[PATCH -next 4/6] md: refactor idle/frozen_sync_thread()

[Yu Kuai <yukuai1@xxxxxxxxxxxxxxx>]

From: Yu Kuai <yukuai3@xxxxxxxxxx>

Our test found a following deadlock in raid10:

1) Issue a normal write, and such write failed:

  raid10_end_write_request
   set_bit(R10BIO_WriteError, &r10_bio->state)
   one_write_done
    reschedule_retry

  // later from md thread
  raid10d
   handle_write_completed
    list_add(&r10_bio->retry_list, &conf->bio_end_io_list)

  // later from md thread
  raid10d
   if (!test_bit(MD_SB_CHANGE_PENDING, &mddev->sb_flags))
    list_move(conf->bio_end_io_list.prev, &tmp)
    r10_bio = list_first_entry(&tmp, struct r10bio, retry_list)
    raid_end_bio_io(r10_bio)

Dependency chain 1: normal io is waiting for updating superblock

2) Trigger a recovery:

  raid10_sync_request
   raise_barrier

Dependency chain 2: sync thread is waiting for normal io

3) echo idle/frozen to sync_action:

  action_store
   mddev_lock
    md_unregister_thread
     kthread_stop

Dependency chain 3: drop 'reconfig_mutex' is waiting for sync thread

4) md thread can't update superblock:

  raid10d
   md_check_recovery
    if (mddev_trylock(mddev))
     md_update_sb

Dependency chain 4: update superblock is waiting for 'reconfig_mutex'

Hence cyclic dependency exist, in order to fix the problem, we must
break one of them. Dependency 1 and 2 can't be broken because they are
foundation design. Dependency 4 may be possible if it can be guaranteed
that no io can be inflight, however, this requires a new mechanism which
seems complex. Dependency 3 is a good choice, because idle/frozen only
requires sync thread to finish, which can be done asynchronously that is
already implemented, and 'reconfig_mutex' is not needed anymore.

This patch switch 'idle' and 'frozen' to wait sync thread to be done
asynchronously, and this patch also add a sequence counter to record how
many times sync thread is done, so that 'idle' won't keep waiting on new
started sync thread.

Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
---
 drivers/md/md.c | 24 ++++++++++++++++++++----
 drivers/md/md.h |  2 ++
 2 files changed, 22 insertions(+), 4 deletions(-)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 223c03149852..ddf33a11d8de 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -662,6 +662,7 @@ void mddev_init(struct mddev *mddev)
 	timer_setup(&mddev->safemode_timer, md_safemode_timeout, 0);
 	atomic_set(&mddev->active, 1);
 	atomic_set(&mddev->openers, 0);
+	atomic_set(&mddev->sync_seq, 0);
 	spin_lock_init(&mddev->lock);
 	atomic_set(&mddev->flush_pending, 0);
 	init_waitqueue_head(&mddev->sb_wait);
@@ -4774,19 +4775,28 @@ static void stop_sync_thread(struct mddev *mddev)
 	if (work_pending(&mddev->del_work))
 		flush_workqueue(md_misc_wq);
 
-	if (mddev->sync_thread) {
-		set_bit(MD_RECOVERY_INTR, &mddev->recovery);
-		md_reap_sync_thread(mddev);
-	}
+	set_bit(MD_RECOVERY_INTR, &mddev->recovery);
+	/*
+	 * Thread might be blocked waiting for metadata update which will now
+	 * never happen.
+	 */
+	if (mddev->sync_thread)
+		wake_up_process(mddev->sync_thread->tsk);
 
 	mddev_unlock(mddev);
 }
 
 static void idle_sync_thread(struct mddev *mddev)
 {
+	int sync_seq = atomic_read(&mddev->sync_seq);
+
 	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) ||
+			!test_bit(MD_RECOVERY_RUNNING, &mddev->recovery));
+
 	mutex_unlock(&mddev->sync_mutex);
 }
 
@@ -4795,6 +4805,10 @@ static void frozen_sync_thread(struct mddev *mddev)
 	mutex_lock(&mddev->sync_mutex);
 	set_bit(MD_RECOVERY_FROZEN, &mddev->recovery);
 	stop_sync_thread(mddev);
+
+	wait_event(resync_wait, mddev->sync_thread == NULL &&
+			!test_bit(MD_RECOVERY_RUNNING, &mddev->recovery));
+
 	mutex_unlock(&mddev->sync_mutex);
 }
 
@@ -9451,6 +9465,8 @@ void md_reap_sync_thread(struct mddev *mddev)
 
 	/* resync has finished, collect result */
 	md_unregister_thread(&mddev->sync_thread);
+	atomic_inc(&mddev->sync_seq);
+
 	if (!test_bit(MD_RECOVERY_INTR, &mddev->recovery) &&
 	    !test_bit(MD_RECOVERY_REQUESTED, &mddev->recovery) &&
 	    mddev->degraded != mddev->raid_disks) {
diff --git a/drivers/md/md.h b/drivers/md/md.h
index 64474e458545..10d425a3daa3 100644
--- a/drivers/md/md.h
+++ b/drivers/md/md.h
@@ -537,6 +537,8 @@ struct mddev {
 
 	/* Used to synchronize idle and frozen for action_store() */
 	struct mutex sync_mutex;
+	/* The sequence number for sync thread */
+	atomic_t sync_seq;
 };
 
 enum recovery_flags {
-- 
2.31.1