Independence for dma_fences!

Hi everyone,

dma_fences have ever lived under the tyranny dictated by the module
lifetime of their issuer, leading to crashes should anybody still holding
a reference to a dma_fence when the module of the issuer was unloaded.

But those days are over! The patch set following this mail finally
implements a way for issuers to release their dma_fence out of this
slavery and outlive the module who originally created them.

Previously various approaches have been discussed, including changing the
locking semantics of the dma_fence callbacks (by me) as well as using the
drm scheduler as intermediate layer (by Sima) to disconnect dma_fences
from their actual users.

Changing the locking semantics turned out to be much more trickier than
originally thought because especially on older drivers (nouveau, radeon,
but also i915) this locking semantics is actually needed for correct
operation.

Using the drm_scheduler as intermediate layer is still a good idea and
should probably be implemented to make live simpler for some drivers, but
doesn't work for all use cases. Especially TLB flush fences, preemption
fences and userqueue fences don't go through the drm scheduler because it
doesn't make sense for them.

Tvrtko did some really nice prerequisite work by protecting the returned
strings of the dma_fence_ops by RCU. This way dma_fence creators where
able to just wait for an RCU grace period after fence signaling before
they could be save to free those data structures.

Now this patch set here goes a step further and protects the whole
dma_fence_ops structure by RCU, so that after the fence signals the
pointer to the dma_fence_ops is set to NULL when there is no wait nor
release callback given. All functionality which use the dma_fence_ops
reference are put inside an RCU critical section, except for the
deprecated issuer specific wait and of course the optional release
callback.

Additional to the RCU changes the lock protecting the dma_fence state
previously had to be allocated external. This set here now changes the
functionality to make that external lock optional and allows dma_fences
to use an inline lock and be self contained.

The new approach is then applied to amdgpu allowing the module to be
unloaded even when dma_fences issued by it are still around.

Please review and comment,
Christian.