[PATCH] block: ensure we hold a queue reference when using queue limits

Jens Axboe
2024-01-12T16:15:17Z

q_usage_counter is the only thing preventing us from the limits changing
under us in __bio_split_to_limits, but blk_mq_submit_bio doesn't hold
it while calling into it.

Move the splitting inside the region where we know we've got a queue
reference. Ideally this could still remain a shared section of code, but
let's keep the fix simple and defer any refactoring here to later.

Reported-by: Christoph Hellwig <hch@lst.de>
Fixes: 9d497e2941c3 ("block: don't protect submit_bio_checks by q_usage_counter")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

---

diff --git a/block/blk-mq.c b/block/blk-mq.c
index f57b86d6de6a..e02c4b1af8c5 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2964,12 +2964,6 @@ void blk_mq_submit_bio(struct bio *bio)
blk_status_t ret;

bio = blk_queue_bounce(bio, q);
- if (bio_may_exceed_limits(bio, &q->limits)) {
- bio = __bio_split_to_limits(bio, &q->limits, &nr_segs);
- if (!bio)
- return;
- }
-
bio_set_ioprio(bio);

if (plug) {
@@ -2978,6 +2972,11 @@ void blk_mq_submit_bio(struct bio *bio)
rq = NULL;
}
if (rq) {
+ if (unlikely(bio_may_exceed_limits(bio, &q->limits))) {
+ bio = __bio_split_to_limits(bio, &q->limits, &nr_segs);
+ if (!bio)
+ return;
+ }
if (!bio_integrity_prep(bio))
return;
if (blk_mq_attempt_bio_merge(q, bio, nr_segs))
@@ -2988,6 +2987,11 @@ void blk_mq_submit_bio(struct bio *bio)
} else {
if (unlikely(bio_queue_enter(bio)))
return;
+ if (unlikely(bio_may_exceed_limits(bio, &q->limits))) {
+ bio = __bio_split_to_limits(bio, &q->limits, &nr_segs);
+ if (!bio)
+ goto fail;
+ }
if (!bio_integrity_prep(bio))
goto fail;
}

--
Jens Axboe