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Subject: Re: INFO: task hung in blk_queue_enter
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From:   Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Message-ID: <25708e84-6f35-04c3-a2e4-6854f0ed9e78@I-love.SAKURA.ne.jp>
Date:   Fri, 1 Jun 2018 19:10:17 +0900
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Tetsuo Handa wrote:
> Since sum of percpu_count did not change after percpu_ref_kill(), this is
> not a race condition while folding percpu counter values into atomic counter
> value. That is, for some reason, someone who is responsible for calling
> percpu_ref_put(&q->q_usage_counter) (presumably via blk_queue_exit()) is
> unable to call percpu_ref_put().
> But I don't know how to find someone who is failing to call percpu_ref_put()...

I found the someone. It was already there in the backtrace...

----------------------------------------
[   62.065852] a.out           D    0  4414   4337 0x00000000
[   62.067677] Call Trace:
[   62.068545]  __schedule+0x40b/0x860
[   62.069726]  schedule+0x31/0x80
[   62.070796]  schedule_timeout+0x1c1/0x3c0
[   62.072159]  ? __next_timer_interrupt+0xd0/0xd0
[   62.073670]  blk_queue_enter+0x218/0x520
[   62.074985]  ? remove_wait_queue+0x70/0x70
[   62.076361]  generic_make_request+0x3d/0x540
[   62.077785]  ? __bio_clone_fast+0x6b/0x80
[   62.079147]  ? bio_clone_fast+0x2c/0x70
[   62.080456]  blk_queue_split+0x29b/0x560
[   62.081772]  ? blk_queue_split+0x29b/0x560
[   62.083162]  blk_mq_make_request+0x7c/0x430
[   62.084562]  generic_make_request+0x276/0x540
[   62.086034]  submit_bio+0x6e/0x140
[   62.087185]  ? submit_bio+0x6e/0x140
[   62.088384]  ? guard_bio_eod+0x9d/0x1d0
[   62.089681]  do_mpage_readpage+0x328/0x730
[   62.091045]  ? __add_to_page_cache_locked+0x12e/0x1a0
[   62.092726]  mpage_readpages+0x120/0x190
[   62.094034]  ? check_disk_change+0x70/0x70
[   62.095454]  ? check_disk_change+0x70/0x70
[   62.096849]  ? alloc_pages_current+0x65/0xd0
[   62.098277]  blkdev_readpages+0x18/0x20
[   62.099568]  __do_page_cache_readahead+0x298/0x360
[   62.101157]  ondemand_readahead+0x1f6/0x490
[   62.102546]  ? ondemand_readahead+0x1f6/0x490
[   62.103995]  page_cache_sync_readahead+0x29/0x40
[   62.105539]  generic_file_read_iter+0x7d0/0x9d0
[   62.107067]  ? futex_wait+0x221/0x240
[   62.108303]  ? trace_hardirqs_on+0xd/0x10
[   62.109654]  blkdev_read_iter+0x30/0x40
[   62.110954]  generic_file_splice_read+0xc5/0x140
[   62.112538]  do_splice_to+0x74/0x90
[   62.113726]  splice_direct_to_actor+0xa4/0x1f0
[   62.115209]  ? generic_pipe_buf_nosteal+0x10/0x10
[   62.116773]  do_splice_direct+0x8a/0xb0
[   62.118056]  do_sendfile+0x1aa/0x390
[   62.119255]  __x64_sys_sendfile64+0x4e/0xc0
[   62.120666]  do_syscall_64+0x6e/0x210
[   62.121909]  entry_SYSCALL_64_after_hwframe+0x49/0xbe
----------------------------------------

The someone is blk_queue_split() from blk_mq_make_request() who depends on an
assumption that blk_queue_enter() from recursively called generic_make_request()
does not get blocked due to percpu_ref_tryget_live(&q->q_usage_counter) failure.

----------------------------------------
generic_make_request(struct bio *bio) {
  if (blk_queue_enter(q, flags) < 0) { /* <= percpu_ref_tryget_live() succeeds. */
    if (!blk_queue_dying(q) && (bio->bi_opf & REQ_NOWAIT))
      bio_wouldblock_error(bio);
    else
      bio_io_error(bio);
    return ret;
  }
(...snipped...)
  ret = q->make_request_fn(q, bio);
(...snipped...)
  if (q)
    blk_queue_exit(q);
}
----------------------------------------

where q->make_request_fn == blk_mq_make_request which does

----------------------------------------
blk_mq_make_request(struct request_queue *q, struct bio *bio) {
   blk_queue_split(q, &bio);
}

blk_queue_split(struct request_queue *q, struct bio **bio) {
  generic_make_request(*bio); /* <= percpu_ref_tryget_live() fails and waits until atomic_read(&q->mq_freeze_depth) becomes 0. */
}
----------------------------------------

and meanwhile atomic_inc_return(&q->mq_freeze_depth) and
percpu_ref_kill() are called by	blk_freeze_queue_start()...

Now, it is up to you about how to fix this race problem.