Re: [PATCH] blk-mq: fix schedule-while-atomic with scheduler attached

[Jens Axboe <axboe@kernel.dk>]

On 04/20/2017 03:30 PM, Omar Sandoval wrote:
> On Thu, Apr 20, 2017 at 03:13:43PM -0600, Jens Axboe wrote:
>> We must have dropped the ctx before we call
>> blk_mq_sched_insert_request() with can_block=true, otherwise we risk
>> that a flush request can block on insertion if we are currently out of
>> tags.
>>
>> [   47.667190] BUG: scheduling while atomic: jbd2/sda2-8/2089/0x00000002
>> [   47.674493] Modules linked in: x86_pkg_temp_thermal btrfs xor zlib_deflate raid6_pq sr_mod cdre
>> [   47.690572] Preemption disabled at:
>> [   47.690584] [<ffffffff81326c7c>] blk_mq_sched_get_request+0x6c/0x280
>> [   47.701764] CPU: 1 PID: 2089 Comm: jbd2/sda2-8 Not tainted 4.11.0-rc7+ #271
>> [   47.709630] Hardware name: Dell Inc. PowerEdge T630/0NT78X, BIOS 2.3.4 11/09/2016
>> [   47.718081] Call Trace:
>> [   47.720903]  dump_stack+0x4f/0x73
>> [   47.724694]  ? blk_mq_sched_get_request+0x6c/0x280
>> [   47.730137]  __schedule_bug+0x6c/0xc0
>> [   47.734314]  __schedule+0x559/0x780
>> [   47.738302]  schedule+0x3b/0x90
>> [   47.741899]  io_schedule+0x11/0x40
>> [   47.745788]  blk_mq_get_tag+0x167/0x2a0
>> [   47.750162]  ? remove_wait_queue+0x70/0x70
>> [   47.754901]  blk_mq_get_driver_tag+0x92/0xf0
>> [   47.759758]  blk_mq_sched_insert_request+0x134/0x170
>> [   47.765398]  ? blk_account_io_start+0xd0/0x270
>> [   47.770679]  blk_mq_make_request+0x1b2/0x850
>> [   47.775766]  generic_make_request+0xf7/0x2d0
>> [   47.780860]  submit_bio+0x5f/0x120
>> [   47.784979]  ? submit_bio+0x5f/0x120
>> [   47.789631]  submit_bh_wbc.isra.46+0x10d/0x130
>> [   47.794902]  submit_bh+0xb/0x10
>> [   47.798719]  journal_submit_commit_record+0x190/0x210
>> [   47.804686]  ? _raw_spin_unlock+0x13/0x30
>> [   47.809480]  jbd2_journal_commit_transaction+0x180a/0x1d00
>> [   47.815925]  kjournald2+0xb6/0x250
>> [   47.820022]  ? kjournald2+0xb6/0x250
>> [   47.824328]  ? remove_wait_queue+0x70/0x70
>> [   47.829223]  kthread+0x10e/0x140
>> [   47.833147]  ? commit_timeout+0x10/0x10
>> [   47.837742]  ? kthread_create_on_node+0x40/0x40
>> [   47.843122]  ret_from_fork+0x29/0x40
>>
>> Fixes: a4d907b6a33b ("blk-mq: streamline blk_mq_make_request")
>> Signed-off-by: Jens Axboe <axboe@fb.com>
>>
>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>> index 9d7645f24b05..323eed50d615 100644
>> --- a/block/blk-mq.c
>> +++ b/block/blk-mq.c
>> @@ -1634,8 +1634,10 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
>>  		blk_mq_try_issue_directly(data.hctx, rq, &cookie);
>>  		return cookie;
>>  	} else if (q->elevator) {
>> +		blk_mq_put_ctx(data.ctx);
>>  		blk_mq_bio_to_request(rq, bio);
>>  		blk_mq_sched_insert_request(rq, false, true, true, true);
>> +		return cookie;
>>  	} else if (!blk_mq_merge_queue_io(data.hctx, data.ctx, rq, bio))
>>  		blk_mq_run_hw_queue(data.hctx, true);
>>  
>>
> 
> I'm confused, the first thing we check in make_request is:
> 
> 	if (unlikely(is_flush_fua)) {
> 		blk_mq_bio_to_request(rq, bio);
> 		if (q->elevator) {
> 			blk_mq_sched_insert_request(rq, false, true, true,
> 					true);
> 		} else {
> 			blk_insert_flush(rq);
> 			blk_mq_run_hw_queue(data.hctx, true);
> 		}
> 	}
> 
> and can_block doesn't do anything in the !flush case, so shouldn't it be
> changed in that one instead?

Just to get closure on this issue, the two cases ends up being folded
into one. So we're really triggering the first case, but it's a jump
to the 2nd one.

Both cases should still be fixed up, the 2nd patch I sent out should be
fine.

-- 
Jens Axboe