block: pop cached rq before potentially blocking rq_qos_throttle()

If rq_qos_throttle() ends up blocking, then we will have invalidated and
flushed our current plug. Since blk_mq_get_cached_request() hasn't
popped the cached request off the plug list just yet, we end holding a
pointer to a request that is no longer valid. This insta-crashes with
rq->mq_hctx being NULL in the validity checks just after.

Pop the request off the cached list before doing rq_qos_throttle() to
avoid using a potentially stale request.

Fixes: 0a5aa8d161d1 ("block: fix blk_mq_attempt_bio_merge and rq_qos_throttle protection")
Reported-by: Dylan Yudaken <dylany@fb.com>
Tested-by: Dylan Yudaken <dylany@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 33145ba..93d9d60 100644 (file)
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2765,15 +2765,20 @@ static inline struct request *blk_mq_get_cached_request(struct request_queue *q,
                return NULL;
        }
 
-       rq_qos_throttle(q, *bio);
-
        if (blk_mq_get_hctx_type((*bio)->bi_opf) != rq->mq_hctx->type)
                return NULL;
        if (op_is_flush(rq->cmd_flags) != op_is_flush((*bio)->bi_opf))
                return NULL;
 
-       rq->cmd_flags = (*bio)->bi_opf;
+       /*
+        * If any qos ->throttle() end up blocking, we will have flushed the
+        * plug and hence killed the cached_rq list as well. Pop this entry
+        * before we throttle.
+        */
        plug->cached_rq = rq_list_next(rq);
+       rq_qos_throttle(q, *bio);
+
+       rq->cmd_flags = (*bio)->bi_opf;
        INIT_LIST_HEAD(&rq->queuelist);
        return rq;
 }