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

Song Liu [off-list ref] 于2021年2月11日周四 上午8:31写道：

On Tue, Feb 9, 2021 at 6:22 PM Guoqing Jiang
[off-list ref] wrote:

quoted

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 <redacted>
Signed-off-by: Guoqing Jiang <redacted>

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.

right.
A simple solution would be add a parameter to md_reap_sync_thread to
indicate if a reconfigure_mutex lock is held.

Regards!
Jack

quoted

---
 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