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

[Robin Murphy]

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@xxxxxxx/

Robin.

> On Thu, Aug 27, 2020 at 04:43:54PM +0800, Shaokun Zhang wrote:
> > From: Yuqi Jin <jinyuqi@xxxxxxxxxx>
> >
> > 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@xxxxxxxxxx>
> > Signed-off-by: Yuqi Jin <jinyuqi@xxxxxxxxxx>
> > Signed-off-by: Shaokun Zhang <zhangshaokun@xxxxxxxxxxxxx>
> > ---
> >   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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