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Date:   Fri, 4 Sep 2020 11:37:32 +0200
From:   Joerg Roedel <joro@8bytes.org>
To:     Shaokun Zhang <zhangshaokun@hisilicon.com>
Cc:     iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org,
        Yuqi Jin <jinyuqi@huawei.com>,
        Robin Murphy <robin.murphy@arm.com>
Subject: Re: [PATCH] iommu/iova: Replace cmpxchg with xchg in queue_iova
Message-ID: <20200904093732.GN6714@8bytes.org>
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Adding Robin.

On Thu, Aug 27, 2020 at 04:43:54PM +0800, 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.
> 
> 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));
>  }
> -- 
> 2.7.4