Commit cd4a4ae4 authored by Jens Axboe's avatar Jens Axboe
Browse files

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



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: default avatar <syzbot+c4f9cebf9d651f6e54de@syzkaller.appspotmail.com>
Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent d00a11df

block/blk-core.c

+3 −1
Original line number Diff line number Diff line
@@ -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

block/blk-merge.c

+10 −0
Original line number Diff line number Diff line
@@ -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);

include/linux/blk_types.h

+2 −0
Original line number Diff line number Diff line
@@ -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 */



/*

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