block/blk-mq: use atomic_t for quiesce_depth to avoid lock contention on RT

block/blk-core.c

@@ -434,6 +434,7 @@ struct request_queue *blk_alloc_queue(struct queue_limits *lim, int node_id)
 	mutex_init(&q->limits_lock);
 	mutex_init(&q->rq_qos_mutex);
 	spin_lock_init(&q->queue_lock);
+	atomic_set(&q->quiesce_depth, 0);
 
 	init_waitqueue_head(&q->mq_freeze_wq);
 	mutex_init(&q->mq_freeze_lock);

block/blk-mq-debugfs.c

@@ -89,7 +89,6 @@ static const char *const blk_queue_flag_name[] = {
 	QUEUE_FLAG_NAME(INIT_DONE),
 	QUEUE_FLAG_NAME(STATS),
 	QUEUE_FLAG_NAME(REGISTERED),
-	QUEUE_FLAG_NAME(QUIESCED),
 	QUEUE_FLAG_NAME(RQ_ALLOC_TIME),
 	QUEUE_FLAG_NAME(HCTX_ACTIVE),
 	QUEUE_FLAG_NAME(SQ_SCHED),

block/blk-mq.c

@@ -260,12 +260,16 @@ EXPORT_SYMBOL_GPL(blk_mq_unfreeze_queue_non_owner);
  */
 void blk_mq_quiesce_queue_nowait(struct request_queue *q)
 {
-	unsigned long flags;
-
-	spin_lock_irqsave(&q->queue_lock, flags);
-	if (!q->quiesce_depth++)
-		blk_queue_flag_set(QUEUE_FLAG_QUIESCED, q);
-	spin_unlock_irqrestore(&q->queue_lock, flags);
+	atomic_inc(&q->quiesce_depth);
+	/*
+	 * Publish the quiesce_depth increment.  Callers must follow this
+	 * with blk_mq_wait_quiesce_done() (synchronize_srcu()/
+	 * synchronize_rcu()), which is what actually guarantees that any
+	 * in-flight dispatcher has finished and that later dispatchers see
+	 * the queue as quiesced; the barrier here only keeps this helper
+	 * self-contained for the few callers that defer the wait.
+	 */
+	smp_mb__after_atomic();
 }
 EXPORT_SYMBOL_GPL(blk_mq_quiesce_queue_nowait);
 
@@ -314,21 +318,30 @@ EXPORT_SYMBOL_GPL(blk_mq_quiesce_queue);
  */
 void blk_mq_unquiesce_queue(struct request_queue *q)
 {
-	unsigned long flags;
-	bool run_queue = false;
+	int depth;
 
-	spin_lock_irqsave(&q->queue_lock, flags);
-	if (WARN_ON_ONCE(q->quiesce_depth <= 0)) {
-		;
-	} else if (!--q->quiesce_depth) {
-		blk_queue_flag_clear(QUEUE_FLAG_QUIESCED, q);
-		run_queue = true;
-	}
-	spin_unlock_irqrestore(&q->queue_lock, flags);
+	depth = atomic_dec_if_positive(&q->quiesce_depth);
+	if (WARN_ON_ONCE(depth < 0))
+		return;
 
-	/* dispatch requests which are inserted during quiescing */
-	if (run_queue)
+	if (depth == 0) {
+		/*
+		 * Full barrier between the quiesce_depth store above and the
+		 * blk_mq_hctx_has_pending() load done from blk_mq_run_hw_queues()
+		 * below.  This pairs with the smp_mb() before the requiesce
+		 * re-check in blk_mq_run_hw_queue(): of the two racing CPUs
+		 * (one inserting a request and then re-checking quiesce state,
+		 * the other unquiescing here and then checking for pending
+		 * work) at least one sees the other's store, so the hw queue
+		 * is not left with a request stranded on a now-running queue.
+		 *
+		 * atomic_dec_if_positive() already orders the decrement on
+		 * success, but spell the barrier out so the pairing is obvious.
+		 */
+		smp_mb__after_atomic();
+		/* dispatch requests which are inserted during quiescing */
 		blk_mq_run_hw_queues(q, true);
+	}
 }
 EXPORT_SYMBOL_GPL(blk_mq_unquiesce_queue);
 
@@ -2362,17 +2375,21 @@ void blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx, bool async)
 
 	need_run = blk_mq_hw_queue_need_run(hctx);
 	if (!need_run) {
-		unsigned long flags;
-
 		/*
-		 * Synchronize with blk_mq_unquiesce_queue(), because we check
-		 * if hw queue is quiesced locklessly above, we need the use
-		 * ->queue_lock to make sure we see the up-to-date status to
-		 * not miss rerunning the hw queue.
+		 * Re-check after a full barrier.  A request may have been
+		 * inserted before this call, while a concurrent
+		 * blk_mq_unquiesce_queue() drops quiesce_depth to zero and
+		 * then runs the hw queues.  This smp_mb() orders the request
+		 * insert (the store that makes blk_mq_hctx_has_pending() true)
+		 * before the requiesce-state load below, and pairs with the
+		 * smp_mb__after_atomic() between the quiesce_depth store and
+		 * the blk_mq_hctx_has_pending() load in blk_mq_unquiesce_queue()
+		 * (and in blk_mq_quiesce_queue_nowait()).  With a full barrier
+		 * on both sides, at least one CPU observes the other's store,
+		 * so the queue is not left both un-quiesced and not rerun.
 		 */
-		spin_lock_irqsave(&hctx->queue->queue_lock, flags);
+		smp_mb();
 		need_run = blk_mq_hw_queue_need_run(hctx);
-		spin_unlock_irqrestore(&hctx->queue->queue_lock, flags);
 
 		if (!need_run)
 			return;

include/linux/blkdev.h

@@ -521,7 +521,8 @@ struct request_queue {
 
 	spinlock_t		queue_lock;
 
-	int			quiesce_depth;
+	/* Atomic quiesce depth - also serves as quiesced indicator (depth > 0) */
+	atomic_t		quiesce_depth;
 
 	struct gendisk		*disk;
 
@@ -666,7 +667,6 @@ enum {
 	QUEUE_FLAG_INIT_DONE,		/* queue is initialized */
 	QUEUE_FLAG_STATS,		/* track IO start and completion times */
 	QUEUE_FLAG_REGISTERED,		/* queue has been registered to a disk */
-	QUEUE_FLAG_QUIESCED,		/* queue has been quiesced */
 	QUEUE_FLAG_RQ_ALLOC_TIME,	/* record rq->alloc_time_ns */
 	QUEUE_FLAG_HCTX_ACTIVE,		/* at least one blk-mq hctx is active */
 	QUEUE_FLAG_SQ_SCHED,		/* single queue style io dispatch */
@@ -704,7 +704,10 @@ void blk_queue_flag_clear(unsigned int flag, struct request_queue *q);
 #define blk_noretry_request(rq) \
 	((rq)->cmd_flags & (REQ_FAILFAST_DEV|REQ_FAILFAST_TRANSPORT| \
 			     REQ_FAILFAST_DRIVER))
-#define blk_queue_quiesced(q)	test_bit(QUEUE_FLAG_QUIESCED, &(q)->queue_flags)
+static inline bool blk_queue_quiesced(struct request_queue *q)
+{
+	return atomic_read(&q->quiesce_depth) > 0;
+}
 #define blk_queue_pm_only(q)	atomic_read(&(q)->pm_only)
 #define blk_queue_registered(q)	test_bit(QUEUE_FLAG_REGISTERED, &(q)->queue_flags)
 #define blk_queue_sq_sched(q)	test_bit(QUEUE_FLAG_SQ_SCHED, &(q)->queue_flags)