Re: [PATCH] dma-fence: fix dma_fence_get_rcu_safe

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On Mon, Sep 11, 2017 at 01:06:32PM +0200, Christian König wrote:
> Am 11.09.2017 um 12:01 schrieb Chris Wilson:
> > [SNIP]
> > > Yeah, but that is illegal with a fence objects.
> > >
> > > When anybody allocates fences this way it breaks at least
> > > reservation_object_get_fences_rcu(),
> > > reservation_object_wait_timeout_rcu() and
> > > reservation_object_test_signaled_single().
> > Many, many months ago I sent patches to fix them all.
>
> Found those after a bit a searching. Yeah, those patches where proposed more
> than a year ago, but never pushed upstream.
>
> Not sure if we really should go this way. dma_fence objects are shared
> between drivers and since we can't judge if it's the correct fence based on
> a criteria in the object (only the read counter which is outside) all
> drivers need to be correct for this.
>
> I would rather go the way and change dma_fence_release() to wrap
> fence->ops->release into call_rcu() to keep the whole RCU handling outside
> of the individual drivers.

Hm, I entirely dropped the ball on this, I kinda assumed that we managed
to get some agreement on this between i915 and dma_fence. Adding a pile
more people.

Joonas, Tvrtko, I guess we need to fix this one way or the other.
-Daniel
-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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