Re: [PATCH 1/1] blk-mq: fix blk_mq_hw_ctx active request accounting

[Jens Axboe <axboe@kernel.dk>]

On 5/13/23 4:12 PM, Tian Lan wrote:
> From: Tian Lan <tian.lan@twosigma.com>
> 
> The nr_active counter continues to increase over time which causes the
> blk_mq_get_tag to hang until the thread is rescheduled to a different
> core despite there are still tags available.
> 
> kernel-stack
> 
>   INFO: task inboundIOReacto:3014879 blocked for more than 2 seconds
>   Not tainted 6.1.15-amd64 #1 Debian 6.1.15~debian11
>   "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>   task:inboundIOReacto state:D stack:0  pid:3014879 ppid:4557 flags:0x00000000
>     Call Trace:
>     <TASK>
>     __schedule+0x351/0xa20
>     scheduler+0x5d/0xe0
>     io_schedule+0x42/0x70
>     blk_mq_get_tag+0x11a/0x2a0
>     ? dequeue_task_stop+0x70/0x70
>     __blk_mq_alloc_requests+0x191/0x2e0
> 
> kprobe output showing RQF_MQ_INFLIGHT bit is not cleared before
> __blk_mq_free_request being called.
> 
>   320    320  kworker/29:1H __blk_mq_free_request rq_flags 0x220c0 in-flight 1
>          b'__blk_mq_free_request+0x1 [kernel]'
>          b'bt_iter+0x50 [kernel]'
>          b'blk_mq_queue_tag_busy_iter+0x318 [kernel]'
>          b'blk_mq_timeout_work+0x7c [kernel]'
>          b'process_one_work+0x1c4 [kernel]'
>          b'worker_thread+0x4d [kernel]'
>          b'kthread+0xe6 [kernel]'
>          b'ret_from_fork+0x1f [kernel]'
> 
> Signed-off-by: Tian Lan <tian.lan@twosigma.com>

I think this needs:

Cc: stable@vger.kernel.org
Fixes: 2e315dc07df0 ("blk-mq: grab rq->refcount before calling ->fn in blk_mq_tagset_busy_iter")

tags, but I'm also now confused as to whether the flush handling part
of that patch. Ming, what am I missing in terms of not honoring the
flush ref on put? What happens if two iterators both grab the
flush at the same time, and then subsequently put them?

-- 
Jens Axboe