linux-kernel - Re: [PATCH v2 1/5] vduse: Remove unnecessary spin lock protection

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Message-ID: <1106133f-ebd4-f6a8-83ab-19e995bcdba6@redhat.com>
Date:   Wed, 13 Jul 2022 13:43:53 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Xie Yongji <xieyongji@...edance.com>, mst@...hat.com,
        xiaodong.liu@...el.com, maxime.coquelin@...hat.com,
        stefanha@...hat.com
Cc:     songmuchun@...edance.com,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/5] vduse: Remove unnecessary spin lock protection


在 2022/7/6 13:04, Xie Yongji 写道:
> Taking iotlb lock to access bounce page in page fault
> handler is meaningless since vduse_domain_free_bounce_pages()
> would only be called during file release.
>
> Signed-off-by: Xie Yongji <xieyongji@...edance.com>
> ---
>   drivers/vdpa/vdpa_user/iova_domain.c | 7 ++-----
>   1 file changed, 2 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/vdpa/vdpa_user/iova_domain.c b/drivers/vdpa/vdpa_user/iova_domain.c
> index 6daa3978d290..bca1f0b8850c 100644
> --- a/drivers/vdpa/vdpa_user/iova_domain.c
> +++ b/drivers/vdpa/vdpa_user/iova_domain.c
> @@ -211,17 +211,14 @@ static struct page *
>   vduse_domain_get_bounce_page(struct vduse_iova_domain *domain, u64 iova)
>   {
>   	struct vduse_bounce_map *map;
> -	struct page *page = NULL;
> +	struct page *page;
>   
> -	spin_lock(&domain->iotlb_lock);
>   	map = &domain->bounce_maps[iova >> PAGE_SHIFT];
>   	if (!map->bounce_page)
> -		goto out;
> +		return NULL;


Interesting, I wonder why we don't serialize with 
vduse_domain_map_bounce_page() with iotlb_lock?

Thanks


>   
>   	page = map->bounce_page;
>   	get_page(page);
> -out:
> -	spin_unlock(&domain->iotlb_lock);
>   
>   	return page;
>   }