[Intel-gfx] [PATCH 12/16] drm/i915: Reduce locking inside swfinish ioctl

[Chris Wilson]

We only need to take the struct_mutex if the object is pinned to the
display engine and so requires checking for clflush. (The race with
userspace pinning the object to a framebuffer is irrelevant.)

v2: Use access once for compiler hints (or not as it is a bitfield)
v3: READ_ONCE, obj->pin_display is not a bitfield anymore

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
---
 drivers/gpu/drm/i915/i915_gem.c | 29 ++++++++++++++++-------------
 1 file changed, 16 insertions(+), 13 deletions(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index f2f70f5ff9f4..51ec5cd1c6ca 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -1510,25 +1510,28 @@ i915_gem_sw_finish_ioctl(struct drm_device *dev, void *data,
 {
 	struct drm_i915_gem_sw_finish *args = data;
 	struct drm_i915_gem_object *obj;
-	int ret = 0;
-
-	ret = i915_mutex_lock_interruptible(dev);
-	if (ret)
-		return ret;
+	int ret;
 
 	obj = i915_gem_object_lookup(file, args->handle);
-	if (!obj) {
-		ret = -ENOENT;
-		goto unlock;
-	}
+	if (!obj)
+		return -ENOENT;
 
 	/* Pinned buffers may be scanout, so flush the cache */
-	if (obj->pin_display)
+	if (READ_ONCE(obj->pin_display)) {
+		ret = i915_mutex_lock_interruptible(dev);
+		if (ret)
+			goto unref;
+
 		i915_gem_object_flush_cpu_write_domain(obj);
 
-	i915_gem_object_put(obj);
-unlock:
-	mutex_unlock(&dev->struct_mutex);
+		i915_gem_object_put(obj);
+		mutex_unlock(&dev->struct_mutex);
+	} else {
+		ret = 0;
+unref:
+		i915_gem_object_put_unlocked(obj);
+	}
+
 	return ret;
 }
 
-- 
2.8.1