FAILED: patch "[PATCH] ublk: wait on ublk_dev_ready() instead of ub->completion" failed to apply to 6.18-stable tree

[email protected]

author maintainer

2026-07-29 13:33 UTC #0 lore raw

The patch below does not apply to the 6.18-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-6.18.y
git checkout FETCH_HEAD
git cherry-pick -x 432a9b2780c0a01caf547bd1fc2fcf28aeb8d173
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026072957-overdue-curing-588b@gregkh' --subject-prefix 'PATCH 6.18.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 432a9b2780c0a01caf547bd1fc2fcf28aeb8d173 Mon Sep 17 00:00:00 2001
From: Ming Lei <[email protected]>
Date: Sun, 19 Jul 2026 08:45:40 -0500
Subject: [PATCH] ublk: wait on ublk_dev_ready() instead of ub->completion

ub->completion is only re-armed by a successful START_USER_RECOVERY. If the ublk server sends END_USER_RECOVERY without one - e.g. its START failed with -EBUSY and the error was ignored - the wait is satisfied by the stale completion of the previous recovery cycle, and the device is marked LIVE and the requeue list kicked while the FETCH stream is still running and ubq->canceling is still set. The kick redispatches a previously requeued request, __ublk_queue_rq_common() sees ->canceling and parks it again via __ublk_abort_rq(), and after the last FETCH clears ->canceling nothing ever kicks the requeue list again: the request is stranded there while holding its tag. If it is the flush machinery's flush_rq, every subsequent fsync piles up in uninterruptible sleep and teardown hangs on tag draining. This matches a report of a lost PREFLUSH with ext4 on top of ublk after daemon crash recovery.

ub->completion is an edge-triggered latch used as a proxy for the level condition "every queue has fetched all I/O commands", which can regress (F_BATCH's UNPREP, daemon death) and whose re-arm can be skipped. Drop it and wait on the real condition instead: the new helper ublk_wait_dev_ready_and_lock() waits on ublk_dev_ready() via wait_var_event_interruptible(), woken from ublk_mark_io_ready(), then re-checks it under ub->mutex, waiting again on regression, and returns with the mutex held and readiness guaranteed.

Readiness becomes true in the same ub->mutex critical section that clears the last queue's ->canceling, so END_USER_RECOVERY marks the device LIVE and kicks the requeue list strictly after ->canceling clears. The wait stays interruptible, so a server whose daemon died can still be signalled out. For ublk_ctrl_start_dev() this replaces the fail-fast -EINVAL on an F_BATCH ready->UNPREP regression with waiting until the device is ready again.

Reported-by George Salisbury <[email protected]>

Fixes 728cbac5fe21 ("ublk: move device reset into ublk_ch_release()")

Cc [email protected]

Signed-off-by Ming Lei <[email protected]>

Link https://patch.msgid.link/[email protected] open here

Signed-off-by Jens Axboe <[email protected]>

drivers/block/ublk_drv.c +30 ~~-17~~

index c2c11f2a01e7..4ca6ec738c93 100644
@@ -19,6 +19,7 @@
 #include <linux/errno.h>
 #include <linux/major.h>
 #include <linux/wait.h>
+#include <linux/wait_bit.h>
 #include <linux/blkdev.h>
 #include <linux/init.h>
 #include <linux/swap.h>

@@ -26,7 +27,6 @@
 #include <linux/compat.h>
 #include <linux/mutex.h>
 #include <linux/writeback.h>
-#include <linux/completion.h>
 #include <linux/highmem.h>
 #include <linux/sysfs.h>
 #include <linux/miscdevice.h>

@@ -327,7 +327,6 @@ struct ublk_device {
 
 	struct ublk_params	params;
 
-	struct completion	completion;
 	u32			nr_queue_ready;
 	bool 			unprivileged_daemons;
 	struct mutex cancel_mutex;

@@ -3054,12 +3053,12 @@ static void ublk_mark_io_ready(struct ublk_device *ub, u16 q_id,
 	if (ublk_dev_ready(ub)) {
 		/*
 		 * All queues ready - clear device-level canceling flag
-		 * and complete the recovery/initialization.
+		 * and wake ublk_dev_ready() waiters.
 		 */
 		mutex_lock(&ub->cancel_mutex);
 		ub->canceling = false;
 		mutex_unlock(&ub->cancel_mutex);
-		complete_all(&ub->completion);
+		wake_up_var(&ub->nr_queue_ready);
 	}
 }

@@ -4273,7 +4272,6 @@ static int ublk_init_queues(struct ublk_device *ub)
 			goto fail;
 	}
 
-	init_completion(&ub->completion);
 	return 0;
 
  fail:

@@ -4417,6 +4415,26 @@ static bool ublk_validate_user_pid(struct ublk_device *ub, pid_t ublksrv_pid)
 	return ub->ublksrv_tgid == ublksrv_pid;
 }
 
+/*
+ * Wait until all queues have fetched their I/O commands, and return with
+ * ub->mutex held and readiness guaranteed: then every queue's ->canceling
+ * is cleared. Ready may regress between wakeup and mutex_lock() (F_BATCH
+ * UNPREP, daemon death), so re-check it under the mutex and wait again.
+ */
+static int ublk_wait_dev_ready_and_lock(struct ublk_device *ub)
+{
+	while (true) {
+		if (wait_var_event_interruptible(&ub->nr_queue_ready,
+						 ublk_dev_ready(ub)))
+			return -EINTR;
+
+		mutex_lock(&ub->mutex);
+		if (ublk_dev_ready(ub))
+			return 0;
+		mutex_unlock(&ub->mutex);
+	}
+}
+
 static int ublk_ctrl_start_dev(struct ublk_device *ub,
 		const struct ublksrv_ctrl_cmd *header)
 {

@@ -4499,15 +4517,10 @@ static int ublk_ctrl_start_dev(struct ublk_device *ub,
 		};
 	}
 
-	if (wait_for_completion_interruptible(&ub->completion) != 0)
+	if (ublk_wait_dev_ready_and_lock(ub))
 		return -EINTR;
 
-	if (!ublk_validate_user_pid(ub, ublksrv_pid))
-		return -EINVAL;
-
-	mutex_lock(&ub->mutex);
-	/* device may become not ready in case of F_BATCH */
-	if (!ublk_dev_ready(ub)) {
+	if (!ublk_validate_user_pid(ub, ublksrv_pid)) {
 		ret = -EINVAL;
 		goto out_unlock;
 	}

@@ -5071,7 +5084,6 @@ static int ublk_ctrl_start_recovery(struct ublk_device *ub)
 		goto out_unlock;
 	}
 	pr_devel("%s: start recovery for dev id %d\n", __func__, ub->ub_number);
-	init_completion(&ub->completion);
 	ret = 0;
  out_unlock:
 	mutex_unlock(&ub->mutex);

@@ -5087,16 +5099,17 @@ static int ublk_ctrl_end_recovery(struct ublk_device *ub,
 	pr_devel("%s: Waiting for all FETCH_REQs, dev id %d...\n", __func__,
 		 header->dev_id);
 
-	if (wait_for_completion_interruptible(&ub->completion))
+	if (ublk_wait_dev_ready_and_lock(ub))
 		return -EINTR;
 
 	pr_devel("%s: All FETCH_REQs received, dev id %d\n", __func__,
 		 header->dev_id);
 
-	if (!ublk_validate_user_pid(ub, ublksrv_pid))
-		return -EINVAL;
+	if (!ublk_validate_user_pid(ub, ublksrv_pid)) {
+		ret = -EINVAL;
+		goto out_unlock;
+	}
 
-	mutex_lock(&ub->mutex);
 	if (ublk_nosrv_should_stop_dev(ub))
 		goto out_unlock;

Ming Lei

maintainer

2026-07-29 14:48 UTC #1 lore raw

PATCH 1/1 ublk: wait on ublk_dev_ready() instead of ub->completion

ub->completion is only re-armed by a successful START_USER_RECOVERY. If the ublk server sends END_USER_RECOVERY without one - e.g. its START failed with -EBUSY and the error was ignored - the wait is satisfied by the stale completion of the previous recovery cycle, and the device is marked LIVE and the requeue list kicked while the FETCH stream is still running and ubq->canceling is still set. The kick redispatches a previously requeued request, __ublk_queue_rq_common() sees ->canceling and parks it again via __ublk_abort_rq(), and after the last FETCH clears ->canceling nothing ever kicks the requeue list again: the request is stranded there while holding its tag. If it is the flush machinery's flush_rq, every subsequent fsync piles up in uninterruptible sleep and teardown hangs on tag draining. This matches a report of a lost PREFLUSH with ext4 on top of ublk after daemon crash recovery.

ub->completion is an edge-triggered latch used as a proxy for the level condition "every queue has fetched all I/O commands", which can regress (F_BATCH's UNPREP, daemon death) and whose re-arm can be skipped. Drop it and wait on the real condition instead: the new helper ublk_wait_dev_ready_and_lock() waits on ublk_dev_ready() via wait_var_event_interruptible(), woken from ublk_mark_io_ready(), then re-checks it under ub->mutex, waiting again on regression, and returns with the mutex held and readiness guaranteed.

Readiness becomes true in the same ub->mutex critical section that clears the last queue's ->canceling, so END_USER_RECOVERY marks the device LIVE and kicks the requeue list strictly after ->canceling clears. The wait stays interruptible, so a server whose daemon died can still be signalled out. For ublk_ctrl_start_dev() this replaces the fail-fast -EINVAL on an F_BATCH ready->UNPREP regression with waiting until the device is ready again.

Reported-by George Salisbury <[email protected]>

Fixes 728cbac5fe21 ("ublk: move device reset into ublk_ch_release()")

Cc [email protected]

Signed-off-by Ming Lei <[email protected]>

Link https://patch.msgid.link/[email protected] open here

Signed-off-by Jens Axboe <[email protected]>

(cherry picked from commit 432a9b2780c0a01caf547bd1fc2fcf28aeb8d173) [ 6.18.y tracks readiness per fetched I/O command in ->nr_io_ready rather
than per queue in ->nr_queue_ready, so wait and wake on that counter; ->canceling is cleared by ublk_reset_io_flags(), which stays in ublk_mark_io_ready(). UBLK_F_BATCH_IO does not exist here, so ublk_ctrl_start_dev() has no F_BATCH readiness re-check to replace, and readiness can only regress via ublk_reset_ch_dev() on daemon death. ]

---
 drivers/block/ublk_drv.c | 52 ++++++++++++++++++++++++++++------------
 1 file changed, 37 insertions(+), 15 deletions(-)

drivers/block/ublk_drv.c +37 ~~-15~~

index c339222513b0..cb31e96f01cd 100644
@@ -19,6 +19,7 @@
 #include <linux/errno.h>
 #include <linux/major.h>
 #include <linux/wait.h>
+#include <linux/wait_bit.h>
 #include <linux/blkdev.h>
 #include <linux/init.h>
 #include <linux/swap.h>

@@ -26,7 +27,6 @@
 #include <linux/compat.h>
 #include <linux/mutex.h>
 #include <linux/writeback.h>
-#include <linux/completion.h>
 #include <linux/highmem.h>
 #include <linux/sysfs.h>
 #include <linux/miscdevice.h>

@@ -230,7 +230,6 @@ struct ublk_device {
 
 	struct ublk_params	params;
 
-	struct completion	completion;
 	u32			nr_io_ready;
 	bool 			unprivileged_daemons;
 	struct mutex cancel_mutex;

@@ -2150,9 +2149,13 @@ static void ublk_mark_io_ready(struct ublk_device *ub)
 
 	ub->nr_io_ready++;
 	if (ublk_dev_ready(ub)) {
-		/* now we are ready for handling ublk io request */
+		/*
+		 * now we are ready for handling ublk io request, clear
+		 * device-level canceling flag and wake ublk_dev_ready()
+		 * waiters
+		 */
 		ublk_reset_io_flags(ub);
-		complete_all(&ub->completion);
+		wake_up_var(&ub->nr_io_ready);
 	}
 }

@@ -2829,7 +2832,6 @@ static int ublk_init_queues(struct ublk_device *ub)
 			goto fail;
 	}
 
-	init_completion(&ub->completion);
 	return 0;
 
  fail:

@@ -2966,6 +2968,26 @@ static bool ublk_validate_user_pid(struct ublk_device *ub, pid_t ublksrv_pid)
 	return ub->ublksrv_tgid == ublksrv_pid;
 }
 
+/*
+ * Wait until all queues have fetched their I/O commands, and return with
+ * ub->mutex held and readiness guaranteed: then every queue's ->canceling
+ * is cleared. Ready may regress between wakeup and mutex_lock() (daemon
+ * death), so re-check it under the mutex and wait again.
+ */
+static int ublk_wait_dev_ready_and_lock(struct ublk_device *ub)
+{
+	while (true) {
+		if (wait_var_event_interruptible(&ub->nr_io_ready,
+						 ublk_dev_ready(ub)))
+			return -EINTR;
+
+		mutex_lock(&ub->mutex);
+		if (ublk_dev_ready(ub))
+			return 0;
+		mutex_unlock(&ub->mutex);
+	}
+}
+
 static int ublk_ctrl_start_dev(struct ublk_device *ub,
 		const struct ublksrv_ctrl_cmd *header)
 {

@@ -3031,13 +3053,13 @@ static int ublk_ctrl_start_dev(struct ublk_device *ub,
 		lim.max_segments = ub->params.seg.max_segments;
 	}
 
-	if (wait_for_completion_interruptible(&ub->completion) != 0)
+	if (ublk_wait_dev_ready_and_lock(ub))
 		return -EINTR;
 
-	if (!ublk_validate_user_pid(ub, ublksrv_pid))
-		return -EINVAL;
-
-	mutex_lock(&ub->mutex);
+	if (!ublk_validate_user_pid(ub, ublksrv_pid)) {
+		ret = -EINVAL;
+		goto out_unlock;
+	}
 	if (ub->dev_info.state == UBLK_S_DEV_LIVE ||
 	    test_bit(UB_STATE_USED, &ub->state)) {
 		ret = -EEXIST;

@@ -3549,7 +3571,6 @@ static int ublk_ctrl_start_recovery(struct ublk_device *ub,
 		goto out_unlock;
 	}
 	pr_devel("%s: start recovery for dev id %d.\n", __func__, header->dev_id);
-	init_completion(&ub->completion);
 	ret = 0;
  out_unlock:
 	mutex_unlock(&ub->mutex);

@@ -3565,16 +3586,17 @@ static int ublk_ctrl_end_recovery(struct ublk_device *ub,
 	pr_devel("%s: Waiting for all FETCH_REQs, dev id %d...\n", __func__,
 		 header->dev_id);
 
-	if (wait_for_completion_interruptible(&ub->completion))
+	if (ublk_wait_dev_ready_and_lock(ub))
 		return -EINTR;
 
 	pr_devel("%s: All FETCH_REQs received, dev id %d\n", __func__,
 		 header->dev_id);
 
-	if (!ublk_validate_user_pid(ub, ublksrv_pid))
-		return -EINVAL;
+	if (!ublk_validate_user_pid(ub, ublksrv_pid)) {
+		ret = -EINVAL;
+		goto out_unlock;
+	}
 
-	mutex_lock(&ub->mutex);
 	if (ublk_nosrv_should_stop_dev(ub))
 		goto out_unlock;

Signature · 1 line

2.55.0

Sasha Levin

maintainer Applied

2026-07-30 02:40 UTC #2 lore raw

Re: [PATCH 6.18.y] ublk: wait on ublk_dev_ready() instead of ub->completion

Quoted 1 line

[PATCH 6.18.y] ublk: wait on ublk_dev_ready() instead of ub->completion

Queued for 6.18, thanks.

Signature · 2 lines

Thanks,
Sasha