Re: [PATCH v3 3/3] md: fix stopping sync thread

[Song Liu]

On Tue, Nov 28, 2023 at 8:32 PM Yu Kuai <yukuai1@xxxxxxxxxxxxxxx> wrote:
>
> From: Yu Kuai <yukuai3@xxxxxxxxxx>
>
> Currently sync thread is stopped from multiple contex:
>  - idle_sync_thread
>  - frozen_sync_thread
>  - __md_stop_writes
>  - md_set_readonly
>  - do_md_stop
>
> And there are some problems:
> 1) sync_work is flushed while reconfig_mutex is grabbed, this can
>    deadlock because the work function will grab reconfig_mutex as well.
> 2) md_reap_sync_thread() can't be called directly while md_do_sync() is
>    not finished yet, for example, commit 130443d60b1b ("md: refactor
>    idle/frozen_sync_thread() to fix deadlock").
> 3) If MD_RECOVERY_RUNNING is not set, there is no need to stop
>    sync_thread at all because sync_thread must not be registered.
>
> Factor out a helper prepare_to_stop_sync_thread(), so that above contex
> will behave the same. Fix 1) by flushing sync_work after reconfig_mutex
> is released, before waiting for sync_thread to be done; Fix 2) bt
> letting daemon thread to unregister sync_thread; Fix 3) by always
> checking MD_RECOVERY_RUNNING first.
>
> Fixes: db5e653d7c9f ("md: delay choosing sync action to md_start_sync()")
> Acked-by: Xiao Ni <xni@xxxxxxxxxx>
>
> Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> ---
>  drivers/md/md.c | 96 +++++++++++++++++++++++--------------------------
>  1 file changed, 45 insertions(+), 51 deletions(-)
>
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index 2d8e45a1af23..05902e36db66 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -4840,26 +4840,9 @@ action_show(struct mddev *mddev, char *page)
>         return sprintf(page, "%s\n", type);
>  }
>
> -static void stop_sync_thread(struct mddev *mddev)
> +static void prepare_to_stop_sync_thread(struct mddev *mddev)
> +       __releases(&mddev->reconfig_mutex)
>  {
> -       if (!test_bit(MD_RECOVERY_RUNNING, &mddev->recovery))
> -               return;
> -
> -       if (mddev_lock(mddev))
> -               return;
> -
> -       /*
> -        * Check again in case MD_RECOVERY_RUNNING is cleared before lock is
> -        * held.
> -        */
> -       if (!test_bit(MD_RECOVERY_RUNNING, &mddev->recovery)) {
> -               mddev_unlock(mddev);
> -               return;
> -       }
> -
> -       if (work_pending(&mddev->sync_work))
> -               flush_workqueue(md_misc_wq);
> -
>         set_bit(MD_RECOVERY_INTR, &mddev->recovery);
>         /*
>          * Thread might be blocked waiting for metadata update which will now
> @@ -4868,6 +4851,8 @@ static void stop_sync_thread(struct mddev *mddev)
>         md_wakeup_thread_directly(mddev->sync_thread);
>
>         mddev_unlock(mddev);
> +       if (work_pending(&mddev->sync_work))
> +               flush_work(&mddev->sync_work);
>  }
>
>  static void idle_sync_thread(struct mddev *mddev)
> @@ -4876,10 +4861,20 @@ static void idle_sync_thread(struct mddev *mddev)
>
>         mutex_lock(&mddev->sync_mutex);
>         clear_bit(MD_RECOVERY_FROZEN, &mddev->recovery);
> -       stop_sync_thread(mddev);
>
> -       wait_event(resync_wait, sync_seq != atomic_read(&mddev->sync_seq) ||
> +       if (mddev_lock(mddev)) {
> +               mutex_unlock(&mddev->sync_mutex);
> +               return;
> +       }
> +
> +       if (test_bit(MD_RECOVERY_RUNNING, &mddev->recovery)) {
> +               prepare_to_stop_sync_thread(mddev);
> +               wait_event(resync_wait,
> +                       sync_seq != atomic_read(&mddev->sync_seq) ||
>                         !test_bit(MD_RECOVERY_RUNNING, &mddev->recovery));
> +       } else {
> +               mddev_unlock(mddev);
> +       }

We have a few patterns like this:

        if (test_bit(MD_RECOVERY_RUNNING, &mddev->recovery)) {
                prepare_to_stop_sync_thread(mddev);
                wait_event(resync_wait,
                        sync_seq != atomic_read(&mddev->sync_seq) ||
                        !test_bit(MD_RECOVERY_RUNNING, &mddev->recovery));
        } else {
                mddev_unlock(mddev);
        }

This is pretty confusing. Maybe try something like (untested)

static void stop_sync_thread(struct mddev *mddev,
                             bool do_unlock, bool check_sync_seq)
{
        if (test_bit(MD_RECOVERY_RUNNING, &mddev->recovery)) {
                if (do_unlock)
                        mddev_unlock(mddev);
                return;
        }
        set_bit(MD_RECOVERY_INTR, &mddev->recovery);
        /*
         * Thread might be blocked waiting for metadata update which will now
         * never happen
         */
        md_wakeup_thread_directly(mddev->sync_thread);

        mddev_unlock(mddev);
        if (work_pending(&mddev->sync_work))
                flush_work(&mddev->sync_work);
        wait_event(resync_wait,
                   !test_bit(MD_RECOVERY_RUNNING, &mddev->recovery) ||
                   (check_sync_seq && sync_seq !=
atomic_read(&mddev->sync_seq)));
        if (!do_unlock)
                mddev_lock_nointr(mddev);
}

Thanks,
Song