From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Sat, 2 Jun 2018 07:49:53 +0800 From: Ming Lei To: Jens Axboe Cc: Tetsuo Handa , Bart.VanAssche@wdc.com, dvyukov@google.com, linux-kernel@vger.kernel.org, linux-block@vger.kernel.org, jthumshirn@suse.de, alan.christopher.jenkins@gmail.com, syzbot+c4f9cebf9d651f6e54de@syzkaller.appspotmail.com, martin.petersen@oracle.com, dan.j.williams@intel.com, hch@lst.de, oleksandr@natalenko.name, martin@lichtvoll.de, hare@suse.com, syzkaller-bugs@googlegroups.com, ross.zwisler@linux.intel.com, keith.busch@intel.com, linux-ext4@vger.kernel.org Subject: Re: INFO: task hung in blk_queue_enter Message-ID: <20180601234946.GA655@ming.t460p> References: <43327033306c3dd2f7c3717d64ce22415b6f3451.camel@wdc.com> <6db16aa3a7c56b6dcca2d10b4e100a780c740081.camel@wdc.com> <201805220652.BFH82351.SMQFFOJOtFOVLH@I-love.SAKURA.ne.jp> <201805222020.FEJ82897.OFtJMFHOVLQOSF@I-love.SAKURA.ne.jp> <25708e84-6f35-04c3-a2e4-6854f0ed9e78@I-love.SAKURA.ne.jp> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: List-ID: On Fri, Jun 01, 2018 at 11:52:45AM -0600, Jens Axboe wrote: > On 6/1/18 4:10 AM, Tetsuo Handa 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... > > > > ---------------------------------------- > > [ 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. > > Ahh, nicely spotted! One idea would be the one below. For this case, > we're recursing, so we can either do a non-block queue enter, or we > can just do a live enter. > > diff --git a/block/blk-core.c b/block/blk-core.c > index 85909b431eb0..7de12e0dcc3d 100644 > --- a/block/blk-core.c > +++ b/block/blk-core.c > @@ -2392,7 +2392,9 @@ blk_qc_t generic_make_request(struct bio *bio) > > if (bio->bi_opf & REQ_NOWAIT) > flags = BLK_MQ_REQ_NOWAIT; > - if (blk_queue_enter(q, flags) < 0) { > + if (bio->bi_opf & REQ_QUEUE_ENTERED) > + blk_queue_enter_live(q); > + else if (blk_queue_enter(q, flags) < 0) { > if (!blk_queue_dying(q) && (bio->bi_opf & REQ_NOWAIT)) > bio_wouldblock_error(bio); > else > diff --git a/block/blk-merge.c b/block/blk-merge.c > index 782940c65d8a..e1063e8f7eda 100644 > --- a/block/blk-merge.c > +++ b/block/blk-merge.c > @@ -210,6 +210,16 @@ void blk_queue_split(struct request_queue *q, struct bio **bio) > /* there isn't chance to merge the splitted bio */ > split->bi_opf |= REQ_NOMERGE; > > + /* > + * Since we're recursing into make_request here, ensure > + * that we mark this bio as already having entered the queue. > + * If not, and the queue is going away, we can get stuck > + * forever on waiting for the queue reference to drop. But > + * that will never happen, as we're already holding a > + * reference to it. > + */ > + (*bio)->bi_opf |= REQ_QUEUE_ENTERED; > + > bio_chain(split, *bio); > trace_block_split(q, split, (*bio)->bi_iter.bi_sector); > generic_make_request(*bio); > diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h > index 17b18b91ebac..a6bc789b83e7 100644 > --- a/include/linux/blk_types.h > +++ b/include/linux/blk_types.h > @@ -282,6 +282,8 @@ enum req_flag_bits { > /* command specific flags for REQ_OP_WRITE_ZEROES: */ > __REQ_NOUNMAP, /* do not free blocks when zeroing */ > > + __REQ_QUEUE_ENTERED, /* queue already entered for this request */ > + > /* for driver use */ > __REQ_DRV, > > @@ -302,9 +304,8 @@ enum req_flag_bits { > #define REQ_RAHEAD (1ULL << __REQ_RAHEAD) > #define REQ_BACKGROUND (1ULL << __REQ_BACKGROUND) > #define REQ_NOWAIT (1ULL << __REQ_NOWAIT) > - > #define REQ_NOUNMAP (1ULL << __REQ_NOUNMAP) > - > +#define REQ_QUEUE_ENTERED (1ULL << __REQ_QUEUE_ENTERED) > #define REQ_DRV (1ULL << __REQ_DRV) > > #define REQ_FAILFAST_MASK \ bio clone inherits .bi_opf, so this way may not work for cloned bio. Given most of bio_split() is run in .make_request_fn(), holding the queue ref may not be needed in case of bio splitting, so another candidate fix might be to introduce one extra parameter of 'need_queue_ref' to generic_make_request(). Thanks, Ming