[Intel-gfx] [CI] drm/i915/gem: Reduce context termination list iteration guard to RCU

[Chris Wilson]

Quoting Chris Wilson (2020-08-06 11:59:54)
> As we now protect the timeline list using RCU, we can drop the
> timeline->mutex for guarding the list iteration during context close, as
> we are searching for an inflight request. Any new request will see the
> context is banned and not be submitted. In doing so, pull the checks for
> a concurrent submission of the request (notably the
> i915_request_completed()) under the engine spinlock, to fully serialise
> with __i915_request_submit()). That is in the case of preempt-to-busy
> where the request may be completed during the __i915_request_submit(),
> we need to be careful that we sample the request status after
> serialising so that we don't miss the request the engine is actually
> submitting.
> 
> Fixes: 4a3174152147 ("drm/i915/gem: Refine occupancy test in kill_context()")
> References: d22d2d073ef8 ("drm/i915: Protect i915_request_await_start from early waits") # rcu protection of timeline->requests
> References: https://gitlab.freedesktop.org/drm/intel/-/issues/1622
Also
References: https://gitlab.freedesktop.org/drm/intel/-/issues/2158
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
-Chris