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>

2 files changed, 13 insertions, 1 deletions

diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
index 0412b0b0a342..eea38d82645c 100644
--- a/drivers/gpu/drm/drm_syncobj.c
+++ b/drivers/gpu/drm/drm_syncobj.c
@@ -105,7 +105,7 @@ int drm_syncobj_find_fence(struct drm_file *file_private,
         if (!syncobj)
                 return -ENOENT;
 
-        *fence = dma_fence_get(syncobj->fence);
+        *fence = drm_syncobj_fence_get(syncobj);
         if (!*fence) {
                 ret = -EINVAL;
         }
diff --git a/include/drm/drm_syncobj.h b/include/drm/drm_syncobj.h
index 7d4ad777132e..ce94d14c5087 100644
--- a/include/drm/drm_syncobj.h
+++ b/include/drm/drm_syncobj.h
@@ -77,6 +77,18 @@ drm_syncobj_put(struct drm_syncobj *obj)
         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,