Quoting Christian König (2017-09-04 14:27:33)
From: Christian König christian.koenig@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