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

[Omar Sandoval <osandov@osandov.com>]

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.