Re: [Update PATCH V3] md: don't unregister sync_thread with reconfig_mutex held

[Song Liu <song@kernel.org>]

On Fri, May 6, 2022 at 4:37 AM Guoqing Jiang <guoqing.jiang@linux.dev> wrote:
>
> From: Guoqing Jiang <guoqing.jiang@cloud.ionos.com>
>
> Unregister sync_thread doesn't need to hold reconfig_mutex since it
> doesn't reconfigure array.
>
> And it could cause deadlock problem for raid5 as follows:
>
> 1. process A tried to reap sync thread with reconfig_mutex held after echo
>    idle to sync_action.
> 2. raid5 sync thread was blocked if there were too many active stripes.
> 3. SB_CHANGE_PENDING was set (because of write IO comes from upper layer)
>    which causes the number of active stripes can't be decreased.
> 4. SB_CHANGE_PENDING can't be cleared since md_check_recovery was not able
>    to hold reconfig_mutex.
>
> More details in the link:
> https://lore.kernel.org/linux-raid/5ed54ffc-ce82-bf66-4eff-390cb23bc1ac@molgen.mpg.de/T/#t
>
> Let's call unregister thread between mddev_unlock and mddev_lock_nointr
> (thanks for the report from kernel test robot <lkp@intel.com>) if the
> reconfig_mutex is held, and mddev_is_locked is introduced accordingly.

mddev_is_locked() feels really hacky to me. It cannot tell whether
mddev is locked
by current thread. So technically, we can unlock reconfigure_mutex for
other thread
by accident, no?


>
> Reported-by: Donald Buczek <buczek@molgen.mpg.de>
> Signed-off-by: Guoqing Jiang <guoqing.jiang@linux.dev>
> ---
>  drivers/md/md.c | 9 ++++++++-
>  drivers/md/md.h | 5 +++++
>  2 files changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index 309b3af906ad..525f65682356 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -9432,10 +9432,17 @@ 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;
>
> +       if (mddev_is_locked(mddev)) {
> +               is_locked = true;
> +               mddev_unlock(mddev);
> +       }
>         /* resync has finished, collect result */
>         md_unregister_thread(&mddev->sync_thread);
> +       if (is_locked)
> +               mddev_lock_nointr(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 6ac283864533..af6f3978b62b 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);
> +}
> +
>  static inline void md_sync_acct(struct block_device *bdev, unsigned long nr_sectors)
>  {
>         atomic_add(nr_sectors, &bdev->bd_disk->sync_io);
> --
> 2.31.1
>