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From:   Suren Baghdasaryan <surenb@google.com>
Date:   Fri, 22 Jan 2021 14:20:56 -0800
Message-ID: <CAJuCfpGRkM+h16SJp7gJOaAhy+gFw1-++WP8kavOjRj3MHcpBQ@mail.gmail.com>
Subject: Re: [RESEND][PATCH 2/3] dma-buf: heaps: Add a WARN_ON should the
 vmap_cnt go negative
To:     Sumit Semwal <sumit.semwal@linaro.org>
Cc:     John Stultz <john.stultz@linaro.org>,
        lkml <linux-kernel@vger.kernel.org>,
        Liam Mark <lmark@codeaurora.org>,
        Laura Abbott <labbott@kernel.org>,
        Brian Starkey <Brian.Starkey@arm.com>,
        Hridya Valsaraju <hridya@google.com>,
        Sandeep Patil <sspatil@google.com>,
        Daniel Mentz <danielmentz@google.com>,
        Chris Goldsworthy <cgoldswo@codeaurora.org>,
        =?UTF-8?Q?=C3=98rjan_Eide?= <orjan.eide@arm.com>,
        Robin Murphy <robin.murphy@arm.com>,
        Ezequiel Garcia <ezequiel@collabora.com>,
        Simon Ser <contact@emersion.fr>,
        James Jones <jajones@nvidia.com>,
        "open list:DMA BUFFER SHARING FRAMEWORK" 
        <linux-media@vger.kernel.org>,
        DRI mailing list <dri-devel@lists.freedesktop.org>
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On Thu, Jan 21, 2021 at 11:56 PM Sumit Semwal <sumit.semwal@linaro.org> wro=
te:
>
> Hi John, Suren,
>
>
> On Wed, 20 Jan 2021 at 02:15, John Stultz <john.stultz@linaro.org> wrote:
> >
> > We shouldn't vunmap more then we vmap, but if we do, make
> > sure we complain loudly.
>
> I was checking the general usage of vunmap in the kernel, and I
> couldn't find many instances where we need to WARN_ON for the vunmap
> count more than vmap count. Is there a specific need for this in the heap=
s?

Hi Sumit,
My worry was that buffer->vmap_cnt could silently go negative. But if
this warning is not consistent with other places we do refcounted
vmap/vunmap then feel free to ignore my suggestion.
Thanks!

>
> Best,
> Sumit.
> >
> > Cc: Sumit Semwal <sumit.semwal@linaro.org>
> > Cc: Liam Mark <lmark@codeaurora.org>
> > Cc: Laura Abbott <labbott@kernel.org>
> > Cc: Brian Starkey <Brian.Starkey@arm.com>
> > Cc: Hridya Valsaraju <hridya@google.com>
> > Cc: Suren Baghdasaryan <surenb@google.com>
> > Cc: Sandeep Patil <sspatil@google.com>
> > Cc: Daniel Mentz <danielmentz@google.com>
> > Cc: Chris Goldsworthy <cgoldswo@codeaurora.org>
> > Cc: =C3=98rjan Eide <orjan.eide@arm.com>
> > Cc: Robin Murphy <robin.murphy@arm.com>
> > Cc: Ezequiel Garcia <ezequiel@collabora.com>
> > Cc: Simon Ser <contact@emersion.fr>
> > Cc: James Jones <jajones@nvidia.com>
> > Cc: linux-media@vger.kernel.org
> > Cc: dri-devel@lists.freedesktop.org
> > Suggested-by: Suren Baghdasaryan <surenb@google.com>
> > Signed-off-by: John Stultz <john.stultz@linaro.org>
> > ---
> >  drivers/dma-buf/heaps/cma_heap.c    | 1 +
> >  drivers/dma-buf/heaps/system_heap.c | 1 +
> >  2 files changed, 2 insertions(+)
> >
> > diff --git a/drivers/dma-buf/heaps/cma_heap.c b/drivers/dma-buf/heaps/c=
ma_heap.c
> > index 364fc2f3e499..0c76cbc3fb11 100644
> > --- a/drivers/dma-buf/heaps/cma_heap.c
> > +++ b/drivers/dma-buf/heaps/cma_heap.c
> > @@ -232,6 +232,7 @@ static void cma_heap_vunmap(struct dma_buf *dmabuf,=
 struct dma_buf_map *map)
> >         struct cma_heap_buffer *buffer =3D dmabuf->priv;
> >
> >         mutex_lock(&buffer->lock);
> > +       WARN_ON(buffer->vmap_cnt =3D=3D 0);
> >         if (!--buffer->vmap_cnt) {
> >                 vunmap(buffer->vaddr);
> >                 buffer->vaddr =3D NULL;
> > diff --git a/drivers/dma-buf/heaps/system_heap.c b/drivers/dma-buf/heap=
s/system_heap.c
> > index 405351aad2a8..2321c91891f6 100644
> > --- a/drivers/dma-buf/heaps/system_heap.c
> > +++ b/drivers/dma-buf/heaps/system_heap.c
> > @@ -273,6 +273,7 @@ static void system_heap_vunmap(struct dma_buf *dmab=
uf, struct dma_buf_map *map)
> >         struct system_heap_buffer *buffer =3D dmabuf->priv;
> >
> >         mutex_lock(&buffer->lock);
> > +       WARN_ON(buffer->vmap_cnt =3D=3D 0);
> >         if (!--buffer->vmap_cnt) {
> >                 vunmap(buffer->vaddr);
> >                 buffer->vaddr =3D NULL;
> > --
> > 2.17.1
> >