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    SubjectRe: INFO: task hung in blk_queue_enter
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    Tetsuo Handa wrote:
    > I couldn't check whether freeze_depth in blk_freeze_queue_start() was 1,
    > but presumably q->mq_freeze_depth > 0 because syz-executor7(PID=5010) is
    > stuck at wait_event() in blk_queue_enter().
    >
    > Since flags == 0, preempt == false. Since stuck at wait_event(), success == false.
    > Thus, atomic_read(&q->mq_freeze_depth) > 0 if blk_queue_dying(q) == false. And I
    > guess blk_queue_dying(q) == false because we are just trying to freeze/unfreeze.
    >

    I was able to reproduce the hung up using modified reproducer, and got values
    using below debug printk() patch.

    --- a/block/blk-core.c
    +++ b/block/blk-core.c
    @@ -950,10 +950,12 @@ int blk_queue_enter(struct request_queue *q, blk_mq_req_flags_t flags)
    */
    smp_rmb();

    - wait_event(q->mq_freeze_wq,
    - (atomic_read(&q->mq_freeze_depth) == 0 &&
    - (preempt || !blk_queue_preempt_only(q))) ||
    - blk_queue_dying(q));
    + while (wait_event_timeout(q->mq_freeze_wq,
    + (atomic_read(&q->mq_freeze_depth) == 0 &&
    + (preempt || !blk_queue_preempt_only(q))) ||
    + blk_queue_dying(q), 10 * HZ) == 0)
    + printk("%s(%u): q->mq_freeze_depth=%d preempt=%d blk_queue_preempt_only(q)=%d blk_queue_dying(q)=%d\n",
    + current->comm, current->pid, atomic_read(&q->mq_freeze_depth), preempt, blk_queue_preempt_only(q), blk_queue_dying(q));
    if (blk_queue_dying(q))
    return -ENODEV;
    }

    [ 75.869126] print_req_error: I/O error, dev loop0, sector 0
    [ 85.983146] a.out(8838): q->mq_freeze_depth=1 preempt=0 blk_queue_preempt_only(q)=0 blk_queue_dying(q)=0
    [ 96.222884] a.out(8838): q->mq_freeze_depth=1 preempt=0 blk_queue_preempt_only(q)=0 blk_queue_dying(q)=0
    [ 106.463322] a.out(8838): q->mq_freeze_depth=1 preempt=0 blk_queue_preempt_only(q)=0 blk_queue_dying(q)=0
    [ 116.702912] a.out(8838): q->mq_freeze_depth=1 preempt=0 blk_queue_preempt_only(q)=0 blk_queue_dying(q)=0

    One ore more threads are waiting for q->mq_freeze_depth to become 0. But the
    thread who incremented q->mq_freeze_depth at blk_freeze_queue_start(q) from
    blk_freeze_queue() is waiting at blk_mq_freeze_queue_wait(). Therefore,
    atomic_read(&q->mq_freeze_depth) == 0 condition for wait_event() in
    blk_queue_enter() will never be satisfied. But what does that wait_event()
    want to do? Isn't "start freezing" a sort of blk_queue_dying(q) == true?
    Since percpu_ref_tryget_live(&q->q_usage_counter) failed and the queue is
    about to be frozen, shouldn't we treat atomic_read(&q->mq_freeze_depth) != 0
    as if blk_queue_dying(q) == true? That is, something like below:

    diff --git a/block/blk-core.c b/block/blk-core.c
    index 85909b4..59e2496 100644
    --- a/block/blk-core.c
    +++ b/block/blk-core.c
    @@ -951,10 +951,10 @@ int blk_queue_enter(struct request_queue *q, blk_mq_req_flags_t flags)
    smp_rmb();

    wait_event(q->mq_freeze_wq,
    - (atomic_read(&q->mq_freeze_depth) == 0 &&
    - (preempt || !blk_queue_preempt_only(q))) ||
    + atomic_read(&q->mq_freeze_depth) ||
    + (preempt || !blk_queue_preempt_only(q)) ||
    blk_queue_dying(q));
    - if (blk_queue_dying(q))
    + if (atomic_read(&q->mq_freeze_depth) || blk_queue_dying(q))
    return -ENODEV;
    }
    }
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