linux-kernel - Re: [PATCH] block: don't decrement flush request refcount if it's state is idle in flush_end

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Message-ID: <e251976b-eab8-1c79-0891-25d48b31d4db@huawei.com>
Date:   Fri, 13 Aug 2021 17:40:18 +0800
From:   "yukuai (C)" <yukuai3@...wei.com>
To:     <axboe@...nel.dk>
CC:     <linux-block@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <yi.zhang@...wei.com>
Subject: Re: [PATCH] block: don't decrement flush request refcount if it's
 state is idle in flush_end_io()

Hi, jens

Can you please consider to apply this patch?

Thanks
Kuai

On 2021/08/08 15:03, Yu Kuai wrote:
> flush_end_io() currently decrement request refcount unconditionally.
> However, it's possible that the request is already idle and it's
> refcount is zero since that flush_end_io() can be called concurrently.
> 
> For example, nbd_clear_que() can be called concurrently with normal
> io completion or io timeout.
> 
> Thus check idle before decrement to avoid refcount_t underflow
> warning.
> 
> Signed-off-by: Yu Kuai <yukuai3@...wei.com>
> ---
>   block/blk-flush.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/block/blk-flush.c b/block/blk-flush.c
> index 1002f6c58181..9b65dc43702c 100644
> --- a/block/blk-flush.c
> +++ b/block/blk-flush.c
> @@ -222,7 +222,8 @@ static void flush_end_io(struct request *flush_rq, blk_status_t error)
>   	/* release the tag's ownership to the req cloned from */
>   	spin_lock_irqsave(&fq->mq_flush_lock, flags);
>   
> -	if (!refcount_dec_and_test(&flush_rq->ref)) {
> +	if (blk_mq_rq_state(flush_rq) == MQ_RQ_IDLE ||
> +	    !refcount_dec_and_test(&flush_rq->ref)) {
>   		fq->rq_status = error;
>   		spin_unlock_irqrestore(&fq->mq_flush_lock, flags);
>   		return;
>