[Intel-gfx] [PATCH 62/70] drm/i915: Reduce locking inside busy ioctl

[Chris Wilson]

We can do the trivial check for an inactive buffer without acquiring the
struct_mutex reducing contention. If we are required to flush the object
and check for retirements, then we do indeed have to resort to taking
the struct_mutex

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
---
 drivers/gpu/drm/i915/i915_gem.c | 49 ++++++++++++++++++++++-------------------
 1 file changed, 26 insertions(+), 23 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index 43baac2c1e20..f9ea8c932a6a 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -4522,34 +4522,37 @@ i915_gem_busy_ioctl(struct drm_device *dev, void *data,
 	struct drm_i915_gem_object *obj;
 	int ret;
 
-	ret = i915_mutex_lock_interruptible(dev);
-	if (ret)
-		return ret;
-
 	obj = to_intel_bo(drm_gem_object_lookup(dev, file, args->handle));
-	if (&obj->base == NULL) {
-		ret = -ENOENT;
-		goto unlock;
-	}
+	if (&obj->base == NULL)
+		return -ENOENT;
 
-	/* Count all active objects as busy, even if they are currently not used
-	 * by the gpu. Users of this interface expect objects to eventually
-	 * become non-busy without any further actions, therefore emit any
-	 * necessary flushes here.
-	 */
-	ret = i915_gem_object_flush_active(obj);
-	if (ret)
-		goto unref;
+	if (obj->active) {
+		ret = i915_mutex_lock_interruptible(dev);
+		if (ret)
+			goto unref;
 
-	BUILD_BUG_ON(I915_NUM_RINGS > 16);
-	args->busy = obj->active << 16;
-	if (obj->last_write_req)
-		args->busy |= obj->last_write_req->ring->id;
+		/* Count all active objects as busy, even if they are
+		 * currently not used by the gpu. Users of this interface
+		 * expect objects to eventually become non-busy without any
+		 * further actions, therefore emit any necessary flushes here.
+		 */
+		ret = i915_gem_object_flush_active(obj);
+		if (ret == 0) {
+			BUILD_BUG_ON(I915_NUM_RINGS > 16);
+			args->busy = obj->active << 16;
+			if (obj->last_write_req)
+				args->busy |= obj->last_write_req->ring->id;
+		}
 
+		drm_gem_object_unreference(&obj->base);
+		mutex_unlock(&dev->struct_mutex);
+	} else {
+		ret = 0;
+		args->busy = 0;
 unref:
-	drm_gem_object_unreference(&obj->base);
-unlock:
-	mutex_unlock(&dev->struct_mutex);
+		drm_gem_object_unreference_unlocked(&obj->base);
+	}
+
 	return ret;
 }
 
-- 
2.1.4