linux-kernel - Re: [PATCH 1/1] sbitmap: Replace cmpxchg with xchg

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Date:   Sat, 29 Jun 2019 08:42:23 -0700
From:   Pavel Begunkov <asml.silence@...il.com>
To:     Jens Axboe <axboe@...nel.dk>, osandov@...com,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] sbitmap: Replace cmpxchg with xchg

Ping?

On 23/05/2019 08:39, Pavel Begunkov (Silence) wrote:
> From: Pavel Begunkov <asml.silence@...il.com>
> 
> cmpxchg() with an immediate value could be replaced with less expensive
> xchg(). The same true if new value don't _depend_ on the old one.
> 
> In the second block, atomic_cmpxchg() return value isn't checked, so
> after atomic_cmpxchg() ->  atomic_xchg() conversion it could be replaced
> with atomic_set(). Comparison with atomic_read() in the second chunk was
> left as an optimisation (if that was the initial intention).
> 
> Signed-off-by: Pavel Begunkov <asml.silence@...il.com>
> ---
>  lib/sbitmap.c | 10 +++-------
>  1 file changed, 3 insertions(+), 7 deletions(-)
> 
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index 155fe38756ec..7d7e0e278523 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -37,9 +37,7 @@ static inline bool sbitmap_deferred_clear(struct sbitmap *sb, int index)
>  	/*
>  	 * First get a stable cleared mask, setting the old mask to 0.
>  	 */
> -	do {
> -		mask = sb->map[index].cleared;
> -	} while (cmpxchg(&sb->map[index].cleared, mask, 0) != mask);
> +	mask = xchg(&sb->map[index].cleared, 0);
>  
>  	/*
>  	 * Now clear the masked bits in our free word
> @@ -527,10 +525,8 @@ static struct sbq_wait_state *sbq_wake_ptr(struct sbitmap_queue *sbq)
>  		struct sbq_wait_state *ws = &sbq->ws[wake_index];
>  
>  		if (waitqueue_active(&ws->wait)) {
> -			int o = atomic_read(&sbq->wake_index);
> -
> -			if (wake_index != o)
> -				atomic_cmpxchg(&sbq->wake_index, o, wake_index);
> +			if (wake_index != atomic_read(&sbq->wake_index))
> +				atomic_set(&sbq->wake_index, wake_index);
>  			return ws;
>  		}
>  
> 

-- 
Yours sincerely,
Pavel Begunkov



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