block: fix concurrent elevator change failure

block/blk-core.c

@@ -473,6 +473,7 @@ struct request_queue *blk_alloc_queue(struct queue_limits *lim, int node_id)
 	refcount_set(&q->refs, 1);
 	mutex_init(&q->debugfs_mutex);
 	mutex_init(&q->elevator_lock);
+	mutex_init(&q->elevator_queue_lock);
 	mutex_init(&q->sysfs_lock);
 	mutex_init(&q->limits_lock);
 	mutex_init(&q->rq_qos_mutex);

block/elevator.c

@@ -665,6 +665,13 @@ static int elevator_change(struct request_queue *q, struct elv_change_ctx *ctx)
 			return ret;
 	}
 
+	/*
+	 * Acquire elevator_queue_lock to serialize the debugfs (un)register
+	 * steps for the same queue. The elevator switch core part is protected
+	 * by queue freezing and ->elevator_lock.
+	 */
+	mutex_lock(&q->elevator_queue_lock);
+
 	memflags = blk_mq_freeze_queue(q);
 	/*
 	 * May be called before adding disk, when there isn't any FS I/O,
@@ -690,6 +697,8 @@ static int elevator_change(struct request_queue *q, struct elv_change_ctx *ctx)
 	if (!ctx->new)
 		blk_mq_free_sched_res(&ctx->res, ctx->type, set);
 
+	mutex_unlock(&q->elevator_queue_lock);
+
 	return ret;
 }
 

include/linux/blkdev.h

@@ -612,6 +612,13 @@ struct request_queue {
 	 */
 	struct mutex		elevator_lock;
 
+	/*
+	 * Serializes the whole elevator change operation for the same queue,
+	 * including the debugfs (un)register steps. Must be acquired before
+	 * freezing the queue and acquiring elevator_lock.
+	 */
+	struct mutex		elevator_queue_lock;
+
 	struct mutex		sysfs_lock;
 	/*
 	 * Protects queue limits and also sysfs attribute read_ahead_kb.