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Subject: Re: [PATCH] iommu/iova: Replace cmpxchg with xchg in queue_iova
To:     Shaokun Zhang <zhangshaokun@hisilicon.com>,
        iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org
Cc:     Yuqi Jin <jinyuqi@huawei.com>
References: <1598517834-30275-1-git-send-email-zhangshaokun@hisilicon.com>
From:   Robin Murphy <robin.murphy@arm.com>
Message-ID: <3afcc7b2-0bfb-b79c-513f-1beb66c5f164@arm.com>
Date:   Thu, 27 Aug 2020 20:41:36 +0100
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On 2020-08-27 09:43, Shaokun Zhang wrote:
> From: Yuqi Jin <jinyuqi@huawei.com>
> 
> The performance of the atomic_xchg is better than atomic_cmpxchg because
> no comparison is required. While the value of @fq_timer_on can only be 0
> or 1. Let's use atomic_xchg instead of atomic_cmpxchg here because we
> only need to check that the value changes from 0 to 1 or from 1 to 1.

Looks reasonable to me - the "compare" part is already covered by the 
separate atomic_read(), and as you say there's no harm if the "exchange" 
part races as long as it's still atomic.

Reviewed-by: Robin Murphy <robin.murphy@arm.com>

> Cc: Joerg Roedel <joro@8bytes.org>
> Signed-off-by: Yuqi Jin <jinyuqi@huawei.com>
> Signed-off-by: Shaokun Zhang <zhangshaokun@hisilicon.com>
> ---
>   drivers/iommu/iova.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
> index 45a251da5453..30d969a4c5fd 100644
> --- a/drivers/iommu/iova.c
> +++ b/drivers/iommu/iova.c
> @@ -579,7 +579,7 @@ void queue_iova(struct iova_domain *iovad,
>   
>   	/* Avoid false sharing as much as possible. */
>   	if (!atomic_read(&iovad->fq_timer_on) &&
> -	    !atomic_cmpxchg(&iovad->fq_timer_on, 0, 1))
> +	    !atomic_xchg(&iovad->fq_timer_on, 1))
>   		mod_timer(&iovad->fq_timer,
>   			  jiffies + msecs_to_jiffies(IOVA_FQ_TIMEOUT));
>   }
>