linux-kernel - Re: [PATCH v4 04/11] md/raid5: Ensure array is suspended for calls to log

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Date:   Wed, 8 Jun 2022 10:59:47 -0700
From:   Song Liu <song@...nel.org>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     open list <linux-kernel@...r.kernel.org>,
        linux-raid <linux-raid@...r.kernel.org>,
        Christoph Hellwig <hch@...radead.org>,
        Donald Buczek <buczek@...gen.mpg.de>,
        Guoqing Jiang <guoqing.jiang@...ux.dev>,
        Xiao Ni <xni@...hat.com>, Stephen Bates <sbates@...thlin.com>,
        Martin Oliveira <Martin.Oliveira@...eticom.com>,
        David Sloan <David.Sloan@...eticom.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v4 04/11] md/raid5: Ensure array is suspended for calls to log_exit()

On Wed, Jun 8, 2022 at 9:28 AM Logan Gunthorpe <logang@...tatee.com> wrote:
>
> The raid5-cache code relies on there being no IO in flight when
> log_exit() is called. There are two places where this is not
> guaranteed so add mddev_suspend() and mddev_resume() calls to these
> sites.
>
> The site in raid5_remove_disk() has a comment saying that it is
> called in raid5d and thus cannot wait for pending writes; however that
> does not appear to be correct anymore (if it ever was) as
> raid5_remove_disk() is called from hot_remove_disk() which only
> appears to be called in the md_ioctl(). Thus, the comment is removed,
> as well as the racy check and replaced with calls to suspend/resume.
>
> The site in raid5_change_consistency_policy() is in the error path,
> and another similar call site already has suspend/resume calls just
> below it; so it should be equally safe to make that change here.
>
> Signed-off-by: Logan Gunthorpe <logang@...tatee.com>
> Reviewed-by: Christoph Hellwig <hch@....de>
> ---
>  drivers/md/raid5.c | 18 ++++++------------
>  1 file changed, 6 insertions(+), 12 deletions(-)
>
> diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
> index 5d09256d7f81..3ad37dd4c5cd 100644
> --- a/drivers/md/raid5.c
> +++ b/drivers/md/raid5.c
> @@ -7938,18 +7938,9 @@ static int raid5_remove_disk(struct mddev *mddev, struct md_rdev *rdev)
>
>         print_raid5_conf(conf);
>         if (test_bit(Journal, &rdev->flags) && conf->log) {
> -               /*
> -                * we can't wait pending write here, as this is called in
> -                * raid5d, wait will deadlock.
> -                * neilb: there is no locking about new writes here,
> -                * so this cannot be safe.
> -                */
> -               if (atomic_read(&conf->active_stripes) ||
> -                   atomic_read(&conf->r5c_cached_full_stripes) ||
> -                   atomic_read(&conf->r5c_cached_partial_stripes)) {
> -                       return -EBUSY;
> -               }
> +               mddev_suspend(mddev);

Unfortunately, the comment about deadlock is still true, and we cannot call
mddev_suspend here. To trigger it:

   # create raid5 with journal:
   mdadm --create /dev/md0  -l 5 -n 3 /dev/nvme[0-2]n1 --write-journal
/dev/nvme3n1
   # start some writes
   fio ...
   # fail the journal
   mdadm --fail /dev/md0 /dev/nvme3n1

This will cause deadlock of the thread:
[<0>] raid5_quiesce+0x2a8/0x5f0
[<0>] mddev_suspend+0x26b/0x530
[<0>] raid5_remove_disk+0x12e/0x6f3
[<0>] remove_and_add_spares+0x5b2/0xef0
[<0>] md_check_recovery+0xe68/0x12b0
[<0>] raid5d+0xf4/0xf30
[<0>] md_thread+0x299/0x350
[<0>] kthread+0x29d/0x340
[<0>] ret_from_fork+0x22/0x30

Thanks,
Song


>                 log_exit(conf);
> +               mddev_resume(mddev);
>                 return 0;
>         }
>         if (rdev == rcu_access_pointer(p->rdev))
> @@ -8697,8 +8688,11 @@ static int raid5_change_consistency_policy(struct mddev *mddev, const char *buf)
>                         err = log_init(conf, NULL, true);
>                         if (!err) {
>                                 err = resize_stripes(conf, conf->pool_size);
> -                               if (err)
> +                               if (err) {
> +                                       mddev_suspend(mddev);
>                                         log_exit(conf);
> +                                       mddev_resume(mddev);
> +                               }
>                         }
>                 } else
>                         err = -EINVAL;
> --
> 2.30.2
>