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Subject[PATCH 4.17 078/101] block: dont use blocking queue entered for recursive bio submits
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4.17-stable review patch.  If anyone has any objections, please let me know.

------------------

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.

Reported-by: syzbot+c4f9cebf9d651f6e54de@syzkaller.appspotmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -2392,7 +2392,9 @@ blk_qc_t generic_make_request(struct 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
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -210,6 +210,16 @@ void blk_queue_split(struct request_queu
/* 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);
--- 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 */

/*

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