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SubjectRe: INFO: task hung in blk_queue_enter
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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.

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