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block: don't use blocking queue entered for recursive bio submits

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If we end up splitting a bio and the queue goes away between
the initial submission and the later split submission, then we
can block forever in blk_queue_enter() waiting for the reference
to drop to zero. This will never happen, since we already hold
a reference.

Mark a split bio as already having entered the queue, so we can
just use the live non-blocking queue enter variant.

Thanks to Tetsuo Handa for the analysis.

Reported-by: syzbot+c4f9cebf9d651f6e54de@syzkaller.appspotmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
This commit is contained in:
Jens Axboe 2018-06-02 14:04:07 -06:00
parent d00a11df69
commit cd4a4ae468
3 changed files with 15 additions and 1 deletions
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4
block/blk-core.c
View file

@ -2377,7 +2377,9 @@ blk_qc_t generic_make_request(struct bio *bio)
if (bio->bi_opf & REQ_NOWAIT)
flags = BLK_MQ_REQ_NOWAIT;
if (blk_queue_enter(q, flags) < 0) {
if (bio_flagged(bio, BIO_QUEUE_ENTERED))
blk_queue_enter_live(q);
else if (blk_queue_enter(q, flags) < 0) {
if (!blk_queue_dying(q) && (bio->bi_opf & REQ_NOWAIT))
bio_wouldblock_error(bio);
else

10
block/blk-merge.c
View file

@ -210,6 +210,16 @@ void blk_queue_split(struct request_queue *q, struct bio **bio)
/* there isn't chance to merge the splitted bio */
split->bi_opf |= REQ_NOMERGE;
/*
* Since we're recursing into make_request here, ensure
* that we mark this bio as already having entered the queue.
* If not, and the queue is going away, we can get stuck
* forever on waiting for the queue reference to drop. But
* that will never happen, as we're already holding a
* reference to it.
*/
bio_set_flag(*bio, BIO_QUEUE_ENTERED);
bio_chain(split, *bio);
trace_block_split(q, split, (*bio)->bi_iter.bi_sector);
generic_make_request(*bio);

2
include/linux/blk_types.h
View file

@ -229,6 +229,8 @@ struct bio {
* throttling rules. Don't do it again. */
#define BIO_TRACE_COMPLETION 10 /* bio_endio() should trace the final completion
* of this bio. */
#define BIO_QUEUE_ENTERED 11 /* can use blk_queue_enter_live() */
/* See BVEC_POOL_OFFSET below before adding new flags */
/*