@@ -628,7 +628,20 @@ static void blk_mq_rq_timer(unsigned long priv)
};
int i;
- if (blk_queue_enter(q, GFP_NOWAIT))
+ /* A deadlock might occur if a request is stuck requiring a
+ * timeout at the same time a queue freeze is waiting
+ * completion, since the timeout code would not be able to
+ * acquire the queue reference here.
+ *
+ * That's why we don't use blk_queue_enter here; instead, we use
+ * percpu_ref_tryget directly, because we need to be able to
+ * obtain a reference even in the short window between the queue
+ * starting to freeze, by dropping the first reference in
+ * blk_mq_freeze_queue_start, and the moment the last request is
+ * consumed, marked by the instant q_usage_counter reaches
+ * zero.
+ */
+ if (!percpu_ref_tryget(&q->q_usage_counter))
return;
blk_mq_queue_tag_busy_iter(q, blk_mq_check_expired, &data);