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Subject: Re: [PATCH 1/5] sbitmap: don't consume nr for inactive waitqueue to
 avoid lost wakeups
To:     Kemeng Shi <shikemeng@huawei.com>, axboe@kernel.dk
Cc:     linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        linfeilong@huawei.com, liuzhiqiang@huawei.com
References: <20221201045408.21908-1-shikemeng@huawei.com>
 <20221201045408.21908-2-shikemeng@huawei.com>
From:   Kemeng Shi <shikemeng@huaweicloud.com>
Message-ID: <81e3f861-f163-c17c-49d4-2408f16c3350@huaweicloud.com>
Date:   Thu, 1 Dec 2022 15:21:05 +0800
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on 12/1/2022 12:54 PM, Kemeng Shi wrote:
> If we decremented queue without waiters, we should not decremente freed
> bits number "nr", or all "nr" could be consumed in a empty queue and no
> wakeup will be called.
> Currently, for case "wait_cnt > 0", "nr" will not be decremented if we
> decremented queue without watiers and retry is returned to avoid lost
> wakeups. However for case "wait_cnt == 0", "nr" will be decremented
> unconditionally and maybe decremented to zero. Although retry is
> returned by active state of queue, it's not actually executed for "nr"
> is zero.
> 
> Fix this by only decrementing "nr" for active queue when "wait_cnt ==
> 0". After this fix, "nr" will always be non-zero when we decremented
> inactive queue for case "wait_cnt == 0", so the need to retry could
> be returned by "nr" and active state of waitqueue returned for the same
> purpose is not needed.
> 
> Signed-off-by: Kemeng Shi <shikemeng@huawei.com>
> ---
>  lib/sbitmap.c | 13 ++++++-------
>  1 file changed, 6 insertions(+), 7 deletions(-)
> 
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index 7280ae8ca88c..e40759bcf821 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -604,7 +604,6 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq, int *nr)
>  	struct sbq_wait_state *ws;
>  	unsigned int wake_batch;
>  	int wait_cnt, cur, sub;
> -	bool ret;
>  
>  	if (*nr <= 0)
>  		return false;
> @@ -632,15 +631,15 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq, int *nr)
>  	if (wait_cnt > 0)
>  		return !waitqueue_active(&ws->wait);
>  
> -	*nr -= sub;
> -
>  	/*
>  	 * When wait_cnt == 0, we have to be particularly careful as we are
>  	 * responsible to reset wait_cnt regardless whether we've actually
> -	 * woken up anybody. But in case we didn't wakeup anybody, we still
> -	 * need to retry.
> +	 * woken up anybody. But in case we didn't wakeup anybody, we should
> +	 * not consume nr and need to retry to avoid lost wakeups.
>  	 */
> -	ret = !waitqueue_active(&ws->wait);
There is a warnning reported by checkpatch.pl which is "WARNING:waitqueue_active
without comment" but I don't know why.
> +	if (waitqueue_active(&ws->wait))
> +		*nr -= sub;
> +
>  	wake_batch = READ_ONCE(sbq->wake_batch);
>  
>  	/*
> @@ -669,7 +668,7 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq, int *nr)
>  	sbq_index_atomic_inc(&sbq->wake_index);
>  	atomic_set(&ws->wait_cnt, wake_batch);
>  
> -	return ret || *nr;
> +	return *nr;
>  }
>  
>  void sbitmap_queue_wake_up(struct sbitmap_queue *sbq, int nr)
> 
Besides, there are some git config problems for my huaweicloud email, I will send
patchset with huaweicloud email when I fix them.

Thanks.
-- 
Best wishes
Kemeng Shi