1/3] btrfs: qgroups, fix rescan worker running races

Quoted 15 lines · On 3.05.2018 00:11, [email protected] wrote:

From: Jeff Mahoney <[email protected]>
Commit 8d9eddad194 (Btrfs: fix qgroup rescan worker initialization) fixed the issue with BTRFS_IOC_QUOTA_RESCAN_WAIT being racy, but ended up reintroducing the hang-on-unmount bug that the commit it intended to fix addressed.
The race this time is between qgroup_rescan_init setting ->qgroup_rescan_running = true and the worker starting. There are many scenarios where we initialize the worker and never start it. The completion btrfs_ioctl_quota_rescan_wait waits for will never come. This can happen even without involving error handling, since mounting the file system read-only returns between initializing the worker and queueing it.

Quoted 8 lines

The right place to do it is when we're queuing the worker. The flag really just means that btrfs_ioctl_quota_rescan_wait should wait for a completion.
Since the BTRFS_QGROUP_STATUS_FLAG_RESCAN flag is overloaded to refer to both runtime behavior and on-disk state, we introduce a new fs_info->qgroup_rescan_ready to indicate that we're initialized and waiting to start.

Am I correct in my understanding that this qgroup_rescan_ready flag is used to avoid qgroup_rescan_init being called AFTER it has already been called but BEFORE queue_rescan_worker ? Why wasn't the initial version of this patch without this flag sufficient?

Quoted 201 lines

This patch introduces a new helper, queue_rescan_worker, that handles most of the initialization, the two flags, and queuing the worker, including races with unmount.

While we're at it, ->qgroup_rescan_running is protected only by the ->qgroup_rescan_mutex. btrfs_ioctl_quota_rescan_wait doesn't need to take the spinlock too.

Fixes 8d9eddad194 (Btrfs: fix qgroup rescan worker initialization)

Signed-off-by Jeff Mahoney <[email protected]>

---
 fs/btrfs/ctree.h  |  2 ++
 fs/btrfs/qgroup.c | 94 +++++++++++++++++++++++++++++++++----------------------
 2 files changed, 58 insertions(+), 38 deletions(-)

fs/btrfs/ctree.h +2 -0

index da308774b8a4..4003498bb714 100644
@@ -1045,6 +1045,8 @@ struct btrfs_fs_info {
 	struct btrfs_workqueue *qgroup_rescan_workers;
 	struct completion qgroup_rescan_completion;
 	struct btrfs_work qgroup_rescan_work;
+	/* qgroup rescan worker is running or queued to run */
+	bool qgroup_rescan_ready;
 	bool qgroup_rescan_running;	/* protected by qgroup_rescan_lock */
 
 	/* filesystem state */

fs/btrfs/qgroup.c +56 ~~-38~~

index aa259d6986e1..466744741873 100644
@@ -101,6 +101,7 @@ static int
 qgroup_rescan_init(struct btrfs_fs_info *fs_info, u64 progress_objectid,
 		   int init_flags);
 static void qgroup_rescan_zero_tracking(struct btrfs_fs_info *fs_info);
+static void btrfs_qgroup_rescan_worker(struct btrfs_work *work);
 
 /* must be called with qgroup_ioctl_lock held */
 static struct btrfs_qgroup *find_qgroup_rb(struct btrfs_fs_info *fs_info,

@@ -2072,6 +2073,46 @@ int btrfs_qgroup_account_extents(struct btrfs_trans_handle *trans,
 	return ret;
 }
 
+static void queue_rescan_worker(struct btrfs_fs_info *fs_info)
+{
+	mutex_lock(&fs_info->qgroup_rescan_lock);
+	if (btrfs_fs_closing(fs_info)) {
+		mutex_unlock(&fs_info->qgroup_rescan_lock);
+		return;
+	}
+
+	if (WARN_ON(!fs_info->qgroup_rescan_ready)) {
+		btrfs_warn(fs_info, "rescan worker not ready");
+		mutex_unlock(&fs_info->qgroup_rescan_lock);
+		return;
+	}
+	fs_info->qgroup_rescan_ready = false;
+
+	if (WARN_ON(fs_info->qgroup_rescan_running)) {
+		btrfs_warn(fs_info, "rescan worker already queued");
+		mutex_unlock(&fs_info->qgroup_rescan_lock);
+		return;
+	}
+
+	/*
+	 * Being queued is enough for btrfs_qgroup_wait_for_completion
+	 * to need to wait.
+	 */
+	fs_info->qgroup_rescan_running = true;
+	init_completion(&fs_info->qgroup_rescan_completion);
+	mutex_unlock(&fs_info->qgroup_rescan_lock);
+
+	memset(&fs_info->qgroup_rescan_work, 0,
+	       sizeof(fs_info->qgroup_rescan_work));
+
+	btrfs_init_work(&fs_info->qgroup_rescan_work,
+			btrfs_qgroup_rescan_helper,
+			btrfs_qgroup_rescan_worker, NULL, NULL);
+
+	btrfs_queue_work(fs_info->qgroup_rescan_workers,
+			 &fs_info->qgroup_rescan_work);
+}
+
 /*
  * called from commit_transaction. Writes all changed qgroups to disk.
  */

@@ -2123,8 +2164,7 @@ int btrfs_run_qgroups(struct btrfs_trans_handle *trans,
 		ret = qgroup_rescan_init(fs_info, 0, 1);
 		if (!ret) {
 			qgroup_rescan_zero_tracking(fs_info);
-			btrfs_queue_work(fs_info->qgroup_rescan_workers,
-					 &fs_info->qgroup_rescan_work);
+			queue_rescan_worker(fs_info);
 		}
 		ret = 0;
 	}

@@ -2607,6 +2647,10 @@ static void btrfs_qgroup_rescan_worker(struct btrfs_work *work)
 	if (!path)
 		goto out;
 
+	mutex_lock(&fs_info->qgroup_rescan_lock);
+	fs_info->qgroup_flags |= BTRFS_QGROUP_STATUS_FLAG_RESCAN;
+	mutex_unlock(&fs_info->qgroup_rescan_lock);
+
 	err = 0;
 	while (!err && !btrfs_fs_closing(fs_info)) {
 		trans = btrfs_start_transaction(fs_info->fs_root, 0);

@@ -2685,47 +2729,27 @@ qgroup_rescan_init(struct btrfs_fs_info *fs_info, u64 progress_objectid,
 {
 	int ret = 0;
 
-	if (!init_flags &&
-	    (!(fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_RESCAN) ||
-	     !(fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_ON))) {
+	if (!test_bit(BTRFS_FS_QUOTA_ENABLED, &fs_info->flags)) {
 		ret = -EINVAL;
 		goto err;
 	}
 
 	mutex_lock(&fs_info->qgroup_rescan_lock);
-	spin_lock(&fs_info->qgroup_lock);
-
-	if (init_flags) {
-		if (fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_RESCAN)
-			ret = -EINPROGRESS;
-		else if (!(fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_ON))
-			ret = -EINVAL;
-
-		if (ret) {
-			spin_unlock(&fs_info->qgroup_lock);
-			mutex_unlock(&fs_info->qgroup_rescan_lock);
-			goto err;
-		}
-		fs_info->qgroup_flags |= BTRFS_QGROUP_STATUS_FLAG_RESCAN;
+	if (fs_info->qgroup_rescan_ready || fs_info->qgroup_rescan_running) {
+		mutex_unlock(&fs_info->qgroup_rescan_lock);
+		ret = -EINPROGRESS;
+		goto err;
 	}
 
 	memset(&fs_info->qgroup_rescan_progress, 0,
 		sizeof(fs_info->qgroup_rescan_progress));
 	fs_info->qgroup_rescan_progress.objectid = progress_objectid;
-	init_completion(&fs_info->qgroup_rescan_completion);
-	fs_info->qgroup_rescan_running = true;
+	fs_info->qgroup_rescan_ready = true;
 
-	spin_unlock(&fs_info->qgroup_lock);
 	mutex_unlock(&fs_info->qgroup_rescan_lock);
 
-	memset(&fs_info->qgroup_rescan_work, 0,
-	       sizeof(fs_info->qgroup_rescan_work));
-	btrfs_init_work(&fs_info->qgroup_rescan_work,
-			btrfs_qgroup_rescan_helper,
-			btrfs_qgroup_rescan_worker, NULL, NULL);
-
-	if (ret) {
 err:
+	if (ret) {
 		btrfs_info(fs_info, "qgroup_rescan_init failed with %d", ret);
 		return ret;
 	}

@@ -2785,9 +2809,7 @@ btrfs_qgroup_rescan(struct btrfs_fs_info *fs_info)
 
 	qgroup_rescan_zero_tracking(fs_info);
 
-	btrfs_queue_work(fs_info->qgroup_rescan_workers,
-			 &fs_info->qgroup_rescan_work);
-
+	queue_rescan_worker(fs_info);
 	return 0;
 }

@@ -2798,9 +2820,7 @@ int btrfs_qgroup_wait_for_completion(struct btrfs_fs_info *fs_info,
 	int ret = 0;
 
 	mutex_lock(&fs_info->qgroup_rescan_lock);
-	spin_lock(&fs_info->qgroup_lock);
 	running = fs_info->qgroup_rescan_running;
-	spin_unlock(&fs_info->qgroup_lock);
 	mutex_unlock(&fs_info->qgroup_rescan_lock);
 
 	if (!running)

@@ -2819,12 +2839,10 @@ int btrfs_qgroup_wait_for_completion(struct btrfs_fs_info *fs_info,
  * this is only called from open_ctree where we're still single threaded, thus
  * locking is omitted here.
  */
-void
-btrfs_qgroup_rescan_resume(struct btrfs_fs_info *fs_info)
+void btrfs_qgroup_rescan_resume(struct btrfs_fs_info *fs_info)
 {
 	if (fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_RESCAN)
-		btrfs_queue_work(fs_info->qgroup_rescan_workers,
-				 &fs_info->qgroup_rescan_work);
+		queue_rescan_worker(fs_info);
 }
 
 /*

Jeff Mahoney

author

2018-05-03 13:39 UTC #3 lore raw

Re: [PATCH 1/3] btrfs: qgroups, fix rescan worker running races

Quoted 27 lines · Nikolay Borisov

Quoted 15 lines · On 3.05.2018 00:11, [email protected] wrote:
From: Jeff Mahoney <[email protected]>
Commit 8d9eddad194 (Btrfs: fix qgroup rescan worker initialization) fixed the issue with BTRFS_IOC_QUOTA_RESCAN_WAIT being racy, but ended up reintroducing the hang-on-unmount bug that the commit it intended to fix addressed.
The race this time is between qgroup_rescan_init setting ->qgroup_rescan_running = true and the worker starting. There are many scenarios where we initialize the worker and never start it. The completion btrfs_ioctl_quota_rescan_wait waits for will never come. This can happen even without involving error handling, since mounting the file system read-only returns between initializing the worker and queueing it.
Quoted 8 lines
The right place to do it is when we're queuing the worker. The flag really just means that btrfs_ioctl_quota_rescan_wait should wait for a completion.
Since the BTRFS_QGROUP_STATUS_FLAG_RESCAN flag is overloaded to refer to both runtime behavior and on-disk state, we introduce a new fs_info->qgroup_rescan_ready to indicate that we're initialized and waiting to start.

Quoted 4 lines

Am I correct in my understanding that this qgroup_rescan_ready flag is used to avoid qgroup_rescan_init being called AFTER it has already been called but BEFORE queue_rescan_worker ? Why wasn't the initial version of this patch without this flag sufficient?

No, the race is between clearing the BTRFS_QGROUP_STATUS_FLAG_RESCAN flag near the end of the worker and clearing the running flag. The rescan lock is dropped in between, so btrfs_rescan_init will let a new rescan request in while we update the status item on disk. We wouldn't have queued another worker since that's what the warning catches, but if there were already tasks waiting for completion, they wouldn't have been woken since the wait queue list would be reinitialized. There's no way to reorder clearing the flag without changing how we handle ->qgroup_flags. I plan on doing that separately. This was just meant to be the simple fix.

That we can use the ready variable to also ensure that we don't let qgroup_rescan_init be called twice without running the rescan is a nice bonus.

-Jeff

Quoted 201 lines

This patch introduces a new helper, queue_rescan_worker, that handles most of the initialization, the two flags, and queuing the worker, including races with unmount.

While we're at it, ->qgroup_rescan_running is protected only by the ->qgroup_rescan_mutex. btrfs_ioctl_quota_rescan_wait doesn't need to take the spinlock too.

Fixes 8d9eddad194 (Btrfs: fix qgroup rescan worker initialization)

Signed-off-by Jeff Mahoney <[email protected]>

---
 fs/btrfs/ctree.h  |  2 ++
 fs/btrfs/qgroup.c | 94 +++++++++++++++++++++++++++++++++----------------------
 2 files changed, 58 insertions(+), 38 deletions(-)

fs/btrfs/ctree.h +2 -0

index da308774b8a4..4003498bb714 100644
@@ -1045,6 +1045,8 @@ struct btrfs_fs_info {
 	struct btrfs_workqueue *qgroup_rescan_workers;
 	struct completion qgroup_rescan_completion;
 	struct btrfs_work qgroup_rescan_work;
+	/* qgroup rescan worker is running or queued to run */
+	bool qgroup_rescan_ready;
 	bool qgroup_rescan_running;	/* protected by qgroup_rescan_lock */
 
 	/* filesystem state */

fs/btrfs/qgroup.c +56 ~~-38~~

index aa259d6986e1..466744741873 100644
@@ -101,6 +101,7 @@ static int
 qgroup_rescan_init(struct btrfs_fs_info *fs_info, u64 progress_objectid,
 		   int init_flags);
 static void qgroup_rescan_zero_tracking(struct btrfs_fs_info *fs_info);
+static void btrfs_qgroup_rescan_worker(struct btrfs_work *work);
 
 /* must be called with qgroup_ioctl_lock held */
 static struct btrfs_qgroup *find_qgroup_rb(struct btrfs_fs_info *fs_info,

@@ -2072,6 +2073,46 @@ int btrfs_qgroup_account_extents(struct btrfs_trans_handle *trans,
 	return ret;
 }
 
+static void queue_rescan_worker(struct btrfs_fs_info *fs_info)
+{
+	mutex_lock(&fs_info->qgroup_rescan_lock);
+	if (btrfs_fs_closing(fs_info)) {
+		mutex_unlock(&fs_info->qgroup_rescan_lock);
+		return;
+	}
+
+	if (WARN_ON(!fs_info->qgroup_rescan_ready)) {
+		btrfs_warn(fs_info, "rescan worker not ready");
+		mutex_unlock(&fs_info->qgroup_rescan_lock);
+		return;
+	}
+	fs_info->qgroup_rescan_ready = false;
+
+	if (WARN_ON(fs_info->qgroup_rescan_running)) {
+		btrfs_warn(fs_info, "rescan worker already queued");
+		mutex_unlock(&fs_info->qgroup_rescan_lock);
+		return;
+	}
+
+	/*
+	 * Being queued is enough for btrfs_qgroup_wait_for_completion
+	 * to need to wait.
+	 */
+	fs_info->qgroup_rescan_running = true;
+	init_completion(&fs_info->qgroup_rescan_completion);
+	mutex_unlock(&fs_info->qgroup_rescan_lock);
+
+	memset(&fs_info->qgroup_rescan_work, 0,
+	       sizeof(fs_info->qgroup_rescan_work));
+
+	btrfs_init_work(&fs_info->qgroup_rescan_work,
+			btrfs_qgroup_rescan_helper,
+			btrfs_qgroup_rescan_worker, NULL, NULL);
+
+	btrfs_queue_work(fs_info->qgroup_rescan_workers,
+			 &fs_info->qgroup_rescan_work);
+}
+
 /*
  * called from commit_transaction. Writes all changed qgroups to disk.
  */

@@ -2123,8 +2164,7 @@ int btrfs_run_qgroups(struct btrfs_trans_handle *trans,
 		ret = qgroup_rescan_init(fs_info, 0, 1);
 		if (!ret) {
 			qgroup_rescan_zero_tracking(fs_info);
-			btrfs_queue_work(fs_info->qgroup_rescan_workers,
-					 &fs_info->qgroup_rescan_work);
+			queue_rescan_worker(fs_info);
 		}
 		ret = 0;
 	}

@@ -2607,6 +2647,10 @@ static void btrfs_qgroup_rescan_worker(struct btrfs_work *work)
 	if (!path)
 		goto out;
 
+	mutex_lock(&fs_info->qgroup_rescan_lock);
+	fs_info->qgroup_flags |= BTRFS_QGROUP_STATUS_FLAG_RESCAN;
+	mutex_unlock(&fs_info->qgroup_rescan_lock);
+
 	err = 0;
 	while (!err && !btrfs_fs_closing(fs_info)) {
 		trans = btrfs_start_transaction(fs_info->fs_root, 0);

@@ -2685,47 +2729,27 @@ qgroup_rescan_init(struct btrfs_fs_info *fs_info, u64 progress_objectid,
 {
 	int ret = 0;
 
-	if (!init_flags &&
-	    (!(fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_RESCAN) ||
-	     !(fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_ON))) {
+	if (!test_bit(BTRFS_FS_QUOTA_ENABLED, &fs_info->flags)) {
 		ret = -EINVAL;
 		goto err;
 	}
 
 	mutex_lock(&fs_info->qgroup_rescan_lock);
-	spin_lock(&fs_info->qgroup_lock);
-
-	if (init_flags) {
-		if (fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_RESCAN)
-			ret = -EINPROGRESS;
-		else if (!(fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_ON))
-			ret = -EINVAL;
-
-		if (ret) {
-			spin_unlock(&fs_info->qgroup_lock);
-			mutex_unlock(&fs_info->qgroup_rescan_lock);
-			goto err;
-		}
-		fs_info->qgroup_flags |= BTRFS_QGROUP_STATUS_FLAG_RESCAN;
+	if (fs_info->qgroup_rescan_ready || fs_info->qgroup_rescan_running) {
+		mutex_unlock(&fs_info->qgroup_rescan_lock);
+		ret = -EINPROGRESS;
+		goto err;
 	}
 
 	memset(&fs_info->qgroup_rescan_progress, 0,
 		sizeof(fs_info->qgroup_rescan_progress));
 	fs_info->qgroup_rescan_progress.objectid = progress_objectid;
-	init_completion(&fs_info->qgroup_rescan_completion);
-	fs_info->qgroup_rescan_running = true;
+	fs_info->qgroup_rescan_ready = true;
 
-	spin_unlock(&fs_info->qgroup_lock);
 	mutex_unlock(&fs_info->qgroup_rescan_lock);
 
-	memset(&fs_info->qgroup_rescan_work, 0,
-	       sizeof(fs_info->qgroup_rescan_work));
-	btrfs_init_work(&fs_info->qgroup_rescan_work,
-			btrfs_qgroup_rescan_helper,
-			btrfs_qgroup_rescan_worker, NULL, NULL);
-
-	if (ret) {
 err:
+	if (ret) {
 		btrfs_info(fs_info, "qgroup_rescan_init failed with %d", ret);
 		return ret;
 	}

@@ -2785,9 +2809,7 @@ btrfs_qgroup_rescan(struct btrfs_fs_info *fs_info)
 
 	qgroup_rescan_zero_tracking(fs_info);
 
-	btrfs_queue_work(fs_info->qgroup_rescan_workers,
-			 &fs_info->qgroup_rescan_work);
-
+	queue_rescan_worker(fs_info);
 	return 0;
 }

@@ -2798,9 +2820,7 @@ int btrfs_qgroup_wait_for_completion(struct btrfs_fs_info *fs_info,
 	int ret = 0;
 
 	mutex_lock(&fs_info->qgroup_rescan_lock);
-	spin_lock(&fs_info->qgroup_lock);
 	running = fs_info->qgroup_rescan_running;
-	spin_unlock(&fs_info->qgroup_lock);
 	mutex_unlock(&fs_info->qgroup_rescan_lock);
 
 	if (!running)

@@ -2819,12 +2839,10 @@ int btrfs_qgroup_wait_for_completion(struct btrfs_fs_info *fs_info,
  * this is only called from open_ctree where we're still single threaded, thus
  * locking is omitted here.
  */
-void
-btrfs_qgroup_rescan_resume(struct btrfs_fs_info *fs_info)
+void btrfs_qgroup_rescan_resume(struct btrfs_fs_info *fs_info)
 {
 	if (fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_RESCAN)
-		btrfs_queue_work(fs_info->qgroup_rescan_workers,
-				 &fs_info->qgroup_rescan_work);
+		queue_rescan_worker(fs_info);
 }
 
 /*

Signature · 2 lines

Jeff Mahoney
SUSE Labs

Nikolay Borisov

2018-05-03 15:52 UTC #4 lore raw

Re: [PATCH 1/3] btrfs: qgroups, fix rescan worker running races

Quoted 33 lines · Jeff Mahoney

Quoted 27 lines · Nikolay Borisov
Quoted 15 lines · On 3.05.2018 00:11, [email protected] wrote:
From: Jeff Mahoney <[email protected]>
Commit 8d9eddad194 (Btrfs: fix qgroup rescan worker initialization) fixed the issue with BTRFS_IOC_QUOTA_RESCAN_WAIT being racy, but ended up reintroducing the hang-on-unmount bug that the commit it intended to fix addressed.
The race this time is between qgroup_rescan_init setting ->qgroup_rescan_running = true and the worker starting. There are many scenarios where we initialize the worker and never start it. The completion btrfs_ioctl_quota_rescan_wait waits for will never come. This can happen even without involving error handling, since mounting the file system read-only returns between initializing the worker and queueing it.
Quoted 8 lines
The right place to do it is when we're queuing the worker. The flag really just means that btrfs_ioctl_quota_rescan_wait should wait for a completion.
Since the BTRFS_QGROUP_STATUS_FLAG_RESCAN flag is overloaded to refer to both runtime behavior and on-disk state, we introduce a new fs_info->qgroup_rescan_ready to indicate that we're initialized and waiting to start.
Quoted 4 lines
Am I correct in my understanding that this qgroup_rescan_ready flag is used to avoid qgroup_rescan_init being called AFTER it has already been called but BEFORE queue_rescan_worker ? Why wasn't the initial version of this patch without this flag sufficient?

Quoted 10 lines

No, the race is between clearing the BTRFS_QGROUP_STATUS_FLAG_RESCAN flag near the end of the worker and clearing the running flag. The rescan lock is dropped in between, so btrfs_rescan_init will let a new rescan request in while we update the status item on disk. We wouldn't have queued another worker since that's what the warning catches, but if there were already tasks waiting for completion, they wouldn't have been woken since the wait queue list would be reinitialized. There's no way to reorder clearing the flag without changing how we handle ->qgroup_flags. I plan on doing that separately. This was just meant to be the simple fix.

Great, I think some of this information should go into the change log, in explaining what the symptoms of the race condition are.

Quoted 210 lines

That we can use the ready variable to also ensure that we don't let qgroup_rescan_init be called twice without running the rescan is a nice bonus.

-Jeff

Quoted 201 lines

This patch introduces a new helper, queue_rescan_worker, that handles most of the initialization, the two flags, and queuing the worker, including races with unmount.

While we're at it, ->qgroup_rescan_running is protected only by the ->qgroup_rescan_mutex. btrfs_ioctl_quota_rescan_wait doesn't need to take the spinlock too.

Fixes 8d9eddad194 (Btrfs: fix qgroup rescan worker initialization)

Signed-off-by Jeff Mahoney <[email protected]>

---
 fs/btrfs/ctree.h  |  2 ++
 fs/btrfs/qgroup.c | 94 +++++++++++++++++++++++++++++++++----------------------
 2 files changed, 58 insertions(+), 38 deletions(-)

fs/btrfs/ctree.h +2 -0

index da308774b8a4..4003498bb714 100644
@@ -1045,6 +1045,8 @@ struct btrfs_fs_info {
 	struct btrfs_workqueue *qgroup_rescan_workers;
 	struct completion qgroup_rescan_completion;
 	struct btrfs_work qgroup_rescan_work;
+	/* qgroup rescan worker is running or queued to run */
+	bool qgroup_rescan_ready;
 	bool qgroup_rescan_running;	/* protected by qgroup_rescan_lock */
 
 	/* filesystem state */

fs/btrfs/qgroup.c +56 ~~-38~~

index aa259d6986e1..466744741873 100644
@@ -101,6 +101,7 @@ static int
 qgroup_rescan_init(struct btrfs_fs_info *fs_info, u64 progress_objectid,
 		   int init_flags);
 static void qgroup_rescan_zero_tracking(struct btrfs_fs_info *fs_info);
+static void btrfs_qgroup_rescan_worker(struct btrfs_work *work);
 
 /* must be called with qgroup_ioctl_lock held */
 static struct btrfs_qgroup *find_qgroup_rb(struct btrfs_fs_info *fs_info,

@@ -2072,6 +2073,46 @@ int btrfs_qgroup_account_extents(struct btrfs_trans_handle *trans,
 	return ret;
 }
 
+static void queue_rescan_worker(struct btrfs_fs_info *fs_info)
+{
+	mutex_lock(&fs_info->qgroup_rescan_lock);
+	if (btrfs_fs_closing(fs_info)) {
+		mutex_unlock(&fs_info->qgroup_rescan_lock);
+		return;
+	}
+
+	if (WARN_ON(!fs_info->qgroup_rescan_ready)) {
+		btrfs_warn(fs_info, "rescan worker not ready");
+		mutex_unlock(&fs_info->qgroup_rescan_lock);
+		return;
+	}
+	fs_info->qgroup_rescan_ready = false;
+
+	if (WARN_ON(fs_info->qgroup_rescan_running)) {
+		btrfs_warn(fs_info, "rescan worker already queued");
+		mutex_unlock(&fs_info->qgroup_rescan_lock);
+		return;
+	}
+
+	/*
+	 * Being queued is enough for btrfs_qgroup_wait_for_completion
+	 * to need to wait.
+	 */
+	fs_info->qgroup_rescan_running = true;
+	init_completion(&fs_info->qgroup_rescan_completion);
+	mutex_unlock(&fs_info->qgroup_rescan_lock);
+
+	memset(&fs_info->qgroup_rescan_work, 0,
+	       sizeof(fs_info->qgroup_rescan_work));
+
+	btrfs_init_work(&fs_info->qgroup_rescan_work,
+			btrfs_qgroup_rescan_helper,
+			btrfs_qgroup_rescan_worker, NULL, NULL);
+
+	btrfs_queue_work(fs_info->qgroup_rescan_workers,
+			 &fs_info->qgroup_rescan_work);
+}
+
 /*
  * called from commit_transaction. Writes all changed qgroups to disk.
  */

@@ -2123,8 +2164,7 @@ int btrfs_run_qgroups(struct btrfs_trans_handle *trans,
 		ret = qgroup_rescan_init(fs_info, 0, 1);
 		if (!ret) {
 			qgroup_rescan_zero_tracking(fs_info);
-			btrfs_queue_work(fs_info->qgroup_rescan_workers,
-					 &fs_info->qgroup_rescan_work);
+			queue_rescan_worker(fs_info);
 		}
 		ret = 0;
 	}

@@ -2607,6 +2647,10 @@ static void btrfs_qgroup_rescan_worker(struct btrfs_work *work)
 	if (!path)
 		goto out;
 
+	mutex_lock(&fs_info->qgroup_rescan_lock);
+	fs_info->qgroup_flags |= BTRFS_QGROUP_STATUS_FLAG_RESCAN;
+	mutex_unlock(&fs_info->qgroup_rescan_lock);
+
 	err = 0;
 	while (!err && !btrfs_fs_closing(fs_info)) {
 		trans = btrfs_start_transaction(fs_info->fs_root, 0);

@@ -2685,47 +2729,27 @@ qgroup_rescan_init(struct btrfs_fs_info *fs_info, u64 progress_objectid,
 {
 	int ret = 0;
 
-	if (!init_flags &&
-	    (!(fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_RESCAN) ||
-	     !(fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_ON))) {
+	if (!test_bit(BTRFS_FS_QUOTA_ENABLED, &fs_info->flags)) {
 		ret = -EINVAL;
 		goto err;
 	}
 
 	mutex_lock(&fs_info->qgroup_rescan_lock);
-	spin_lock(&fs_info->qgroup_lock);
-
-	if (init_flags) {
-		if (fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_RESCAN)
-			ret = -EINPROGRESS;
-		else if (!(fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_ON))
-			ret = -EINVAL;
-
-		if (ret) {
-			spin_unlock(&fs_info->qgroup_lock);
-			mutex_unlock(&fs_info->qgroup_rescan_lock);
-			goto err;
-		}
-		fs_info->qgroup_flags |= BTRFS_QGROUP_STATUS_FLAG_RESCAN;
+	if (fs_info->qgroup_rescan_ready || fs_info->qgroup_rescan_running) {
+		mutex_unlock(&fs_info->qgroup_rescan_lock);
+		ret = -EINPROGRESS;
+		goto err;
 	}
 
 	memset(&fs_info->qgroup_rescan_progress, 0,
 		sizeof(fs_info->qgroup_rescan_progress));
 	fs_info->qgroup_rescan_progress.objectid = progress_objectid;
-	init_completion(&fs_info->qgroup_rescan_completion);
-	fs_info->qgroup_rescan_running = true;
+	fs_info->qgroup_rescan_ready = true;
 
-	spin_unlock(&fs_info->qgroup_lock);
 	mutex_unlock(&fs_info->qgroup_rescan_lock);
 
-	memset(&fs_info->qgroup_rescan_work, 0,
-	       sizeof(fs_info->qgroup_rescan_work));
-	btrfs_init_work(&fs_info->qgroup_rescan_work,
-			btrfs_qgroup_rescan_helper,
-			btrfs_qgroup_rescan_worker, NULL, NULL);
-
-	if (ret) {
 err:
+	if (ret) {
 		btrfs_info(fs_info, "qgroup_rescan_init failed with %d", ret);
 		return ret;
 	}

@@ -2785,9 +2809,7 @@ btrfs_qgroup_rescan(struct btrfs_fs_info *fs_info)
 
 	qgroup_rescan_zero_tracking(fs_info);
 
-	btrfs_queue_work(fs_info->qgroup_rescan_workers,
-			 &fs_info->qgroup_rescan_work);
-
+	queue_rescan_worker(fs_info);
 	return 0;
 }

@@ -2798,9 +2820,7 @@ int btrfs_qgroup_wait_for_completion(struct btrfs_fs_info *fs_info,
 	int ret = 0;
 
 	mutex_lock(&fs_info->qgroup_rescan_lock);
-	spin_lock(&fs_info->qgroup_lock);
 	running = fs_info->qgroup_rescan_running;
-	spin_unlock(&fs_info->qgroup_lock);
 	mutex_unlock(&fs_info->qgroup_rescan_lock);
 
 	if (!running)

@@ -2819,12 +2839,10 @@ int btrfs_qgroup_wait_for_completion(struct btrfs_fs_info *fs_info,
  * this is only called from open_ctree where we're still single threaded, thus
  * locking is omitted here.
  */
-void
-btrfs_qgroup_rescan_resume(struct btrfs_fs_info *fs_info)
+void btrfs_qgroup_rescan_resume(struct btrfs_fs_info *fs_info)
 {
 	if (fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_RESCAN)
-		btrfs_queue_work(fs_info->qgroup_rescan_workers,
-				 &fs_info->qgroup_rescan_work);
+		queue_rescan_worker(fs_info);
 }
 
 /*

Jeff Mahoney

author

2018-05-03 15:57 UTC #5 lore raw

Re: [PATCH 1/3] btrfs: qgroups, fix rescan worker running races

Quoted 47 lines · Nikolay Borisov

Quoted 33 lines · Jeff Mahoney
Quoted 27 lines · Nikolay Borisov
Quoted 15 lines · On 3.05.2018 00:11, [email protected] wrote:
From: Jeff Mahoney <[email protected]>
Commit 8d9eddad194 (Btrfs: fix qgroup rescan worker initialization) fixed the issue with BTRFS_IOC_QUOTA_RESCAN_WAIT being racy, but ended up reintroducing the hang-on-unmount bug that the commit it intended to fix addressed.
The race this time is between qgroup_rescan_init setting ->qgroup_rescan_running = true and the worker starting. There are many scenarios where we initialize the worker and never start it. The completion btrfs_ioctl_quota_rescan_wait waits for will never come. This can happen even without involving error handling, since mounting the file system read-only returns between initializing the worker and queueing it.
Quoted 8 lines
The right place to do it is when we're queuing the worker. The flag really just means that btrfs_ioctl_quota_rescan_wait should wait for a completion.
Since the BTRFS_QGROUP_STATUS_FLAG_RESCAN flag is overloaded to refer to both runtime behavior and on-disk state, we introduce a new fs_info->qgroup_rescan_ready to indicate that we're initialized and waiting to start.
Quoted 4 lines
Am I correct in my understanding that this qgroup_rescan_ready flag is used to avoid qgroup_rescan_init being called AFTER it has already been called but BEFORE queue_rescan_worker ? Why wasn't the initial version of this patch without this flag sufficient?
Quoted 10 lines
No, the race is between clearing the BTRFS_QGROUP_STATUS_FLAG_RESCAN flag near the end of the worker and clearing the running flag. The rescan lock is dropped in between, so btrfs_rescan_init will let a new rescan request in while we update the status item on disk. We wouldn't have queued another worker since that's what the warning catches, but if there were already tasks waiting for completion, they wouldn't have been woken since the wait queue list would be reinitialized. There's no way to reorder clearing the flag without changing how we handle ->qgroup_flags. I plan on doing that separately. This was just meant to be the simple fix.

Quoted 2 lines

Great, I think some of this information should go into the change log, in explaining what the symptoms of the race condition are.

You're right. I was treating as a race that my patch introduced but it didn't. It just complained about it.

-Jeff

Signature · 2 lines

Jeff Mahoney
SUSE Labs

Jeff Mahoney

author

2018-05-10 19:49 UTC #6 lore raw

Re: [PATCH 1/3] btrfs: qgroups, fix rescan worker running races

In reply to [email protected]

Quoted 209 lines · On 5/2/18 5:11 PM, [email protected] wrote:

From: Jeff Mahoney <[email protected]>

The right place to do it is when we're queuing the worker. The flag really just means that btrfs_ioctl_quota_rescan_wait should wait for a completion.

This patch introduces a new helper, queue_rescan_worker, that handles most of the initialization, the two flags, and queuing the worker, including races with unmount.

While we're at it, ->qgroup_rescan_running is protected only by the ->qgroup_rescan_mutex. btrfs_ioctl_quota_rescan_wait doesn't need to take the spinlock too.

Fixes 8d9eddad194 (Btrfs: fix qgroup rescan worker initialization)

Signed-off-by Jeff Mahoney <[email protected]>

---
 fs/btrfs/ctree.h  |  2 ++
 fs/btrfs/qgroup.c | 94 +++++++++++++++++++++++++++++++++----------------------
 2 files changed, 58 insertions(+), 38 deletions(-)

fs/btrfs/ctree.h +2 -0

index da308774b8a4..4003498bb714 100644
@@ -1045,6 +1045,8 @@ struct btrfs_fs_info {
 	struct btrfs_workqueue *qgroup_rescan_workers;
 	struct completion qgroup_rescan_completion;
 	struct btrfs_work qgroup_rescan_work;
+	/* qgroup rescan worker is running or queued to run */
+	bool qgroup_rescan_ready;
 	bool qgroup_rescan_running;	/* protected by qgroup_rescan_lock */
 
 	/* filesystem state */

fs/btrfs/qgroup.c +54 ~~-34~~

index aa259d6986e1..466744741873 100644
@@ -101,6 +101,7 @@ static int
 qgroup_rescan_init(struct btrfs_fs_info *fs_info, u64 progress_objectid,
 		   int init_flags);
 static void qgroup_rescan_zero_tracking(struct btrfs_fs_info *fs_info);
+static void btrfs_qgroup_rescan_worker(struct btrfs_work *work);
 
 /* must be called with qgroup_ioctl_lock held */
 static struct btrfs_qgroup *find_qgroup_rb(struct btrfs_fs_info *fs_info,

@@ -2072,6 +2073,46 @@ int btrfs_qgroup_account_extents(struct btrfs_trans_handle *trans,
 	return ret;
 }
 
+static void queue_rescan_worker(struct btrfs_fs_info *fs_info)
+{
+	mutex_lock(&fs_info->qgroup_rescan_lock);
+	if (btrfs_fs_closing(fs_info)) {
+		mutex_unlock(&fs_info->qgroup_rescan_lock);
+		return;
+	}
+
+	if (WARN_ON(!fs_info->qgroup_rescan_ready)) {
+		btrfs_warn(fs_info, "rescan worker not ready");
+		mutex_unlock(&fs_info->qgroup_rescan_lock);
+		return;
+	}
+	fs_info->qgroup_rescan_ready = false;
+
+	if (WARN_ON(fs_info->qgroup_rescan_running)) {
+		btrfs_warn(fs_info, "rescan worker already queued");
+		mutex_unlock(&fs_info->qgroup_rescan_lock);
+		return;
+	}
+
+	/*
+	 * Being queued is enough for btrfs_qgroup_wait_for_completion
+	 * to need to wait.
+	 */
+	fs_info->qgroup_rescan_running = true;
+	init_completion(&fs_info->qgroup_rescan_completion);
+	mutex_unlock(&fs_info->qgroup_rescan_lock);
+
+	memset(&fs_info->qgroup_rescan_work, 0,
+	       sizeof(fs_info->qgroup_rescan_work));
+
+	btrfs_init_work(&fs_info->qgroup_rescan_work,
+			btrfs_qgroup_rescan_helper,
+			btrfs_qgroup_rescan_worker, NULL, NULL);
+
+	btrfs_queue_work(fs_info->qgroup_rescan_workers,
+			 &fs_info->qgroup_rescan_work);
+}
+
 /*
  * called from commit_transaction. Writes all changed qgroups to disk.
  */

@@ -2123,8 +2164,7 @@ int btrfs_run_qgroups(struct btrfs_trans_handle *trans,
 		ret = qgroup_rescan_init(fs_info, 0, 1);
 		if (!ret) {
 			qgroup_rescan_zero_tracking(fs_info);
-			btrfs_queue_work(fs_info->qgroup_rescan_workers,
-					 &fs_info->qgroup_rescan_work);
+			queue_rescan_worker(fs_info);
 		}
 		ret = 0;
 	}

@@ -2607,6 +2647,10 @@ static void btrfs_qgroup_rescan_worker(struct btrfs_work *work)
 	if (!path)
 		goto out;
 
+	mutex_lock(&fs_info->qgroup_rescan_lock);
+	fs_info->qgroup_flags |= BTRFS_QGROUP_STATUS_FLAG_RESCAN;
+	mutex_unlock(&fs_info->qgroup_rescan_lock);
+
 	err = 0;
 	while (!err && !btrfs_fs_closing(fs_info)) {
 		trans = btrfs_start_transaction(fs_info->fs_root, 0);

@@ -2685,47 +2729,27 @@ qgroup_rescan_init(struct btrfs_fs_info *fs_info, u64 progress_objectid,
 {
 	int ret = 0;
 
-	if (!init_flags &&
-	    (!(fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_RESCAN) ||
-	     !(fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_ON))) {
+	if (!test_bit(BTRFS_FS_QUOTA_ENABLED, &fs_info->flags)) {
 		ret = -EINVAL;
 		goto err;
 	}
 
 	mutex_lock(&fs_info->qgroup_rescan_lock);
-	spin_lock(&fs_info->qgroup_lock);
-
-	if (init_flags) {
-		if (fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_RESCAN)
-			ret = -EINPROGRESS;
-		else if (!(fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_ON))
-			ret = -EINVAL;
-
-		if (ret) {
-			spin_unlock(&fs_info->qgroup_lock);
-			mutex_unlock(&fs_info->qgroup_rescan_lock);
-			goto err;
-		}
-		fs_info->qgroup_flags |= BTRFS_QGROUP_STATUS_FLAG_RESCAN;
+	if (fs_info->qgroup_rescan_ready || fs_info->qgroup_rescan_running) {
+		mutex_unlock(&fs_info->qgroup_rescan_lock);
+		ret = -EINPROGRESS;
+		goto err;
 	}
 
 	memset(&fs_info->qgroup_rescan_progress, 0,
 		sizeof(fs_info->qgroup_rescan_progress));
 	fs_info->qgroup_rescan_progress.objectid = progress_objectid;
-	init_completion(&fs_info->qgroup_rescan_completion);
-	fs_info->qgroup_rescan_running = true;
+	fs_info->qgroup_rescan_ready = true;
 
-	spin_unlock(&fs_info->qgroup_lock);
 	mutex_unlock(&fs_info->qgroup_rescan_lock);
 
-	memset(&fs_info->qgroup_rescan_work, 0,
-	       sizeof(fs_info->qgroup_rescan_work));
-	btrfs_init_work(&fs_info->qgroup_rescan_work,
-			btrfs_qgroup_rescan_helper,
-			btrfs_qgroup_rescan_worker, NULL, NULL);
-
-	if (ret) {
 err:
+	if (ret) {
 		btrfs_info(fs_info, "qgroup_rescan_init failed with %d", ret);
 		return ret;
 	}

@@ -2785,9 +2809,7 @@ btrfs_qgroup_rescan(struct btrfs_fs_info *fs_info)
 
 	qgroup_rescan_zero_tracking(fs_info);
 
-	btrfs_queue_work(fs_info->qgroup_rescan_workers,
-			 &fs_info->qgroup_rescan_work);
-
+	queue_rescan_worker(fs_info);
 	return 0;
 }

@@ -2798,9 +2820,7 @@ int btrfs_qgroup_wait_for_completion(struct btrfs_fs_info *fs_info,
 	int ret = 0;
 
 	mutex_lock(&fs_info->qgroup_rescan_lock);
-	spin_lock(&fs_info->qgroup_lock);
 	running = fs_info->qgroup_rescan_running;
-	spin_unlock(&fs_info->qgroup_lock);
 	mutex_unlock(&fs_info->qgroup_rescan_lock);
 
 	if (!running)

fs/btrfs/qgroup.c int btrfs_qgroup_wait_for_completion(struct btrfs_fs_info *fs_info, · line 2844

Quoted 8 lines

fs/btrfs/qgroup.c +1 -2

@@ -2819,12 +2839,10 @@ int btrfs_qgroup_wait_for_completion(struct btrfs_fs_info *fs_info,
  * this is only called from open_ctree where we're still single threaded, thus
  * locking is omitted here.
  */
-void
-btrfs_qgroup_rescan_resume(struct btrfs_fs_info *fs_info)
+void btrfs_qgroup_rescan_resume(struct btrfs_fs_info *fs_info)
 {
 	if (fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_RESCAN)

This check will never be true since the worker is now responsible for setting it.

-Jeff

Quoted 7 lines

fs/btrfs/qgroup.c +1 -2

-		btrfs_queue_work(fs_info->qgroup_rescan_workers,
-				 &fs_info->qgroup_rescan_work);
+		queue_rescan_worker(fs_info);
 }
 
 /*

Signature · 2 lines

Jeff Mahoney
SUSE Labs

Jeff Mahoney

author

2018-05-10 23:04 UTC #7 lore raw

Re: [PATCH 1/3] btrfs: qgroups, fix rescan worker running races

In reply to [email protected]

Quoted 154 lines · On 5/2/18 5:11 PM, [email protected] wrote:

From: Jeff Mahoney <[email protected]>

The right place to do it is when we're queuing the worker. The flag really just means that btrfs_ioctl_quota_rescan_wait should wait for a completion.

This patch introduces a new helper, queue_rescan_worker, that handles most of the initialization, the two flags, and queuing the worker, including races with unmount.

While we're at it, ->qgroup_rescan_running is protected only by the ->qgroup_rescan_mutex. btrfs_ioctl_quota_rescan_wait doesn't need to take the spinlock too.

Fixes 8d9eddad194 (Btrfs: fix qgroup rescan worker initialization)

Signed-off-by Jeff Mahoney <[email protected]>

---
 fs/btrfs/ctree.h  |  2 ++
 fs/btrfs/qgroup.c | 94 +++++++++++++++++++++++++++++++++----------------------
 2 files changed, 58 insertions(+), 38 deletions(-)

fs/btrfs/ctree.h +2 -0

index da308774b8a4..4003498bb714 100644
@@ -1045,6 +1045,8 @@ struct btrfs_fs_info {
 	struct btrfs_workqueue *qgroup_rescan_workers;
 	struct completion qgroup_rescan_completion;
 	struct btrfs_work qgroup_rescan_work;
+	/* qgroup rescan worker is running or queued to run */
+	bool qgroup_rescan_ready;
 	bool qgroup_rescan_running;	/* protected by qgroup_rescan_lock */
 
 	/* filesystem state */

fs/btrfs/qgroup.c +47 ~~-14~~

index aa259d6986e1..466744741873 100644
@@ -101,6 +101,7 @@ static int
 qgroup_rescan_init(struct btrfs_fs_info *fs_info, u64 progress_objectid,
 		   int init_flags);
 static void qgroup_rescan_zero_tracking(struct btrfs_fs_info *fs_info);
+static void btrfs_qgroup_rescan_worker(struct btrfs_work *work);
 
 /* must be called with qgroup_ioctl_lock held */
 static struct btrfs_qgroup *find_qgroup_rb(struct btrfs_fs_info *fs_info,

@@ -2072,6 +2073,46 @@ int btrfs_qgroup_account_extents(struct btrfs_trans_handle *trans,
 	return ret;
 }
 
+static void queue_rescan_worker(struct btrfs_fs_info *fs_info)
+{
+	mutex_lock(&fs_info->qgroup_rescan_lock);
+	if (btrfs_fs_closing(fs_info)) {
+		mutex_unlock(&fs_info->qgroup_rescan_lock);
+		return;
+	}
+
+	if (WARN_ON(!fs_info->qgroup_rescan_ready)) {
+		btrfs_warn(fs_info, "rescan worker not ready");
+		mutex_unlock(&fs_info->qgroup_rescan_lock);
+		return;
+	}
+	fs_info->qgroup_rescan_ready = false;
+
+	if (WARN_ON(fs_info->qgroup_rescan_running)) {
+		btrfs_warn(fs_info, "rescan worker already queued");
+		mutex_unlock(&fs_info->qgroup_rescan_lock);
+		return;
+	}
+
+	/*
+	 * Being queued is enough for btrfs_qgroup_wait_for_completion
+	 * to need to wait.
+	 */
+	fs_info->qgroup_rescan_running = true;
+	init_completion(&fs_info->qgroup_rescan_completion);
+	mutex_unlock(&fs_info->qgroup_rescan_lock);
+
+	memset(&fs_info->qgroup_rescan_work, 0,
+	       sizeof(fs_info->qgroup_rescan_work));
+
+	btrfs_init_work(&fs_info->qgroup_rescan_work,
+			btrfs_qgroup_rescan_helper,
+			btrfs_qgroup_rescan_worker, NULL, NULL);
+
+	btrfs_queue_work(fs_info->qgroup_rescan_workers,
+			 &fs_info->qgroup_rescan_work);
+}
+
 /*
  * called from commit_transaction. Writes all changed qgroups to disk.
  */

@@ -2123,8 +2164,7 @@ int btrfs_run_qgroups(struct btrfs_trans_handle *trans,
 		ret = qgroup_rescan_init(fs_info, 0, 1);
 		if (!ret) {
 			qgroup_rescan_zero_tracking(fs_info);
-			btrfs_queue_work(fs_info->qgroup_rescan_workers,
-					 &fs_info->qgroup_rescan_work);
+			queue_rescan_worker(fs_info);
 		}
 		ret = 0;
 	}

@@ -2607,6 +2647,10 @@ static void btrfs_qgroup_rescan_worker(struct btrfs_work *work)
 	if (!path)
 		goto out;
 
+	mutex_lock(&fs_info->qgroup_rescan_lock);
+	fs_info->qgroup_flags |= BTRFS_QGROUP_STATUS_FLAG_RESCAN;
+	mutex_unlock(&fs_info->qgroup_rescan_lock);
+
 	err = 0;
 	while (!err && !btrfs_fs_closing(fs_info)) {
 		trans = btrfs_start_transaction(fs_info->fs_root, 0);

@@ -2685,47 +2729,27 @@ qgroup_rescan_init(struct btrfs_fs_info *fs_info, u64 progress_objectid,
 {
 	int ret = 0;
 
-	if (!init_flags &&
-	    (!(fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_RESCAN) ||
-	     !(fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_ON))) {
+	if (!test_bit(BTRFS_FS_QUOTA_ENABLED, &fs_info->flags)) {
 		ret = -EINVAL;
 		goto err;
 	}
 
 	mutex_lock(&fs_info->qgroup_rescan_lock);
-	spin_lock(&fs_info->qgroup_lock);
-
-	if (init_flags) {
-		if (fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_RESCAN)
-			ret = -EINPROGRESS;
-		else if (!(fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_ON))
-			ret = -EINVAL;
-
-		if (ret) {

fs/btrfs/qgroup.c qgroup_rescan_init(struct btrfs_fs_info *fs_info, u64 progress_objectid, · line 2742

Quoted 10 lines

fs/btrfs/qgroup.c +4 -5

-			spin_unlock(&fs_info->qgroup_lock);
-			mutex_unlock(&fs_info->qgroup_rescan_lock);
-			goto err;
-		}
-		fs_info->qgroup_flags |= BTRFS_QGROUP_STATUS_FLAG_RESCAN;
+	if (fs_info->qgroup_rescan_ready || fs_info->qgroup_rescan_running) {
+		mutex_unlock(&fs_info->qgroup_rescan_lock);
+		ret = -EINPROGRESS;
+		goto err;
 	}

Without checking for these flags when deciding whether we want to do accounting, we'll end up doing the accounting until we start the rescan thread. This may not matter normally, but I'm working on a patch as a workaround to 824d8dff8846533c9f1f9b1eabb0c03959e989ca doing all qgroup accounting for entire trees in single transactions.

-Jeff

Quoted 59 lines

fs/btrfs/qgroup.c +5 ~~-19~~

 	memset(&fs_info->qgroup_rescan_progress, 0,
 		sizeof(fs_info->qgroup_rescan_progress));
 	fs_info->qgroup_rescan_progress.objectid = progress_objectid;
-	init_completion(&fs_info->qgroup_rescan_completion);
-	fs_info->qgroup_rescan_running = true;
+	fs_info->qgroup_rescan_ready = true;
 
-	spin_unlock(&fs_info->qgroup_lock);
 	mutex_unlock(&fs_info->qgroup_rescan_lock);
 
-	memset(&fs_info->qgroup_rescan_work, 0,
-	       sizeof(fs_info->qgroup_rescan_work));
-	btrfs_init_work(&fs_info->qgroup_rescan_work,
-			btrfs_qgroup_rescan_helper,
-			btrfs_qgroup_rescan_worker, NULL, NULL);
-
-	if (ret) {
 err:
+	if (ret) {
 		btrfs_info(fs_info, "qgroup_rescan_init failed with %d", ret);
 		return ret;
 	}

@@ -2785,9 +2809,7 @@ btrfs_qgroup_rescan(struct btrfs_fs_info *fs_info)
 
 	qgroup_rescan_zero_tracking(fs_info);
 
-	btrfs_queue_work(fs_info->qgroup_rescan_workers,
-			 &fs_info->qgroup_rescan_work);
-
+	queue_rescan_worker(fs_info);
 	return 0;
 }

@@ -2798,9 +2820,7 @@ int btrfs_qgroup_wait_for_completion(struct btrfs_fs_info *fs_info,
 	int ret = 0;
 
 	mutex_lock(&fs_info->qgroup_rescan_lock);
-	spin_lock(&fs_info->qgroup_lock);
 	running = fs_info->qgroup_rescan_running;
-	spin_unlock(&fs_info->qgroup_lock);
 	mutex_unlock(&fs_info->qgroup_rescan_lock);
 
 	if (!running)

@@ -2819,12 +2839,10 @@ int btrfs_qgroup_wait_for_completion(struct btrfs_fs_info *fs_info,
  * this is only called from open_ctree where we're still single threaded, thus
  * locking is omitted here.
  */
-void
-btrfs_qgroup_rescan_resume(struct btrfs_fs_info *fs_info)
+void btrfs_qgroup_rescan_resume(struct btrfs_fs_info *fs_info)
 {
 	if (fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_RESCAN)
-		btrfs_queue_work(fs_info->qgroup_rescan_workers,
-				 &fs_info->qgroup_rescan_work);
+		queue_rescan_worker(fs_info);
 }
 
 /*

Signature · 2 lines

Jeff Mahoney
SUSE Labs

Qu Wenruo

2020-01-16 06:41 UTC #8 lore raw

Re: [PATCH 1/3] btrfs: qgroups, fix rescan worker running races

In reply to [email protected]

Hi Jeff,

Would you like to share more info about the how the race happened?

Some stackdump or race graph, or even just reproducer would help greatly. Especially when I guess the latest qgroup rescan code is no longer affected, but without firm reproducer/race cause, I can't rule it out completely.

Thanks,
Qu

Quoted 224 lines · On 2018/5/3 上午5:11, [email protected] wrote:

From: Jeff Mahoney <[email protected]>

The right place to do it is when we're queuing the worker. The flag really just means that btrfs_ioctl_quota_rescan_wait should wait for a completion.

This patch introduces a new helper, queue_rescan_worker, that handles most of the initialization, the two flags, and queuing the worker, including races with unmount.

While we're at it, ->qgroup_rescan_running is protected only by the ->qgroup_rescan_mutex. btrfs_ioctl_quota_rescan_wait doesn't need to take the spinlock too.

Fixes 8d9eddad194 (Btrfs: fix qgroup rescan worker initialization)

Signed-off-by Jeff Mahoney <[email protected]>

---
 fs/btrfs/ctree.h  |  2 ++
 fs/btrfs/qgroup.c | 94 +++++++++++++++++++++++++++++++++----------------------
 2 files changed, 58 insertions(+), 38 deletions(-)

fs/btrfs/ctree.h +2 -0

index da308774b8a4..4003498bb714 100644
@@ -1045,6 +1045,8 @@ struct btrfs_fs_info {
 	struct btrfs_workqueue *qgroup_rescan_workers;
 	struct completion qgroup_rescan_completion;
 	struct btrfs_work qgroup_rescan_work;
+	/* qgroup rescan worker is running or queued to run */
+	bool qgroup_rescan_ready;
 	bool qgroup_rescan_running;	/* protected by qgroup_rescan_lock */
 
 	/* filesystem state */

fs/btrfs/qgroup.c +56 ~~-38~~

index aa259d6986e1..466744741873 100644
@@ -101,6 +101,7 @@ static int
 qgroup_rescan_init(struct btrfs_fs_info *fs_info, u64 progress_objectid,
 		   int init_flags);
 static void qgroup_rescan_zero_tracking(struct btrfs_fs_info *fs_info);
+static void btrfs_qgroup_rescan_worker(struct btrfs_work *work);
 
 /* must be called with qgroup_ioctl_lock held */
 static struct btrfs_qgroup *find_qgroup_rb(struct btrfs_fs_info *fs_info,

@@ -2072,6 +2073,46 @@ int btrfs_qgroup_account_extents(struct btrfs_trans_handle *trans,
 	return ret;
 }
 
+static void queue_rescan_worker(struct btrfs_fs_info *fs_info)
+{
+	mutex_lock(&fs_info->qgroup_rescan_lock);
+	if (btrfs_fs_closing(fs_info)) {
+		mutex_unlock(&fs_info->qgroup_rescan_lock);
+		return;
+	}
+
+	if (WARN_ON(!fs_info->qgroup_rescan_ready)) {
+		btrfs_warn(fs_info, "rescan worker not ready");
+		mutex_unlock(&fs_info->qgroup_rescan_lock);
+		return;
+	}
+	fs_info->qgroup_rescan_ready = false;
+
+	if (WARN_ON(fs_info->qgroup_rescan_running)) {
+		btrfs_warn(fs_info, "rescan worker already queued");
+		mutex_unlock(&fs_info->qgroup_rescan_lock);
+		return;
+	}
+
+	/*
+	 * Being queued is enough for btrfs_qgroup_wait_for_completion
+	 * to need to wait.
+	 */
+	fs_info->qgroup_rescan_running = true;
+	init_completion(&fs_info->qgroup_rescan_completion);
+	mutex_unlock(&fs_info->qgroup_rescan_lock);
+
+	memset(&fs_info->qgroup_rescan_work, 0,
+	       sizeof(fs_info->qgroup_rescan_work));
+
+	btrfs_init_work(&fs_info->qgroup_rescan_work,
+			btrfs_qgroup_rescan_helper,
+			btrfs_qgroup_rescan_worker, NULL, NULL);
+
+	btrfs_queue_work(fs_info->qgroup_rescan_workers,
+			 &fs_info->qgroup_rescan_work);
+}
+
 /*
  * called from commit_transaction. Writes all changed qgroups to disk.
  */

@@ -2123,8 +2164,7 @@ int btrfs_run_qgroups(struct btrfs_trans_handle *trans,
 		ret = qgroup_rescan_init(fs_info, 0, 1);
 		if (!ret) {
 			qgroup_rescan_zero_tracking(fs_info);
-			btrfs_queue_work(fs_info->qgroup_rescan_workers,
-					 &fs_info->qgroup_rescan_work);
+			queue_rescan_worker(fs_info);
 		}
 		ret = 0;
 	}

@@ -2607,6 +2647,10 @@ static void btrfs_qgroup_rescan_worker(struct btrfs_work *work)
 	if (!path)
 		goto out;
 
+	mutex_lock(&fs_info->qgroup_rescan_lock);
+	fs_info->qgroup_flags |= BTRFS_QGROUP_STATUS_FLAG_RESCAN;
+	mutex_unlock(&fs_info->qgroup_rescan_lock);
+
 	err = 0;
 	while (!err && !btrfs_fs_closing(fs_info)) {
 		trans = btrfs_start_transaction(fs_info->fs_root, 0);

@@ -2685,47 +2729,27 @@ qgroup_rescan_init(struct btrfs_fs_info *fs_info, u64 progress_objectid,
 {
 	int ret = 0;
 
-	if (!init_flags &&
-	    (!(fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_RESCAN) ||
-	     !(fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_ON))) {
+	if (!test_bit(BTRFS_FS_QUOTA_ENABLED, &fs_info->flags)) {
 		ret = -EINVAL;
 		goto err;
 	}
 
 	mutex_lock(&fs_info->qgroup_rescan_lock);
-	spin_lock(&fs_info->qgroup_lock);
-
-	if (init_flags) {
-		if (fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_RESCAN)
-			ret = -EINPROGRESS;
-		else if (!(fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_ON))
-			ret = -EINVAL;
-
-		if (ret) {
-			spin_unlock(&fs_info->qgroup_lock);
-			mutex_unlock(&fs_info->qgroup_rescan_lock);
-			goto err;
-		}
-		fs_info->qgroup_flags |= BTRFS_QGROUP_STATUS_FLAG_RESCAN;
+	if (fs_info->qgroup_rescan_ready || fs_info->qgroup_rescan_running) {
+		mutex_unlock(&fs_info->qgroup_rescan_lock);
+		ret = -EINPROGRESS;
+		goto err;
 	}
 
 	memset(&fs_info->qgroup_rescan_progress, 0,
 		sizeof(fs_info->qgroup_rescan_progress));
 	fs_info->qgroup_rescan_progress.objectid = progress_objectid;
-	init_completion(&fs_info->qgroup_rescan_completion);
-	fs_info->qgroup_rescan_running = true;
+	fs_info->qgroup_rescan_ready = true;
 
-	spin_unlock(&fs_info->qgroup_lock);
 	mutex_unlock(&fs_info->qgroup_rescan_lock);
 
-	memset(&fs_info->qgroup_rescan_work, 0,
-	       sizeof(fs_info->qgroup_rescan_work));
-	btrfs_init_work(&fs_info->qgroup_rescan_work,
-			btrfs_qgroup_rescan_helper,
-			btrfs_qgroup_rescan_worker, NULL, NULL);
-
-	if (ret) {
 err:
+	if (ret) {
 		btrfs_info(fs_info, "qgroup_rescan_init failed with %d", ret);
 		return ret;
 	}

@@ -2785,9 +2809,7 @@ btrfs_qgroup_rescan(struct btrfs_fs_info *fs_info)
 
 	qgroup_rescan_zero_tracking(fs_info);
 
-	btrfs_queue_work(fs_info->qgroup_rescan_workers,
-			 &fs_info->qgroup_rescan_work);
-
+	queue_rescan_worker(fs_info);
 	return 0;
 }

@@ -2798,9 +2820,7 @@ int btrfs_qgroup_wait_for_completion(struct btrfs_fs_info *fs_info,
 	int ret = 0;
 
 	mutex_lock(&fs_info->qgroup_rescan_lock);
-	spin_lock(&fs_info->qgroup_lock);
 	running = fs_info->qgroup_rescan_running;
-	spin_unlock(&fs_info->qgroup_lock);
 	mutex_unlock(&fs_info->qgroup_rescan_lock);
 
 	if (!running)

@@ -2819,12 +2839,10 @@ int btrfs_qgroup_wait_for_completion(struct btrfs_fs_info *fs_info,
  * this is only called from open_ctree where we're still single threaded, thus
  * locking is omitted here.
  */
-void
-btrfs_qgroup_rescan_resume(struct btrfs_fs_info *fs_info)
+void btrfs_qgroup_rescan_resume(struct btrfs_fs_info *fs_info)
 {
 	if (fs_info->qgroup_flags & BTRFS_QGROUP_STATUS_FLAG_RESCAN)
-		btrfs_queue_work(fs_info->qgroup_rescan_workers,
-				 &fs_info->qgroup_rescan_work);
+		queue_rescan_worker(fs_info);
 }
 
 /*

[email protected]

author

2018-05-02 21:12 UTC #9 lore raw

PATCH v3 2/3 btrfs: qgroups, remove unnecessary memset before btrfs_init_work

In reply to [email protected]

From: Jeff Mahoney <[email protected]>

btrfs_init_work clears the work struct except for ->wq, so the memset before calling btrfs_init_work in qgroup_rescan_init is unnecessary.

We'll also initialize ->wq in btrfs_init_work so that it's obvious.

Signed-off-by Jeff Mahoney <[email protected]>

---
 fs/btrfs/async-thread.c | 1 +
 fs/btrfs/qgroup.c       | 3 ---
 2 files changed, 1 insertion(+), 3 deletions(-)

fs/btrfs/async-thread.c +1 -0

index d5540749f0e5..c614fb7b9b9d 100644
@@ -354,6 +354,7 @@ void btrfs_init_work(struct btrfs_work *work, btrfs_work_func_t uniq_func,
 	INIT_WORK(&work->normal_work, uniq_func);
 	INIT_LIST_HEAD(&work->ordered_list);
 	work->flags = 0;
+	work->wq = NULL;
 }
 
 static inline void __btrfs_queue_work(struct __btrfs_workqueue *wq,

fs/btrfs/qgroup.c +0 -3

index 466744741873..3d47700c6a30 100644
@@ -2102,9 +2102,6 @@ static void queue_rescan_worker(struct btrfs_fs_info *fs_info)
 	init_completion(&fs_info->qgroup_rescan_completion);
 	mutex_unlock(&fs_info->qgroup_rescan_lock);
 
-	memset(&fs_info->qgroup_rescan_work, 0,
-	       sizeof(fs_info->qgroup_rescan_work));
-
 	btrfs_init_work(&fs_info->qgroup_rescan_work,
 			btrfs_qgroup_rescan_helper,
 			btrfs_qgroup_rescan_worker, NULL, NULL);

Signature · 1 line

2.12.3

[email protected]

author

2018-05-02 21:12 UTC #10 lore raw

PATCH v3 3/3 btrfs: qgroup, don't try to insert status item after ENOMEM in rescan worker

In reply to [email protected]

From: Jeff Mahoney <[email protected]>

If we fail to allocate memory for a path, don't bother trying to insert the qgroup status item. We haven't done anything yet and it'll fail also. Just print an error and be done with it.

Signed-off-by Jeff Mahoney <[email protected]>

---
 fs/btrfs/qgroup.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

fs/btrfs/qgroup.c +4 -5

index 3d47700c6a30..44d5e3da835a 100644
@@ -2666,7 +2666,6 @@ static void btrfs_qgroup_rescan_worker(struct btrfs_work *work)
 			btrfs_end_transaction(trans);
 	}
 
-out:
 	btrfs_free_path(path);
 
 	mutex_lock(&fs_info->qgroup_rescan_lock);

@@ -2702,13 +2701,13 @@ static void btrfs_qgroup_rescan_worker(struct btrfs_work *work)
 
 	if (btrfs_fs_closing(fs_info)) {
 		btrfs_info(fs_info, "qgroup scan paused");
-	} else if (err >= 0) {
+		err = 0;
+	} else if (err >= 0)
 		btrfs_info(fs_info, "qgroup scan completed%s",
 			err > 0 ? " (inconsistency flag cleared)" : "");
-	} else {
+out:
+	if (err < 0)
 		btrfs_err(fs_info, "qgroup scan failed with %d", err);
-	}
-
 done:
 	mutex_lock(&fs_info->qgroup_rescan_lock);
 	fs_info->qgroup_rescan_running = false;

Signature · 1 line

2.12.3

Nikolay Borisov

2018-05-03 06:23 UTC #11 lore raw

In reply to [email protected]

Quoted 4 lines · On 3.05.2018 00:11, [email protected] wrote:

From: Jeff Mahoney <[email protected]>
Hi Dave -

Quoted 8 lines

Here's the updated patchset for the rescan races. This fixes the issue where we'd try to start multiple workers. It introduces a new "ready" bool that we set during initialization and clear while queuing the worker. The queuer is also now responsible for most of the initialization.
I have a separate patch set start that gets rid of the racy mess surrounding the rescan worker startup. We can handle it in btrfs_run_qgroups and just set a flag to start it everywhere else.

I'd be interested in seeing those patches. Some time ago I did send a patch which cleaned up the way qgroup rescan was initiated. It was done from "btrfs_run_qgroups" and I think this is messy. Whatever we do we ought to really have well-defined semantics when qgroups rescan are run, preferably we shouldn't be conflating rescan + run (unless there is _really_ good reason to do). In the past the rescan from scan was used only during qgroup enabling.

Quoted 16 lines

-Jeff
---
Jeff Mahoney (3):
btrfs: qgroups, fix rescan worker running races
btrfs: qgroups, remove unnecessary memset before btrfs_init_work
btrfs: qgroup, don't try to insert status item after ENOMEM in rescan
worker
 fs/btrfs/async-thread.c |   1 +
 fs/btrfs/ctree.h        |   2 +
 fs/btrfs/qgroup.c       | 100 +++++++++++++++++++++++++++---------------------
 3 files changed, 60 insertions(+), 43 deletions(-)

Jeff Mahoney

author

2018-05-03 22:27 UTC #12 lore raw

Quoted 15 lines · Nikolay Borisov

Quoted 4 lines · On 3.05.2018 00:11, [email protected] wrote:
From: Jeff Mahoney <[email protected]>
Hi Dave -
Quoted 8 lines
Here's the updated patchset for the rescan races. This fixes the issue where we'd try to start multiple workers. It introduces a new "ready" bool that we set during initialization and clear while queuing the worker. The queuer is also now responsible for most of the initialization.
I have a separate patch set start that gets rid of the racy mess surrounding the rescan worker startup. We can handle it in btrfs_run_qgroups and just set a flag to start it everywhere else.

Quoted 7 lines

I'd be interested in seeing those patches. Some time ago I did send a patch which cleaned up the way qgroup rescan was initiated. It was done from "btrfs_run_qgroups" and I think this is messy. Whatever we do we ought to really have well-defined semantics when qgroups rescan are run, preferably we shouldn't be conflating rescan + run (unless there is _really_ good reason to do). In the past the rescan from scan was used only during qgroup enabling.

I think btrfs_run_qgroups is the place to do it. Here's why:

2773 int
2774 btrfs_qgroup_rescan(struct btrfs_fs_info *fs_info) 2775 {
2776 int ret = 0;
2777 struct btrfs_trans_handle *trans;
2778
2779 ret = qgroup_rescan_init(fs_info, 0, 1);
2780 if (ret)
2781 return ret;
2782
2783 /*

2784          * We have set the rescan_progress to 0, which means no more
2785          * delayed refs will be accounted by btrfs_qgroup_account_ref.
2786          * However, btrfs_qgroup_account_ref may be right after its call
2787          * to btrfs_find_all_roots, in which case it would still do the
2788          * accounting.
2789          * To solve this, we're committing the transaction, which will
2790          * ensure we run all delayed refs and only after that, we are
2791          * going to clear all tracking information for a clean start.
2792          */

2793
2794 trans = btrfs_join_transaction(fs_info->fs_root);
2795 if (IS_ERR(trans)) {
2796 fs_info->qgroup_flags &= ~BTRFS_QGROUP_STATUS_FLAG_RESCAN;
2797 return PTR_ERR(trans);

2798         }
2799         ret = btrfs_commit_transaction(trans);
2800         if (ret) {

2801 fs_info->qgroup_flags &= ~BTRFS_QGROUP_STATUS_FLAG_RESCAN;
2802 return ret;
2803 }
2804
2805 qgroup_rescan_zero_tracking(fs_info);
2806
2807 queue_rescan_worker(fs_info);
2808 return 0;
2809 }

The delayed ref race should exist anywhere we initiate a rescan outside of initially enabling qgroups. We already zero the tracking and queue the rescan worker in btrfs_run_qgroups for when we enable qgroups. Why not just always queue the worker there so the initialization and execution has a clear starting point?

There are a few other races I'd like to fix as well. We call btrfs_run_qgroups directly from btrfs_ioctl_qgroup_assign, which is buggy since btrfs_add_qgroup_relation only checks to see if the quota_root exists. It will exist as soon as btrfs_quota_enable runs but we won't have committed the transaction yet. The call will end up enabling quotas in the middle of a transaction.

-Jeff

Signature · 2 lines

Jeff Mahoney
SUSE Labs

Nikolay Borisov

2018-05-04 05:59 UTC #13 lore raw

Quoted 64 lines · Jeff Mahoney

Quoted 15 lines · Nikolay Borisov
Quoted 4 lines · On 3.05.2018 00:11, [email protected] wrote:
From: Jeff Mahoney <[email protected]>
Hi Dave -
Quoted 8 lines
Here's the updated patchset for the rescan races. This fixes the issue where we'd try to start multiple workers. It introduces a new "ready" bool that we set during initialization and clear while queuing the worker. The queuer is also now responsible for most of the initialization.
I have a separate patch set start that gets rid of the racy mess surrounding the rescan worker startup. We can handle it in btrfs_run_qgroups and just set a flag to start it everywhere else.
Quoted 7 lines
I'd be interested in seeing those patches. Some time ago I did send a patch which cleaned up the way qgroup rescan was initiated. It was done from "btrfs_run_qgroups" and I think this is messy. Whatever we do we ought to really have well-defined semantics when qgroups rescan are run, preferably we shouldn't be conflating rescan + run (unless there is _really_ good reason to do). In the past the rescan from scan was used only during qgroup enabling.
I think btrfs_run_qgroups is the place to do it. Here's why:
2773 int
2774 btrfs_qgroup_rescan(struct btrfs_fs_info *fs_info) 2775 {
2776 int ret = 0;
2777 struct btrfs_trans_handle *trans;
2778
2779 ret = qgroup_rescan_init(fs_info, 0, 1);
2780 if (ret)
2781 return ret;
2782
2783 /*
2784          * We have set the rescan_progress to 0, which means no more
2785          * delayed refs will be accounted by btrfs_qgroup_account_ref.
2786          * However, btrfs_qgroup_account_ref may be right after its call
2787          * to btrfs_find_all_roots, in which case it would still do the
2788          * accounting.
2789          * To solve this, we're committing the transaction, which will
2790          * ensure we run all delayed refs and only after that, we are
2791          * going to clear all tracking information for a clean start.
2792          */
2793
2794 trans = btrfs_join_transaction(fs_info->fs_root);
2795 if (IS_ERR(trans)) {
2796 fs_info->qgroup_flags &= ~BTRFS_QGROUP_STATUS_FLAG_RESCAN;
2797 return PTR_ERR(trans);
2798         }
2799         ret = btrfs_commit_transaction(trans);
2800         if (ret) {
2801 fs_info->qgroup_flags &= ~BTRFS_QGROUP_STATUS_FLAG_RESCAN;
2802 return ret;
2803 }
2804
2805 qgroup_rescan_zero_tracking(fs_info);
2806
2807 queue_rescan_worker(fs_info);
2808 return 0;
2809 }

Quoted 4 lines

The delayed ref race should exist anywhere we initiate a rescan outside of initially enabling qgroups. We already zero the tracking and queue the rescan worker in btrfs_run_qgroups for when we enable qgroups. Why not just always queue the worker there so the initialization and execution has a clear starting point?

This is no longer true in upstream as of commit 5d23515be669 ("btrfs:
Move qgroup rescan on quota enable to btrfs_quota_enable"). Hence my asking about this. I guess if we make it unconditional it won't increase the complexity, but the original code which was only run during qgroup enable was rather iffy I Just don't want to repeat this.

Quoted 8 lines

There are a few other races I'd like to fix as well. We call btrfs_run_qgroups directly from btrfs_ioctl_qgroup_assign, which is buggy since btrfs_add_qgroup_relation only checks to see if the quota_root exists. It will exist as soon as btrfs_quota_enable runs but we won't have committed the transaction yet. The call will end up enabling quotas in the middle of a transaction.
-Jeff

Jeff Mahoney

author

2018-05-04 13:32 UTC #14 lore raw

Quoted 72 lines · Nikolay Borisov

Quoted 64 lines · Jeff Mahoney
Quoted 15 lines · Nikolay Borisov
Quoted 4 lines · On 3.05.2018 00:11, [email protected] wrote:
From: Jeff Mahoney <[email protected]>
Hi Dave -
Quoted 8 lines
Here's the updated patchset for the rescan races. This fixes the issue where we'd try to start multiple workers. It introduces a new "ready" bool that we set during initialization and clear while queuing the worker. The queuer is also now responsible for most of the initialization.
I have a separate patch set start that gets rid of the racy mess surrounding the rescan worker startup. We can handle it in btrfs_run_qgroups and just set a flag to start it everywhere else.
Quoted 7 lines
I'd be interested in seeing those patches. Some time ago I did send a patch which cleaned up the way qgroup rescan was initiated. It was done from "btrfs_run_qgroups" and I think this is messy. Whatever we do we ought to really have well-defined semantics when qgroups rescan are run, preferably we shouldn't be conflating rescan + run (unless there is _really_ good reason to do). In the past the rescan from scan was used only during qgroup enabling.
I think btrfs_run_qgroups is the place to do it. Here's why:
2773 int
2774 btrfs_qgroup_rescan(struct btrfs_fs_info *fs_info) 2775 {
2776 int ret = 0;
2777 struct btrfs_trans_handle *trans;
2778
2779 ret = qgroup_rescan_init(fs_info, 0, 1);
2780 if (ret)
2781 return ret;
2782
2783 /*
2784          * We have set the rescan_progress to 0, which means no more
2785          * delayed refs will be accounted by btrfs_qgroup_account_ref.
2786          * However, btrfs_qgroup_account_ref may be right after its call
2787          * to btrfs_find_all_roots, in which case it would still do the
2788          * accounting.
2789          * To solve this, we're committing the transaction, which will
2790          * ensure we run all delayed refs and only after that, we are
2791          * going to clear all tracking information for a clean start.
2792          */
2793
2794 trans = btrfs_join_transaction(fs_info->fs_root);
2795 if (IS_ERR(trans)) {
2796 fs_info->qgroup_flags &= ~BTRFS_QGROUP_STATUS_FLAG_RESCAN;
2797 return PTR_ERR(trans);
2798         }
2799         ret = btrfs_commit_transaction(trans);
2800         if (ret) {
2801 fs_info->qgroup_flags &= ~BTRFS_QGROUP_STATUS_FLAG_RESCAN;
2802 return ret;
2803 }
2804
2805 qgroup_rescan_zero_tracking(fs_info);
2806
2807 queue_rescan_worker(fs_info);
2808 return 0;
2809 }
Quoted 4 lines
The delayed ref race should exist anywhere we initiate a rescan outside of initially enabling qgroups. We already zero the tracking and queue the rescan worker in btrfs_run_qgroups for when we enable qgroups. Why not just always queue the worker there so the initialization and execution has a clear starting point?

Quoted 5 lines

This is no longer true in upstream as of commit 5d23515be669 ("btrfs:
Move qgroup rescan on quota enable to btrfs_quota_enable"). Hence my asking about this. I guess if we make it unconditional it won't increase the complexity, but the original code which was only run during qgroup enable was rather iffy I Just don't want to repeat this.

Ah, ok. My repo is still using v4.16. How does this work with the race that is described in btrfs_qgroup_rescan?

-Jeff

Signature · 2 lines

Jeff Mahoney
SUSE Labs

Nikolay Borisov

2018-05-04 13:41 UTC #15 lore raw

Quoted 73 lines · Jeff Mahoney

Quoted 72 lines · Nikolay Borisov
Quoted 68 lines · Jeff Mahoney
Quoted 15 lines · Nikolay Borisov
Quoted 4 lines · On 3.05.2018 00:11, [email protected] wrote:
From: Jeff Mahoney <[email protected]>
Hi Dave -
Quoted 8 lines
Here's the updated patchset for the rescan races. This fixes the issue where we'd try to start multiple workers. It introduces a new "ready" bool that we set during initialization and clear while queuing the worker. The queuer is also now responsible for most of the initialization.
I have a separate patch set start that gets rid of the racy mess surrounding the rescan worker startup. We can handle it in btrfs_run_qgroups and just set a flag to start it everywhere else.
Quoted 7 lines
I'd be interested in seeing those patches. Some time ago I did send a patch which cleaned up the way qgroup rescan was initiated. It was done from "btrfs_run_qgroups" and I think this is messy. Whatever we do we ought to really have well-defined semantics when qgroups rescan are run, preferably we shouldn't be conflating rescan + run (unless there is _really_ good reason to do). In the past the rescan from scan was used only during qgroup enabling.
I think btrfs_run_qgroups is the place to do it. Here's why:
2773 int
2774 btrfs_qgroup_rescan(struct btrfs_fs_info *fs_info) 2775 {
2776 int ret = 0;
2777 struct btrfs_trans_handle *trans;
2778
2779 ret = qgroup_rescan_init(fs_info, 0, 1);
2780 if (ret)
2781 return ret;
2782
2783 /*
2784          * We have set the rescan_progress to 0, which means no more
2785          * delayed refs will be accounted by btrfs_qgroup_account_ref.
2786          * However, btrfs_qgroup_account_ref may be right after its call
2787          * to btrfs_find_all_roots, in which case it would still do the
2788          * accounting.
2789          * To solve this, we're committing the transaction, which will
2790          * ensure we run all delayed refs and only after that, we are
2791          * going to clear all tracking information for a clean start.
2792          */
2793
2794 trans = btrfs_join_transaction(fs_info->fs_root);
2795 if (IS_ERR(trans)) {
2796 fs_info->qgroup_flags &= ~BTRFS_QGROUP_STATUS_FLAG_RESCAN;
2797 return PTR_ERR(trans);
2798         }
2799         ret = btrfs_commit_transaction(trans);
2800         if (ret) {
2801 fs_info->qgroup_flags &= ~BTRFS_QGROUP_STATUS_FLAG_RESCAN;
2802 return ret;
2803 }
2804
2805 qgroup_rescan_zero_tracking(fs_info);
2806
2807 queue_rescan_worker(fs_info);
2808 return 0;
2809 }
The delayed ref race should exist anywhere we initiate a rescan outside of initially enabling qgroups. We already zero the tracking and queue the rescan worker in btrfs_run_qgroups for when we enable qgroups. Why not just always queue the worker there so the initialization and execution has a clear starting point?

Quoted 8 lines

Quoted 5 lines
This is no longer true in upstream as of commit 5d23515be669 ("btrfs:
Move qgroup rescan on quota enable to btrfs_quota_enable"). Hence my asking about this. I guess if we make it unconditional it won't increase the complexity, but the original code which was only run during qgroup enable was rather iffy I Just don't want to repeat this.
Ah, ok. My repo is still using v4.16. How does this work with the race that is described in btrfs_qgroup_rescan?

TBH I didn't even consider it. It seems the qgroups code is just a minefield ;\. So the original code only ever queued the rescan from btrfs_run_qgroups if we were enabling qgroups i.e once. So I just moved the code to queue the scan during the ioctl (btrfs_quota_enable) execution. Prior to my patch it seems that the rescan following qgroup enable was triggered during the first transaction commit.

Quoted 3 lines

-Jeff

Qu Wenruo

2019-11-28 03:30 UTC #16 lore raw

In reply to [email protected]

Any feedback and update on this patchset?

It looks like the first patch is going to fix a bug of btrfs unable to unmount the fs due to deadlock rescan.

Thanks,
Qu

Quoted 28 lines · On 2018/5/3 上午5:11, [email protected] wrote:

From: Jeff Mahoney <[email protected]>
Hi Dave -
Here's the updated patchset for the rescan races. This fixes the issue where we'd try to start multiple workers. It introduces a new "ready" bool that we set during initialization and clear while queuing the worker. The queuer is also now responsible for most of the initialization.
I have a separate patch set start that gets rid of the racy mess surrounding the rescan worker startup. We can handle it in btrfs_run_qgroups and just set a flag to start it everywhere else.
-Jeff
---
Jeff Mahoney (3):
btrfs: qgroups, fix rescan worker running races
btrfs: qgroups, remove unnecessary memset before btrfs_init_work
btrfs: qgroup, don't try to insert status item after ENOMEM in rescan
worker
 fs/btrfs/async-thread.c |   1 +
 fs/btrfs/ctree.h        |   2 +
 fs/btrfs/qgroup.c       | 100 +++++++++++++++++++++++++++---------------------
 3 files changed, 60 insertions(+), 43 deletions(-)

David Sterba

2019-12-03 19:32 UTC #17 lore raw

Quoted 4 lines · Qu Wenruo

Any feedback and update on this patchset?
It looks like the first patch is going to fix a bug of btrfs unable to unmount the fs due to deadlock rescan.

The patchset was posted 1.5 year ago, it would be better to resend. I think there were some changes in the quota rescan, there were also some replies to the thread so fresh iteration can continue the discussion. Thanks.