blk-mq: Replace tags->lock with SRCU for tag iterators

JIRA: https://issues.redhat.com/browse/RHEL-120078

commit 995412e
Author: Ming Lei <ming.lei@redhat.com>
Date:   Sat Aug 30 10:18:23 2025 +0800

    blk-mq: Replace tags->lock with SRCU for tag iterators

    Replace the spinlock in blk_mq_find_and_get_req() with an SRCU read lock
    around the tag iterators.

    This is done by:

    - Holding the SRCU read lock in blk_mq_queue_tag_busy_iter(),
    blk_mq_tagset_busy_iter(), and blk_mq_hctx_has_requests().

    - Removing the now-redundant tags->lock from blk_mq_find_and_get_req().

    This change fixes lockup issue in scsi_host_busy() in case of shost->host_blocked.

    Also avoids big tags->lock when reading disk sysfs attribute `inflight`.

    Reviewed-by: Hannes Reinecke <hare@suse.de>
    Signed-off-by: Ming Lei <ming.lei@redhat.com>
    Reviewed-by: Yu Kuai <yukuai3@huawei.com>
    Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
    Signed-off-by: Jens Axboe <axboe@kernel.dk>

Signed-off-by: Ming Lei <ming.lei@redhat.com>

Author: Ming Lei

block/blk-mq-tag.c

@@ -256,13 +256,10 @@ static struct request *blk_mq_find_and_get_req(struct blk_mq_tags *tags,
 		unsigned int bitnr)
 {
 	struct request *rq;
-	unsigned long flags;
 
-	spin_lock_irqsave(&tags->lock, flags);
 	rq = tags->rqs[bitnr];
 	if (!rq || rq->tag != bitnr || !req_ref_inc_not_zero(rq))
 		rq = NULL;
-	spin_unlock_irqrestore(&tags->lock, flags);
 	return rq;
 }
 
@@ -440,7 +437,9 @@ void blk_mq_tagset_busy_iter(struct blk_mq_tag_set *tagset,
 		busy_tag_iter_fn *fn, void *priv)
 {
 	unsigned int flags = tagset->flags;
-	int i, nr_tags;
+	int i, nr_tags, srcu_idx;
+
+	srcu_idx = srcu_read_lock(&tagset->tags_srcu);
 
 	nr_tags = blk_mq_is_shared_tags(flags) ? 1 : tagset->nr_hw_queues;
 
@@ -449,6 +448,7 @@ void blk_mq_tagset_busy_iter(struct blk_mq_tag_set *tagset,
 			__blk_mq_all_tag_iter(tagset->tags[i], fn, priv,
 					      BT_TAG_ITER_STARTED);
 	}
+	srcu_read_unlock(&tagset->tags_srcu, srcu_idx);
 }
 EXPORT_SYMBOL(blk_mq_tagset_busy_iter);
 
@@ -499,6 +499,8 @@ EXPORT_SYMBOL(blk_mq_tagset_wait_completed_request);
 void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_tag_iter_fn *fn,
 		void *priv)
 {
+	int srcu_idx;
+
 	/*
 	 * __blk_mq_update_nr_hw_queues() updates nr_hw_queues and hctx_table
 	 * while the queue is frozen. So we can use q_usage_counter to avoid
@@ -507,6 +509,7 @@ void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_tag_iter_fn *fn,
 	if (!percpu_ref_tryget(&q->q_usage_counter))
 		return;
 
+	srcu_idx = srcu_read_lock(&q->tag_set->tags_srcu);
 	if (blk_mq_is_shared_tags(q->tag_set->flags)) {
 		struct blk_mq_tags *tags = q->tag_set->shared_tags;
 		struct sbitmap_queue *bresv = &tags->breserved_tags;
@@ -536,6 +539,7 @@ void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_tag_iter_fn *fn,
 			bt_for_each(hctx, q, btags, fn, priv, false);
 		}
 	}
+	srcu_read_unlock(&q->tag_set->tags_srcu, srcu_idx);
 	blk_queue_exit(q);
 }
 

block/blk-mq.c

@@ -3411,7 +3411,6 @@ static void blk_mq_clear_rq_mapping(struct blk_mq_tags *drv_tags,
 				    struct blk_mq_tags *tags)
 {
 	struct page *page;
-	unsigned long flags;
 
 	/*
 	 * There is no need to clear mapping if driver tags is not initialized
@@ -3435,15 +3434,6 @@ static void blk_mq_clear_rq_mapping(struct blk_mq_tags *drv_tags,
 			}
 		}
 	}
-
-	/*
-	 * Wait until all pending iteration is done.
-	 *
-	 * Request reference is cleared and it is guaranteed to be observed
-	 * after the ->lock is released.
-	 */
-	spin_lock_irqsave(&drv_tags->lock, flags);
-	spin_unlock_irqrestore(&drv_tags->lock, flags);
 }
 
 void blk_mq_free_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
@@ -3666,8 +3656,12 @@ static bool blk_mq_hctx_has_requests(struct blk_mq_hw_ctx *hctx)
 	struct rq_iter_data data = {
 		.hctx	= hctx,
 	};
+	int srcu_idx;
 
+	srcu_idx = srcu_read_lock(&hctx->queue->tag_set->tags_srcu);
 	blk_mq_all_tag_iter(tags, blk_mq_has_request, &data);
+	srcu_read_unlock(&hctx->queue->tag_set->tags_srcu, srcu_idx);
+
 	return data.has_rq;
 }
 
@@ -3887,7 +3881,6 @@ static void blk_mq_clear_flush_rq_mapping(struct blk_mq_tags *tags,
 		unsigned int queue_depth, struct request *flush_rq)
 {
 	int i;
-	unsigned long flags;
 
 	/* The hw queue may not be mapped yet */
 	if (!tags)
@@ -3897,15 +3890,6 @@ static void blk_mq_clear_flush_rq_mapping(struct blk_mq_tags *tags,
 
 	for (i = 0; i < queue_depth; i++)
 		cmpxchg(&tags->rqs[i], flush_rq, NULL);
-
-	/*
-	 * Wait until all pending iteration is done.
-	 *
-	 * Request reference is cleared and it is guaranteed to be observed
-	 * after the ->lock is released.
-	 */
-	spin_lock_irqsave(&tags->lock, flags);
-	spin_unlock_irqrestore(&tags->lock, flags);
 }
 
 static void blk_free_flush_queue_callback(struct rcu_head *head)