linux-kernel - Re: [PATCH v3 1/5] vduse: Remove unnecessary spin lock protection

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Date:   Wed, 20 Jul 2022 14:02:40 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Xie Yongji <xieyongji@...edance.com>
Cc:     mst <mst@...hat.com>, Liu Xiaodong <xiaodong.liu@...el.com>,
        Maxime Coquelin <maxime.coquelin@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        songmuchun@...edance.com,
        virtualization <virtualization@...ts.linux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/5] vduse: Remove unnecessary spin lock protection

On Wed, Jul 20, 2022 at 12:42 PM Xie Yongji <xieyongji@...edance.com> wrote:
>
> Now we use domain->iotlb_lock to protect two different
> variables: domain->bounce_maps->bounce_page and
> domain->iotlb. But for domain->bounce_maps->bounce_page,
> we actually don't need any synchronization between
> vduse_domain_get_bounce_page() and vduse_domain_free_bounce_pages()
> since vduse_domain_get_bounce_page() will only be called in
> page fault handler and vduse_domain_free_bounce_pages() will
> be called during file release.
>
> So let's remove the unnecessary spin lock protection in
> vduse_domain_get_bounce_page(). Then the usage of
> domain->iotlb_lock could be more clear: the lock will be
> only used to protect the domain->iotlb.
>
> Signed-off-by: Xie Yongji <xieyongji@...edance.com>

Acked-by: Jason Wang <jasowang@...hat.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;
>
>         page = map->bounce_page;
>         get_page(page);
> -out:
> -       spin_unlock(&domain->iotlb_lock);
>
>         return page;
>  }
> --
> 2.20.1
>