Re: [PATCH] block: fix potential IO hang when turning off io_poll

[Joseph Qi <joseph.qi@linux.alibaba.com>]

On 2/22/21 2:54 PM, Jeffle Xu wrote:
> QUEUE_FLAG_POLL flag will be cleared when turning off 'io_poll', while
> at that moment there may be IOs stuck in hw queue uncompleted. The
> following polling routine won't help reap these IOs, since blk_poll()
> will return immediately because of cleared QUEUE_FLAG_POLL flag. Thus
> these IOs will hang until they finnaly time out. The hang out can be
> observed by 'fio --engine=io_uring iodepth=1', while turning off
> 'io_poll' at the same time.
> 
> To fix this, freeze and flush the request queue first when turning off
> 'io_poll'.
> 
> Signed-off-by: Jeffle Xu <jefflexu@linux.alibaba.com>
> ---
>  block/blk-sysfs.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> index b513f1683af0..10d74741c002 100644
> --- a/block/blk-sysfs.c
> +++ b/block/blk-sysfs.c
> @@ -430,8 +430,11 @@ static ssize_t queue_poll_store(struct request_queue *q, const char *page,
>  
>  	if (poll_on)
>  		blk_queue_flag_set(QUEUE_FLAG_POLL, q);
> -	else
> +	else {
> +		blk_mq_freeze_queue(q);
>  		blk_queue_flag_clear(QUEUE_FLAG_POLL, q);
> +		blk_mq_unfreeze_queue(q);
> +	}
>  
>  	return ret;
>  }
> 

Better to place brace to 'if' as well.

Thanks,
Joseph