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

Jens Axboe
2017-04-20T22:42:43Z

On 04/20/2017 04:41 PM, Omar Sandoval wrote:
> On Thu, Apr 20, 2017 at 04:39:10PM -0600, Jens Axboe wrote:
>> 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.
>
> You can add
>
> Reviewed-by: Omar Sandoval <osandov@fb.com>
>
> for the 2nd patch.

Thanks Omar, queued up for 4.12.

--
Jens Axboe