drm/i915/gt: Avoid resetting ring->head outside of its timeline mutex

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@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Andi Shyti <andi.shyti@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200211120131.958949-1-chris@chris-wilson.co.uk
(cherry picked from commit 42827350f75c56d0fe9f15d8425a1390528958b6)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>

1 files changed, 3 insertions, 3 deletions

diff --git a/drivers/gpu/drm/i915/gt/intel_ring_types.h b/drivers/gpu/drm/i915/gt/intel_ring_types.h
index 3cd7fec7fd8d..1a189ea00fd8 100644
--- a/drivers/gpu/drm/i915/gt/intel_ring_types.h
+++ b/drivers/gpu/drm/i915/gt/intel_ring_types.h
@@ -39,9 +39,9 @@ struct intel_ring {
 	 */
 	atomic_t pin_count;
 
-	u32 head;
-	u32 tail;
-	u32 emit;
+	u32 head; /* updated during retire, loosely tracks RING_HEAD */
+	u32 tail; /* updated on submission, used for RING_TAIL */
+	u32 emit; /* updated during request construction */
 
 	u32 space;
 	u32 size;