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

Omar Sandoval
2017-04-20T21:30:40Z

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?

Alternatively, blk_mq_get_tag() knows how to drop the ctx before it
sleeps, but blk_mq_get_driver_tag() doesn't set data.ctx. I'm not sure
if that'll do what we want, and this is making my head hurt.