raid1: prevent freeze_array/wait_all_barriers deadlock

If freeze_array is attempted in the middle of close_sync/
wait_all_barriers, deadlock can occur.

freeze_array will wait for nr_pending and nr_queued to line up.
wait_all_barriers increments nr_pending for each barrier bucket, one
at a time, but doesn't actually issue IO that could be counted in
nr_queued. So freeze_array is blocked until wait_all_barriers
completes and allow_all_barriers runs. At the same time, when
_wait_barrier sees array_frozen == 1, it stops and waits for
freeze_array to complete.

Prevent the deadlock by making close_sync call _wait_barrier and
_allow_barrier for one bucket at a time, instead of deferring the
_allow_barrier calls until after all _wait_barriers are complete.

Signed-off-by: Nate Dailey <nate.dailey@stratus.com>
Fix: fd76863e37fe(RAID1: a new I/O barrier implementation to remove resync window)
Reviewed-by: Coly Li <colyli@suse.de>
Cc: stable@vger.kernel.org (v4.11)
Signed-off-by: Shaohua Li <shli@fb.com>

drivers/md/raid1.c

@@ -989,14 +989,6 @@ static void wait_barrier(struct r1conf *conf, sector_t sector_nr)
 	_wait_barrier(conf, idx);
 }
 
-static void wait_all_barriers(struct r1conf *conf)
-{
-	int idx;
-
-	for (idx = 0; idx < BARRIER_BUCKETS_NR; idx++)
-		_wait_barrier(conf, idx);
-}
-
 static void _allow_barrier(struct r1conf *conf, int idx)
 {
 	atomic_dec(&conf->nr_pending[idx]);
@@ -1010,14 +1002,6 @@ static void allow_barrier(struct r1conf *conf, sector_t sector_nr)
 	_allow_barrier(conf, idx);
 }
 
-static void allow_all_barriers(struct r1conf *conf)
-{
-	int idx;
-
-	for (idx = 0; idx < BARRIER_BUCKETS_NR; idx++)
-		_allow_barrier(conf, idx);
-}
-
 /* conf->resync_lock should be held */
 static int get_unqueued_pending(struct r1conf *conf)
 {
@@ -1645,8 +1629,12 @@ static void print_conf(struct r1conf *conf)
 
 static void close_sync(struct r1conf *conf)
 {
-	wait_all_barriers(conf);
-	allow_all_barriers(conf);
+	int idx;
+
+	for (idx = 0; idx < BARRIER_BUCKETS_NR; idx++) {
+		_wait_barrier(conf, idx);
+		_allow_barrier(conf, idx);
+	}
 
 	mempool_destroy(conf->r1buf_pool);
 	conf->r1buf_pool = NULL;