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

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Date:   Wed, 13 Jul 2022 01:57:23 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Xie Yongji <xieyongji@...edance.com>
Cc:     jasowang@...hat.com, xiaodong.liu@...el.com,
        maxime.coquelin@...hat.com, stefanha@...hat.com,
        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

On Wed, Jul 06, 2022 at 01:04:59PM +0800, Xie Yongji wrote:
> 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>

vduse_domain_free_bounce_pages is not the
only one taking this lock. This commit log needs more
analysis documenting all points of access to bounce_maps
and why vduse_domain_get_bounce_page and file
release are the only two.

> ---
>  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;
>  
>  	page = map->bounce_page;
>  	get_page(page);
> -out:
> -	spin_unlock(&domain->iotlb_lock);
>  
>  	return page;
>  }
> -- 
> 2.20.1