linux-kernel - Re: Subject: [PATCH] md: avoid deadlock when raid5 array has unack badblocks during md_stop

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Date:	Tue, 10 Sep 2013 15:33:47 +1000
From:	NeilBrown <neilb@...e.de>
To:	y b <ycbzzjlby@...il.com>
Cc:	linux-raid@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Subject: [PATCH] md: avoid deadlock when raid5 array has unack
 badblocks during md_stop_writes.

On Tue, 10 Sep 2013 13:00:52 +0800 y b <ycbzzjlby@...il.com> wrote:

> When raid5 hit a fresh badblock, this badblock will flagged as unack
> badblock until md_update_sb is called.
> But md_stop/reboot/md_set_readonly will avoid raid5d call md_update_sb
> in md_check_recovery, the badblock will always be unack, so raid5d
> thread enter a infinite loop and never can unregister sync_thread
> that cause deadlock.
> 
> To solve this, before md_stop_writes call md_unregister_thread, set
> MD_STOPPING_WRITES on mddev->flags. In raid5.c analyse_stripe judge
> MD_STOPPING_WRITES bit on mddev->flags, if setted don't block rdev
> to wait md_update_sb. so raid5d thread can be finished.
> Signed-off-by: Bian Yu <bianyu@...acom.com>

Have you actually seen this deadlock happen?  Because I don't think it can
happen.

By the time we get to md_stop or md_set_readonly all dirty buffers should
have been flushed and there should be no pending writes so nothing to wait
for an unacked bad block.

If you have seen this happen, any details you can give about the exact state
of the RAID5 when it deadlocked, the stack trace of any relevant processes
etc would be very helpful.

Thanks,
NeilBrown


> ---
>  drivers/md/md.c    |    2 ++
>  drivers/md/md.h    |    3 +++
>  drivers/md/raid5.c |    3 ++-
>  3 files changed, 7 insertions(+), 1 deletions(-)
> 
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index adf4d7e..54ef71f 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -5278,6 +5278,7 @@ static void md_clean(struct mddev *mddev)
>  static void __md_stop_writes(struct mddev *mddev)
>  {
>      set_bit(MD_RECOVERY_FROZEN, &mddev->recovery);
> +    set_bit(MD_STOPPING_WRITES, &mddev->flags);
>      if (mddev->sync_thread) {
>          set_bit(MD_RECOVERY_INTR, &mddev->recovery);
>          md_reap_sync_thread(mddev);
> @@ -5294,6 +5295,7 @@ static void __md_stop_writes(struct mddev *mddev)
>          mddev->in_sync = 1;
>          md_update_sb(mddev, 1);
>      }
> +    clear_bit(MD_STOPPING_WRITES, &mddev->flags);
>  }
> 
>  void md_stop_writes(struct mddev *mddev)
> diff --git a/drivers/md/md.h b/drivers/md/md.h
> index 608050c..c998b82 100644
> --- a/drivers/md/md.h
> +++ b/drivers/md/md.h
> @@ -214,6 +214,9 @@ struct mddev {
>  #define MD_STILL_CLOSED    4    /* If set, then array has not been opened since
>                   * md_ioctl checked on it.
>                   */
> +#define MD_STOPPING_WRITES 5     /* If set, raid5 shouldn't set unacknowledged
> +                  * badblock blocked in analyse_stripe to avoid infinite loop
> +                  */
> 
>      int                suspended;
>      atomic_t            active_io;
> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> index f9972e2..ff1aecf 100644
> --- a/drivers/md/raid5.c
> +++ b/drivers/md/raid5.c
> @@ -3446,7 +3446,8 @@ static void analyse_stripe(struct stripe_head
> *sh, struct stripe_head_state *s)
>          if (rdev) {
>              is_bad = is_badblock(rdev, sh->sector, STRIPE_SECTORS,
>                           &first_bad, &bad_sectors);
> -            if (s->blocked_rdev == NULL
> +            if (!test_bit(MD_STOPPING_WRITES, &conf->mddev->flags)
> +                && s->blocked_rdev == NULL
>                  && (test_bit(Blocked, &rdev->flags)
>                  || is_bad < 0)) {
>                  if (is_bad < 0)


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