From: Jens Axboe axboe@kernel.dk

commit cd4a4ae4683dc2e09380118e205e057896dcda2b upstream

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.

We're running fio tests and the tasks get stuck in a D state forever when systemd-udevd tries to read the partition table. This patch solves it. Please apply to 4.17 stable.

Reported-by: syzbot+c4f9cebf9d651f6e54de@syzkaller.appspotmail.com Signed-off-by: Jens Axboe axboe@kernel.dk Signed-off-by: Alexandru Moise 00moses.alexander00@gmail.com --- v2: Fixed "From:" v3: Added "From: Jens Axboe" above commit sha

block/blk-core.c | 4 +++- block/blk-merge.c | 10 ++++++++++ include/linux/blk_types.h | 2 ++ 3 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/block/blk-core.c b/block/blk-core.c index b559b9d4f1a2..47ab2d9d02d9 100644 --- a/block/blk-core.c +++ b/block/blk-core.c @@ -2392,7 +2392,9 @@ 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 diff --git a/block/blk-merge.c b/block/blk-merge.c index 782940c65d8a..481dc02668f9 100644 --- a/block/blk-merge.c +++ b/block/blk-merge.c @@ -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); diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h index 17b18b91ebac..1602bf4ab4cd 100644 --- a/include/linux/blk_types.h +++ b/include/linux/blk_types.h @@ -186,6 +186,8 @@ 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 */

/*