[Intel-gfx] [PATCH] drm/i915/gt: Avoid resetting ring->head outside of its timeline mutex

[Chris Wilson]

Quoting Chris Wilson (2020-02-10 16:42:15)
> We manipulate ring->head while active in i915_request_retire underneath
> the timeline manipulation. We cannot rely on a stable ring->head outside
> of the timeline->mutex, in particular while setting up the context for
> resume and reset.
> 
> Closes: https://gitlab.freedesktop.org/drm/intel/issues/1126
Fixes: 0881954965e3 ("drm/i915: Introduce intel_context.pin_mutex for pin management")
Fixes: e5dadff4b093 ("drm/i915: Protect request retirement with timeline->mutex")
> References: f3c0efc9fe7a ("drm/i915/execlists: Leave resetting ring to intel_ring")
> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
> Cc: Matthew Auld <matthew.auld at intel.com>
> Cc: Tvrtko Ursulin <tvrtko.ursulin at intel.com>
> Cc: Mika Kuoppala <mika.kuoppala at linux.intel.com>
-Chris