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

Jens Axboe
2017-04-20T22:39:10Z

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