On Fri, Jun 1, 2018 at 12:10 PM, Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx> 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. >
