Re: [PATCH v2 1/4] dma-buf: Add dma_buf_try_get()

[Jason Gunthorpe <jgg@nvidia.com>]

On Thu, Sep 01, 2022 at 09:55:08AM +0200, Christian König wrote:
> Am 01.09.22 um 01:12 schrieb Jason Gunthorpe:
> > Used to increment the refcount of the dma buf's struct file, only if the
> > refcount is not zero. Useful to allow the struct file's lifetime to
> > control the lifetime of the dmabuf while still letting the driver to keep
> > track of created dmabufs.
> > 
> > Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> > ---
> >   include/linux/dma-buf.h | 13 +++++++++++++
> >   1 file changed, 13 insertions(+)
> > 
> > diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> > index 71731796c8c3a8..a35f1554f2fb36 100644
> > --- a/include/linux/dma-buf.h
> > +++ b/include/linux/dma-buf.h
> > @@ -618,6 +618,19 @@ int dma_buf_fd(struct dma_buf *dmabuf, int flags);
> >   struct dma_buf *dma_buf_get(int fd);
> >   void dma_buf_put(struct dma_buf *dmabuf);
> > +/**
> > + * dma_buf_try_get - try to get a reference on a dmabuf
> > + * @dmabuf - the dmabuf to get
> > + *
> > + * Returns true if a reference was successfully obtained. The caller must
> > + * interlock with the dmabuf's release function in some way, such as RCU, to
> > + * ensure that this is not called on freed memory.
> 
> I still have a bad feeling about this, but I also see that we can only
> choose between evils here.
> 
> Could you just call get_file_rcu() from the exporter with a comment
> explaining why this works instead?

I guess, are you sure? It seems very hacky.

Jason