[Intel-gfx] [PATCH 2/2] drm/i915: Tighten atomicity of i915_active_acquire vs i915_active_release

[Chris Wilson]

As we use a mutex to serialise the first acquire (as it may be a lengthy
operation), but only an atomic decrement for the release, we have to
be careful in case a second thread races and completes both
acquire/release as the first finishes its acquire.

Fixes: c9ad602feabe ("drm/i915: Split i915_active.mutex into an irq-safe spinlock for the rbtree")
Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_active.c | 16 +++++++++-------
 1 file changed, 9 insertions(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_active.c b/drivers/gpu/drm/i915/i915_active.c
index ace55d5d4ca7..9d6830885d2e 100644
--- a/drivers/gpu/drm/i915/i915_active.c
+++ b/drivers/gpu/drm/i915/i915_active.c
@@ -416,13 +416,15 @@ int i915_active_acquire(struct i915_active *ref)
 	if (err)
 		return err;
 
-	if (!atomic_read(&ref->count) && ref->active)
-		err = ref->active(ref);
-	if (!err) {
-		spin_lock_irq(&ref->tree_lock); /* vs __active_retire() */
-		debug_active_activate(ref);
-		atomic_inc(&ref->count);
-		spin_unlock_irq(&ref->tree_lock);
+	if (likely(!i915_active_acquire_if_busy(ref))) {
+		if (ref->active)
+			err = ref->active(ref);
+		if (!err) {
+			spin_lock_irq(&ref->tree_lock); /* __active_retire() */
+			debug_active_activate(ref);
+			atomic_inc(&ref->count);
+			spin_unlock_irq(&ref->tree_lock);
+		}
 	}
 
 	mutex_unlock(&ref->mutex);
-- 
2.25.0