linux-kernel - Re: [PATCH] blk-mq: avoid potential infinite loop in __blk_mq_alloc

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Date:   Thu, 21 Jul 2022 22:33:36 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Liu Song <liusong@...ux.alibaba.com>
Cc:     axboe@...nel.dk, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] blk-mq: avoid potential infinite loop in
 __blk_mq_alloc_request

On Fri, Jul 22, 2022 at 01:22:23PM +0800, Liu Song wrote:
> From: Liu Song <liusong@...ux.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.

That means the driver calling blk_mq_alloc_request has a bug, and
we should not work round that in the low level tag allocation path.

If we want to be nice we can add a WARN_ON before going all the way
down into the tag allocator, something like:

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 92aae03103b74..d6c7e2ece025f 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -520,6 +520,10 @@ struct request *blk_mq_alloc_request(struct request_queue *q, unsigned int op,
 	struct request *rq;
 	int ret;
 
+	if (WARN_ON_ONCE((flags & BLK_MQ_REQ_RESERVED) &&
+			!q->tag_set->reserved_tags))
+		return ERR_PTR(-EINVAL);
+
 	ret = blk_queue_enter(q, flags);
 	if (ret)
 		return ERR_PTR(ret);