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

[Song Liu <song@kernel.org>]

On Tue, Feb 9, 2021 at 6:22 PM Guoqing Jiang
<guoqing.jiang@cloud.ionos.com> wrote:
>
> 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:
> issu://lore.kernel.org/linux-raid/5ed54ffc-ce82-bf66-4eff-390cb23bc1ac@molgen.mpg.de/T/#t
>
> Reported-and-tested-by: Donald Buczek <buczek@molgen.mpg.de>
> Signed-off-by: Guoqing Jiang <guoqing.jiang@cloud.ionos.com>

Thanks for debugging the issue. However, I am not sure whether this is
the proper
fix. For example, would this break dm-raid.c:raid_message()? IIUC,
raid_message()
calls md_reap_sync_thread() without holding reconfigure_mutex, no?

Thanks,
Song

> ---
>  drivers/md/md.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/drivers/md/md.c b/drivers/md/md.c
> index ca40942..eec8c27 100644
> --- a/drivers/md/md.c
> +++ b/drivers/md/md.c
> @@ -9365,13 +9365,18 @@ void md_check_recovery(struct mddev *mddev)
>  EXPORT_SYMBOL(md_check_recovery);
>
>  void md_reap_sync_thread(struct mddev *mddev)
> +       __releases(&mddev->reconfig_mutex)
> +       __acquires(&mddev->reconfig_mutex)
> +
>  {
>         struct md_rdev *rdev;
>         sector_t old_dev_sectors = mddev->dev_sectors;
>         bool is_reshaped = false;
>
>         /* resync has finished, collect result */
> +       mddev_unlock(mddev);
>         md_unregister_thread(&mddev->sync_thread);
> +       mddev_lock_nointr(mddev);
>         if (!test_bit(MD_RECOVERY_INTR, &mddev->recovery) &&
>             !test_bit(MD_RECOVERY_REQUESTED, &mddev->recovery) &&
>             mddev->degraded != mddev->raid_disks) {
> --
> 2.7.4
>