[RFC PATCH v2 0/3] Solve dma_fence's problems by locking

[Philipp Stanner <phasta@kernel.org>]

In our Rust code bases, I'm currently experiencing what we also
experience in C at many places: that the dma_fence contract's RCU
requirements make the API more fragile and put an unnecessary burden on
the API user. Most notably, in C the API behaving correctly now depends
on the API *user* waiting before free()s, waiting before unloads – all
because dma_fence is not properly synchronized.

The patches commit messages give more details about where I want to go
with this particular RFC.

My rough "vision" for dma_fence, where I think it should develop to, can
be described like this:

1. All users should use inline-lock.
2. The fence-signaled state should be our decoupling point.
3. The fence state must be consistently locked.
4. This means all callbacks are only invoked for unsignaled fences, and
   the fence cannot signal while a callback runs.
5. All fence API functions should be "atomically correct": they can
   always be called without any manual RCU guards in their proximity.
6. The fence lock should, in the long-term, become inaccessible for
   users.

Point №6 means that this:

dma_fence_lock_irqsave(…);
dma_fence_set_error(f, err);
dma_fence_signal_locked(f);
dma_fence_unlock_irqrestore(…);
synchronize_rcu();
kfree(driver_foo);

should simply become this:

dma_fence_signal(f, err);
kfree(driver_foo);

So we would end up with a simpler, more robust and more maintainable
API.

I gave this basic testing on a Nouveau desktop and it runs fine.
dma-buf-tests also run fine. I can do more tests on Xe and amdgpu, maybe
on Mali, too.

Since the series is mostly about locking the callbacks, all of which
except for set_deadline() are *already* locked, I am confident that this
can be done.

Feedback welcome

P.

Philipp Stanner (3):
  dma-fence: Remove RCU protection for fence->ops
  dma-fence: use lock to protect against module-unload
  dma-fence: Support unload for ops->get_XXX_name()

 drivers/dma-buf/dma-fence.c                 | 144 +++++++-------------
 drivers/dma-buf/sync_file.c                 |  29 ++--
 drivers/gpu/drm/i915/gt/intel_gt_requests.c |  16 +--
 drivers/gpu/drm/i915/i915_request.c         |   9 +-
 drivers/gpu/drm/i915/i915_sw_fence.c        |  15 +-
 include/linux/dma-fence.h                   |  63 ++++-----
 include/trace/events/dma_fence.h            |  12 +-
 7 files changed, 113 insertions(+), 175 deletions(-)

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2.56.0