On Wed, 29 Nov 2023, Sasha Levin wrote:
> On Wed, Nov 29, 2023 at 06:28:16PM +0100, Mikulas Patocka wrote:
> >
> >
> >On Wed, 29 Nov 2023, Christian Loehle wrote:
> >
> >> Hi Mikulas,
> >> Agreed and thanks for fixing.
> >> Has this been selected for stable because of:
> >> 6fc45b6ed921 ("dm-delay: fix a race between delay_presuspend and
> >> delay_bio")
> >> If so, I would volunteer do the backports for that for you at least.
> >
> >I wouldn't backport this patch - it is an enhancement, not a bugfix, so it
> >doesn't qualify for the stable kernel backports.
>
> Right - this watch was selected as a dependency for 6fc45b6ed921
> ("dm-delay: fix a race between delay_presuspend and delay_bio").
>
> In general, unless it's impractical, we'd rather take a dependency chain
> rather than deal with a non-trivial backport as those tend to have
> issues longer term.
>
> --
> Thanks,
> Sasha
The patch 70bbeb29fab0 ("dm delay: for short delays, use kthread instead
of timers and wq") changes behavior of dm-delay from using timers to
polling, so it may cause problems to people running legacy kernels - the
polling consumes more CPU time than the timers - so I think it shouldn't
go to the stable kernels where users expect that there will be no
functional change.
Here I'm submitting the patch 6fc45b6ed921 backported for 6.6.3.
Mikulas
From: Mikulas Patocka <[email protected]>
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 theread executes
delay_presuspend, it sets, dc->may_delay to zero, deletes the timer and
flushes pending bios. Now, the first thread continues and adds the bio to
delayed->list despite the fact that dc->may_delay is false.
In order to fix this bug, we change may_delay's type from atomic_t to bool
and we read and write it only 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]
---
drivers/md/dm-delay.c | 17 ++++++++++++-----
1 file changed, 12 insertions(+), 5 deletions(-)
Index: linux-stable/drivers/md/dm-delay.c
===================================================================
--- linux-stable.orig/drivers/md/dm-delay.c 2023-11-29 19:03:03.000000000
+0100
+++ linux-stable/drivers/md/dm-delay.c 2023-11-29 19:03:03.000000000 +0100
@@ -31,7 +31,7 @@ struct delay_c {
struct workqueue_struct *kdelayd_wq;
struct work_struct flush_expired_bios;
struct list_head delayed_bios;
- atomic_t may_delay;
+ bool may_delay;
struct delay_class read;
struct delay_class write;
@@ -192,7 +192,7 @@ static int delay_ctr(struct dm_target *t
INIT_WORK(&dc->flush_expired_bios, flush_expired_bios);
INIT_LIST_HEAD(&dc->delayed_bios);
mutex_init(&dc->timer_lock);
- atomic_set(&dc->may_delay, 1);
+ dc->may_delay = true;
dc->argc = argc;
ret = delay_class_ctr(ti, &dc->read, argv);
@@ -247,7 +247,7 @@ static int delay_bio(struct delay_c *dc,
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));
@@ -256,6 +256,10 @@ static int delay_bio(struct delay_c *dc,
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);
@@ -269,7 +273,10 @@ static void delay_presuspend(struct dm_t
{
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);
+
del_timer_sync(&dc->delay_timer);
flush_bios(flush_delayed_bios(dc, 1));
}
@@ -278,7 +285,7 @@ static void delay_resume(struct dm_targe
{
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)
