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

["Christian König" <christian.koenig@amd.com>]

Am 06.09.22 um 18:44 schrieb Jason Gunthorpe:
> 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.

Yes, it's still better than exposing a dma_buf_try_get() interface to 
everyone.

Keep in mind that those functions here are mostly supposed to be used by 
the importer and not the exporter.

Christian.

>
> Jason