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

Chris Wilson
2016-08-01T18:23:06Z

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@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@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