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-rw-r--r--block/blk-mq.c15
1 files changed, 14 insertions, 1 deletions
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 576e7112f807..6a63da101bc4 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -672,7 +672,20 @@ static void blk_mq_timeout_work(struct work_struct *work)
};
int i;
- if (blk_queue_enter(q, true))
+ /* 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);