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FAILED: patch "[PATCH] dm-delay: fix a race between delay_presuspend and delay_bio" failed to apply to 6.5-stable tree

PATCH [email protected] 1 participant 1 message Nov 24, 2023

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The patch below does not apply to the 6.5-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-6.5.y
git checkout FETCH_HEAD
git cherry-pick -x 6fc45b6ed921dc00dfb264dc08c7d67ee63d2656
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2023112421-cathedral-speckled-9fa0@gregkh' --subject-prefix 'PATCH 6.5.y' HEAD^..

Possible dependencies:

6fc45b6ed921 ("dm-delay: fix a race between delay_presuspend and delay_bio") 70bbeb29fab0 ("dm delay: for short delays, use kthread instead of timers and wq")

thanks,

greg k-h

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

From 6fc45b6ed921dc00dfb264dc08c7d67ee63d2656 Mon Sep 17 00:00:00 2001
From: Mikulas Patocka <[email protected]>
Date: Fri, 17 Nov 2023 18:21:14 +0100
Subject: [PATCH] dm-delay: fix a race between delay_presuspend and delay_bio

In delay_presuspend, we set the atomic variable may_delay and then stop the timer and flush pending bios. The intention here is to prevent the delay target from re-arming the timer again.

However, this test is racy. Suppose that one thread goes to delay_bio, sees that dc->may_delay is one and proceeds; now, another thread executes delay_presuspend, it sets dc->may_delay to zero, deletes the timer and flushes pending bios. Then, the first thread continues and adds the bio to delayed->list despite the fact that dc->may_delay is false.

Fix this bug by changing may_delay's type from atomic_t to bool and only access it while holding the delayed_bios_lock mutex. Note that we don't have to grab the mutex in delay_resume because there are no bios in flight at this point.

Signed-off-by Mikulas Patocka <[email protected]>
Cc [email protected]
Signed-off-by Mike Snitzer <[email protected]>
drivers/md/dm-delay.c +11 -5
index efd510984e25..2d6b900e4353 100644
@@ -33,7 +33,7 @@ struct delay_c {
 	struct work_struct flush_expired_bios;
 	struct list_head delayed_bios;
 	struct task_struct *worker;
-	atomic_t may_delay;
+	bool may_delay;
 
 	struct delay_class read;
 	struct delay_class write;

@@ -236,7 +236,7 @@ static int delay_ctr(struct dm_target *ti, unsigned int argc, char **argv)
 
 	ti->private = dc;
 	INIT_LIST_HEAD(&dc->delayed_bios);
-	atomic_set(&dc->may_delay, 1);
+	dc->may_delay = true;
 	dc->argc = argc;
 
 	ret = delay_class_ctr(ti, &dc->read, argv);

@@ -312,7 +312,7 @@ static int delay_bio(struct delay_c *dc, struct delay_class *c, struct bio *bio)
 	struct dm_delay_info *delayed;
 	unsigned long expires = 0;
 
-	if (!c->delay || !atomic_read(&dc->may_delay))
+	if (!c->delay)
 		return DM_MAPIO_REMAPPED;
 
 	delayed = dm_per_bio_data(bio, sizeof(struct dm_delay_info));

@@ -321,6 +321,10 @@ static int delay_bio(struct delay_c *dc, struct delay_class *c, struct bio *bio)
 	delayed->expires = expires = jiffies + msecs_to_jiffies(c->delay);
 
 	mutex_lock(&delayed_bios_lock);
+	if (unlikely(!dc->may_delay)) {
+		mutex_unlock(&delayed_bios_lock);
+		return DM_MAPIO_REMAPPED;
+	}
 	c->ops++;
 	list_add_tail(&delayed->list, &dc->delayed_bios);
 	mutex_unlock(&delayed_bios_lock);

@@ -337,7 +341,9 @@ static void delay_presuspend(struct dm_target *ti)
 {
 	struct delay_c *dc = ti->private;
 
-	atomic_set(&dc->may_delay, 0);
+	mutex_lock(&delayed_bios_lock);
+	dc->may_delay = false;
+	mutex_unlock(&delayed_bios_lock);
 
 	if (delay_is_fast(dc))
 		flush_delayed_bios_fast(dc, true);

@@ -351,7 +357,7 @@ static void delay_resume(struct dm_target *ti)
 {
 	struct delay_c *dc = ti->private;
 
-	atomic_set(&dc->may_delay, 1);
+	dc->may_delay = true;
 }
 
 static int delay_map(struct dm_target *ti, struct bio *bio)