[dm-devel] [PATCH V2] md: don't unregister sync_thread with reconfig_mutex held

[Guoqing Jiang]

On 12/9/21 12:35 AM, Heinz Mauelshagen wrote:
> NACK, see details below.
>
> On Wed, Dec 8, 2021 at 3:24 PM Guoqing Jiang <guoqing.jiang at linux.dev 
> <mailto:guoqing.jiang at linux.dev>> wrote:
>
>
>
>     On 12/1/21 1:27 AM, Paul Menzel wrote:
>     >
>     >>>>>>> diff --git a/drivers/md/dm-raid.c b/drivers/md/dm-raid.c
>     >>>>>>> index cab12b2..0c4cbba 100644
>     >>>>>>> --- a/drivers/md/dm-raid.c
>     >>>>>>> +++ b/drivers/md/dm-raid.c
>     >>>>>>> @@ -3668,7 +3668,7 @@ static int raid_message(struct
>     dm_target
>     >>>>>>> *ti, unsigned int argc, char **argv,
>     >>>>>>>         if (!strcasecmp(argv[0], "idle") ||
>     !strcasecmp(argv[0],
>     >>>>>>> "frozen")) {
>     >>>>>>>                 if (mddev->sync_thread) {
>     >>>>>>> set_bit(MD_RECOVERY_INTR,
>     >>>>>>> &mddev->recovery);
>     >>>>>>> - md_reap_sync_thread(mddev);
>     >>>>>>> + md_reap_sync_thread(mddev, false);
>     >>>>>
>     >>>>> I think we can add mddev_lock() and mddev_unlock() here and
>     then
>     >>>>> we don't
>     >>>>> need the extra parameter?
>     >>>>
>     >>>> I thought it too, but I would prefer get the input from DM
>     people
>     >>>> first.
>     >>>>
>     >>>> @ Mike or Alasdair
>     >>>
>     >>> Hi Mike and Alasdair,
>     >>>
>     >>> Could you please comment on this option: adding mddev_lock() and
>     >>> mddev_unlock()
>     >>> to raid_message() around md_reap_sync_thread()?
>
>     Add Heinz and Jonathan, could you comment about this? Thanks.
>
>     >>
>     >> The issue is unfortunately still unresolved (at least Linux
>     5.10.82).
>     >> How can we move forward?
>
>     If it is not applicable to change dm-raid, another alternative
>     could be
>     like this
>
>     --- a/drivers/md/md.c
>     +++ b/drivers/md/md.c
>     @@ -9409,8 +9409,12 @@ void md_reap_sync_thread(struct mdev *mddev)
>              sector_t old_dev_sectors = mddev->dev_sectors;
>              bool is_reshaped = false;
>
>     +       if (mddev_is_locked(mddev))
>     +               mddev_unlock(mddev);
>              /* resync has finished, collect result */
>              md_unregister_thread(&mddev->sync_thread);
>     +       if (mddev_is_locked(mddev))
>     +               mddev_lock(mddev);
>              if (!test_bit(MD_RECOVERY_INTR, &mddev->recovery) &&
>                  !test_bit(MD_RECOVERY_REQUESTED, &mddev->recovery) &&
>                  mddev->degraded != mddev->raid_disks) {
>     diff --git a/drivers/md/md.h b/drivers/md/md.h
>     index 53ea7a6961de..96a88b7681d6 100644
>     --- a/drivers/md/md.h
>     +++ b/drivers/md/md.h
>     @@ -549,6 +549,11 @@ static inline int mddev_trylock(struct mddev
>     *mddev)
>       }
>       extern void mddev_unlock(struct mddev *mddev);
>
>     +static inline int mddev_is_locked(struct mddev *mddev)
>     +{
>     +       return mutex_is_locked(&mddev->reconfig_mutex);
>     +}
>     +
>
>
> Patch is bogus relative to the proposed mddev_unlock/mddev_lock logic 
> in md.c around the
> md_unregister_thread() as it's failing to lock again if it was holding 
> the mutex before as it again
> calls mddev_locked() after having the mutex unlocked just before the 
> md_unregister_thread() call.
>
> If that patch to md.c holds up in further analysis, it has to keep the 
> mddev_is_locked() result
> and unlock/lock conditionally based on its result.
>

Yes, that was my intention too, thanks for pointing it out.

@@ -9407,10 +9407,16 @@ void md_reap_sync_thread(struct mddev *mddev)
  {
         struct md_rdev *rdev;
         sector_t old_dev_sectors = mddev->dev_sectors;
-       bool is_reshaped = false;
+       bool is_reshaped = false, is_locked = false;

         /* resync has finished, collect result */
+       if (mddev_is_locked(mddev)) {
+               is_locked = true;
+               mddev_unlock(mddev);
+       }
         md_unregister_thread(&mddev->sync_thread);
+       if (is_locked)
+               mddev_lock(mddev);

Thanks,
Guoqing