Re: INFO: task hung in blk_queue_enter

Dmitry Vyukov
2018-06-04T11:46:21Z

On Fri, Jun 1, 2018 at 12:10 PM, Tetsuo Handa
<penguin-kernel@i-love.sakura.ne.jp> wrote:
> 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...

Nice!

Do I understand it correctly that this bug is probably the root cause
of a whole lot of syzbot "task hung" reports? E.g. this one too?
https://syzkaller.appspot.com/bug?id=cdc4add60bb95a4da3fec27c5fe6d75196b7f976
I guess we will need to sweep close everything related to
filesystems/block devices when this is committed?

> ----------------------------------------
> [ 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.
>