linux-kernel - Re: [PATCH] iommu/iova: Replace cmpxchg with xchg in queue_iova

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Date:   Fri, 4 Sep 2020 10:58:14 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Joerg Roedel <joro@...tes.org>,
        Shaokun Zhang <zhangshaokun@...ilicon.com>
Cc:     Yuqi Jin <jinyuqi@...wei.com>, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] iommu/iova: Replace cmpxchg with xchg in queue_iova

Hi Joerg,

On 2020-09-04 10:37, Joerg Roedel wrote:
> Adding Robin.

Did you miss that I've reviewed this already? :)

https://lore.kernel.org/linux-iommu/3afcc7b2-0bfb-b79c-513f-1beb66c5f164@arm.com/

Robin.

> On Thu, Aug 27, 2020 at 04:43:54PM +0800, Shaokun Zhang wrote:
>> From: Yuqi Jin <jinyuqi@...wei.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@...tes.org>
>> Signed-off-by: Yuqi Jin <jinyuqi@...wei.com>
>> Signed-off-by: Shaokun Zhang <zhangshaokun@...ilicon.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
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