[PATCH] blk-mq: avoid potential infinite loop in __blk_mq_alloc_request

[Liu Song <liusong@linux.alibaba.com>]

From: Liu Song <liusong@linux.alibaba.com>

If "blk_mq_get_tag" returns BLK_MQ_NO_TAG because the value of
"tags->nr_reserved_tags" is 0, it will fall into an infinite loop in
"__blk_mq_alloc_requests", so borrow BLK_MQ_REQ_NOWAIT to exit the loop.

Because "blk_mq_alloc_data" objects are allocated on the stack, changing
the content of flags will not generate extra impact.

Signed-off-by: Liu Song <liusong@linux.alibaba.com>
---
 block/blk-mq-tag.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 2dcd738..6f1d6e6 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -139,6 +139,7 @@ unsigned int blk_mq_get_tag(struct blk_mq_alloc_data *data)
 	if (data->flags & BLK_MQ_REQ_RESERVED) {
 		if (unlikely(!tags->nr_reserved_tags)) {
 			WARN_ON_ONCE(1);
+			data->flags |= BLK_MQ_REQ_NOWAIT;
 			return BLK_MQ_NO_TAG;
 		}
 		bt = &tags->breserved_tags;
-- 
1.8.3.1