Re: [PATCH v10 01/16] videobuf2: Make struct vb2_buffer refcounted

[Ezequiel Garcia <ezequiel@collabora.com>]

On Fri, 2018-05-25 at 12:11 +0530, sathyam panda wrote:
> Hello,
> 
> On 5/21/18, Ezequiel Garcia <ezequiel@collabora.com> wrote:
> > The in-fence implementation involves having a per-buffer fence callback,
> > that triggers on the fence signal. The fence callback is called
> > asynchronously
> > and needs a valid reference to the associated ideobuf2 buffer.
> > 
> > Allow this by making the vb2_buffer refcounted, so it can be passed
> > to other contexts.
> > 
> 
> -Is it really required, because when a queued buffer with an in_fence
> is deallocated, firstly queue is cancelled.
> -And __vb2_dqbuf is called which calls dma_fence_remove_callback.
> -So if fence callback has been called -__vb2_dqbuf will wait to
> acquire fence lock.
> -So during execution of fence callback, buffers and queue are still valid.
> -And if __vb2_dqbuf remove callback first ,then dma_fence_signal will
> wait for lock
> - so there won't be any fence callback to call for that buffer when
> dma_fence_signal resumes.
> 

Hi Sathyam,

Thanks for your review! The refcount is definitely required,
as the fence callback only schedules a workqueue, which is
completely asynchronous with respect to the rest of the
ioctls.

In particular, the workqueue is not synchronized with
vb2_core_queue_release.

Also, another subtle detail, dma_fence_remove_callback
can fail to remove the callback.

Thanks,
Eze