md: always hold reconfig_mutex when calling mddev_suspend()

Most often mddev_suspend() is called with
reconfig_mutex held.  Make this a requirement in
preparation a subsequent patch.  Also require
reconfig_mutex to be held for mddev_resume(),
partly for symmetry and partly to guarantee
no races with incr/decr of mddev->suspend.

Taking the mutex in r5c_disable_writeback_async() is
a little tricky as this is called from a work queue
via log->disable_writeback_work, and flush_work()
is called on that while holding ->reconfig_mutex.
If the work item hasn't run before flush_work()
is called, the work function will not be able to
get the mutex.

So we use mddev_trylock() inside the wait_event() call, and have that
abort when conf->log is set to NULL, which happens before
flush_work() is called.
We wait in mddev->sb_wait and ensure this is woken
when any of the conditions change.  This requires
waking mddev->sb_wait in mddev_unlock().  This is only
like to trigger extra wake_ups of threads that needn't
be woken when metadata is being written, and that
doesn't happen often enough that the cost would be
noticeable.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>

1 files changed, 13 insertions, 5 deletions

diff --git a/drivers/md/raid5-cache.c b/drivers/md/raid5-cache.c
index 2b450eee21fa..59af7cf35092 100644
--- a/drivers/md/raid5-cache.c
+++ b/drivers/md/raid5-cache.c
@@ -693,6 +693,8 @@ static void r5c_disable_writeback_async(struct work_struct *work)
 	struct r5l_log *log = container_of(work, struct r5l_log,
 					   disable_writeback_work);
 	struct mddev *mddev = log->rdev->mddev;
+	struct r5conf *conf = mddev->private;
+	int locked = 0;
 
 	if (log->r5c_journal_mode == R5C_JOURNAL_MODE_WRITE_THROUGH)
 		return;
@@ -701,11 +703,15 @@ static void r5c_disable_writeback_async(struct work_struct *work)
 
 	/* wait superblock change before suspend */
 	wait_event(mddev->sb_wait,
-		   !test_bit(MD_SB_CHANGE_PENDING, &mddev->sb_flags));
-
-	mddev_suspend(mddev);
-	log->r5c_journal_mode = R5C_JOURNAL_MODE_WRITE_THROUGH;
-	mddev_resume(mddev);
+		   conf->log == NULL ||
+		   (!test_bit(MD_SB_CHANGE_PENDING, &mddev->sb_flags) &&
+		    (locked = mddev_trylock(mddev))));
+	if (locked) {
+		mddev_suspend(mddev);
+		log->r5c_journal_mode = R5C_JOURNAL_MODE_WRITE_THROUGH;
+		mddev_resume(mddev);
+		mddev_unlock(mddev);
+	}
 }
 
 static void r5l_submit_current_io(struct r5l_log *log)
@@ -3165,6 +3171,8 @@ void r5l_exit_log(struct r5conf *conf)
 	conf->log = NULL;
 	synchronize_rcu();
 
+	/* Ensure disable_writeback_work wakes up and exits */
+	wake_up(&conf->mddev->sb_wait);
 	flush_work(&log->disable_writeback_work);
 	md_unregister_thread(&log->reclaim_thread);
 	mempool_destroy(log->meta_pool);