md/raid10: fix task hung in raid10d

commit fe630de ("md/raid10: avoid deadlock on recovery.") allowed
normal io and sync io to exist at the same time. Task hung will occur as
below:

T1                      T2		T3		T4
raid10d
 handle_read_error
  allow_barrier
   conf->nr_pending--
    -> 0
                        //submit sync io
                        raid10_sync_request
                         raise_barrier
			  ->will not be blocked
			  ...
			//submit to drivers
  raid10_read_request
   wait_barrier
    conf->nr_pending++
     -> 1
					//retry read fail
					raid10_end_read_request
					 reschedule_retry
					  add to retry_list
					  conf->nr_queued++
					   -> 1
							//sync io fail
							end_sync_read
							 __end_sync_read
							  reschedule_retry
							   add to retry_list
					                    conf->nr_queued++
							     -> 2
 ...
 handle_read_error
 get form retry_list
 conf->nr_queued--
  freeze_array
   wait nr_pending == nr_queued+1
        ->1	      ->2
   //task hung

retry read and sync io will be added to retry_list(nr_queued->2) if they
fails. raid10d() called handle_read_error() and hung in freeze_array().
nr_queued will not decrease because raid10d is blocked, nr_pending will
not increase because conf->barrier is not released.

Fix it by moving allow_barrier() after raid10_read_request().
raise_barrier() will wait for nr_waiting to become 0. Therefore, sync io
and regular io will not be issued at the same time.

Also remove the check of nr_queued in stop_waiting_barrier. It can be 0
but don't need to be blocking. Remove the check for MD_RECOVERY_RUNNING as
the check is redundent.

Fixes: fe630de ("md/raid10: avoid deadlock on recovery.")
Signed-off-by: Li Nan <linan122@huawei.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20230222041000.3341651-2-linan666@huaweicloud.com

Author: Li Nan

drivers/md/raid10.c

@@ -995,11 +995,15 @@ static bool stop_waiting_barrier(struct r10conf *conf)
 	    (!bio_list_empty(&bio_list[0]) || !bio_list_empty(&bio_list[1])))
 		return true;
 
-	/* move on if recovery thread is blocked by us */
-	if (conf->mddev->thread->tsk == current &&
-	    test_bit(MD_RECOVERY_RUNNING, &conf->mddev->recovery) &&
-	    conf->nr_queued > 0)
+	/*
+	 * move on if io is issued from raid10d(), nr_pending is not released
+	 * from original io(see handle_read_error()). All raise barrier is
+	 * blocked until this io is done.
+	 */
+	if (conf->mddev->thread->tsk == current) {
+		WARN_ON_ONCE(atomic_read(&conf->nr_pending) == 0);
 		return true;
+	}
 
 	return false;
 }
@@ -2978,9 +2982,13 @@ static void handle_read_error(struct mddev *mddev, struct r10bio *r10_bio)
 		md_error(mddev, rdev);
 
 	rdev_dec_pending(rdev, mddev);
-	allow_barrier(conf);
 	r10_bio->state = 0;
 	raid10_read_request(mddev, r10_bio->master_bio, r10_bio);
+	/*
+	 * allow_barrier after re-submit to ensure no sync io
+	 * can be issued while regular io pending.
+	 */
+	allow_barrier(conf);
 }
 
 static void handle_write_completed(struct r10conf *conf, struct r10bio *r10_bio)