[Intel-gfx] [PATCH 3/6] 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.

Thread A			Thread B
i915_active_acquire		i915_active_acquire
  atomic_read() == 0		  atomic_read() == 0
  mutex_lock()			  mutex_lock()
				  atomic_read() == 0
				    ref->active();
				  atomic_inc()
				  mutex_unlock()
  atomic_read() == 1
				i915_active_release
				  atomic_dec_and_test() -> 0
				    ref->retire()
  atomic_inc() -> 1
  mutex_unlock()

So thread A has acquired the ref->active_count but since the ref was
still active at the time, it did not initialise it. By switching the
check inside the mutex to an atomic increment only if already active, we
close the race.

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