[PATCH] dma-fence: fix dma_fence_get_rcu_safe

[Maarten Lankhorst]

Op 21-09-17 om 09:00 schreef Christian König:
> Am 20.09.2017 um 20:20 schrieb Daniel Vetter:
>> 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.
>
> For the meantime I've send a v2 of this patch to fix at least the buggy return of NULL when we fail to grab the RCU reference but keeping the extra checking for now.
>
> Can I get an rb on this please so that we fix at least the bug at hand?
>
> Thanks,
> Christian. 
Done.