drm/syncobj: Add a race-free drm_syncobj_fence_get helper (v2)
The atomic exchange operation in drm_syncobj_replace_fence is sufficient
for the case where it races with itself. However, if you have a race
between a replace_fence and dma_fence_get(syncobj->fence), you may end
up with the entire replace_fence happening between the point in time
where the one thread gets the syncobj->fence pointer and when it calls
dma_fence_get() on it. If this happens, then the reference may be
dropped before we get a chance to get a new one. The new helper uses
dma_fence_get_rcu_safe to get rid of the race.
This is also needed because it allows us to do a bit more than just get
a reference in drm_syncobj_fence_get should we wish to do so.
v2:
- RCU isn't that scary
- Call rcu_read_lock/unlock
- Don't rename fence to _fence
- Make the helper static inline
Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
Acked-by: Christian König <christian.koenig@amd.com> (v1)
Signed-off-by: Dave Airlie <airlied@redhat.com>
diff --git a/include/drm/drm_syncobj.h b/include/drm/drm_syncobj.h
index 7d4ad77..ce94d14 100644
--- a/include/drm/drm_syncobj.h
+++ b/include/drm/drm_syncobj.h
@@ -77,6 +77,18 @@
kref_put(&obj->refcount, drm_syncobj_free);
}
+static inline struct dma_fence *
+drm_syncobj_fence_get(struct drm_syncobj *syncobj)
+{
+ struct dma_fence *fence;
+
+ rcu_read_lock();
+ fence = dma_fence_get_rcu_safe(&syncobj->fence);
+ rcu_read_unlock();
+
+ return fence;
+}
+
struct drm_syncobj *drm_syncobj_find(struct drm_file *file_private,
u32 handle);
void drm_syncobj_replace_fence(struct drm_syncobj *syncobj,