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    Subject[PATCH 4.17 078/101] block: dont use blocking queue entered for recursive bio submits
    Date
    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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