[PATCH] dma-fence: fix dma_fence_get_rcu_safe

[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.

>
> Joonas, Tvrtko, I guess we need to fix this one way or the other.
> -Daniel