[PATCH] dma-fence: fix dma_fence_get_rcu_safe

[Chris Wilson]

Quoting Christian König (2017-09-04 14:27:33)
> From: Christian König <christian.koenig at amd.com>
> 
> The logic is buggy and unnecessary complex. When dma_fence_get_rcu() fails to
> acquire a reference it doesn't necessary mean that there is no fence at all.
> 
> It usually mean that the fence was replaced by a new one and in this situation
> we certainly want to have the new one as result and *NOT* NULL.

Which is not guaranteed by the code you wrote either.

The point of the comment is that the mb is only inside the successful
kref_atomic_inc_unless_zero, and that only after that mb do you know
whether or not you have the current fence.

You can argue that you want to replace the
	if (!dma_fence_get_rcu())
		return NULL
with
	if (!dma_fence_get_rcu()
		continue;
but it would be incorrect to say that by simply ignoring the
post-condition check that you do have the right fence.
-Chris