From: Jens Axboe Subject: Re: INFO: task hung in blk_queue_enter Date: Fri, 1 Jun 2018 18:56:31 -0600 Message-ID: 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> <20180601234946.GA655@ming.t460p> <95c419d8-7f19-f9c0-a53f-3d381fe93176@kernel.dk> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit 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 To: Ming Lei Return-path: In-Reply-To: <95c419d8-7f19-f9c0-a53f-3d381fe93176@kernel.dk> Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On 6/1/18 6:49 PM, Jens Axboe wrote: > On 6/1/18 5:49 PM, Ming Lei wrote: >> 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(). > > I did consider that, but that's pretty silly for a corner case and you'd > have to change a crap ton of calls to it. I think it'd be cleaner to > clear the bit when we need to, potentially even adding a debug check to > blk_queue_enter_live() that complains if the ref was not already > elevated. Though that would be expensive, compared to the percpu inc > now. Not saying the bit is necessarily the best way forward, but I do > like it a LOT more than adding an argument to generic_make_request. > > It should have been a bio flag to begin with, that better captures the > scope and also avoids clones inheriting anything they shouldn't. It's > also not applicable to a request. A little too much inheritance from the previous patch, better version below. diff --git a/block/blk-core.c b/block/blk-core.c index 85909b431eb0..71cb228d7374 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_flagged(bio, BIO_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..481dc02668f9 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_set_flag(*bio, BIO_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..1602bf4ab4cd 100644 --- a/include/linux/blk_types.h +++ b/include/linux/blk_types.h @@ -186,6 +186,8 @@ struct bio { * throttling rules. Don't do it again. */ #define BIO_TRACE_COMPLETION 10 /* bio_endio() should trace the final completion * of this bio. */ +#define BIO_QUEUE_ENTERED 11 /* can use blk_queue_enter_live() */ + /* See BVEC_POOL_OFFSET below before adding new flags */ /* -- Jens Axboe