blk-mq: fix race condition in active queue accounting

If multiple CPUs are sharing the same hardware queue, it can
cause leak in the active queue counter tracking when __blk_mq_tag_busy()
is executed simultaneously.

Fixes: ee78ec1 ("blk-mq: blk_mq_tag_busy is no need to return a value")
Signed-off-by: Tian Lan <[email protected]>
Reviewed-by: Ming Lei <[email protected]>
Reviewed-by: Damien Le Moal <[email protected]>
Reviewed-by: John Garry <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jens Axboe <[email protected]>

Author: tilan7663

block/blk-mq-tag.c

@@ -39,16 +39,20 @@ void __blk_mq_tag_busy(struct blk_mq_hw_ctx *hctx)
 {
 	unsigned int users;
 
+	/*
+	 * calling test_bit() prior to test_and_set_bit() is intentional,
+	 * it avoids dirtying the cacheline if the queue is already active.
+	 */
 	if (blk_mq_is_shared_tags(hctx->flags)) {
 		struct request_queue *q = hctx->queue;
 
-		if (test_bit(QUEUE_FLAG_HCTX_ACTIVE, &q->queue_flags))
+		if (test_bit(QUEUE_FLAG_HCTX_ACTIVE, &q->queue_flags) ||
+		    test_and_set_bit(QUEUE_FLAG_HCTX_ACTIVE, &q->queue_flags))
 			return;
-		set_bit(QUEUE_FLAG_HCTX_ACTIVE, &q->queue_flags);
 	} else {
-		if (test_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
+		if (test_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state) ||
+		    test_and_set_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
 			return;
-		set_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state);
 	}
 
 	users = atomic_inc_return(&hctx->tags->active_queues);