[PATCH 2/9] drm/syncobj: Add a race-free drm_syncobj_fence_get helper

[Jason Ekstrand <jason@jlekstrand.net>]

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.

Signed-off-by: Jason Ekstrand <jason@jlekstrand.net>
---
 drivers/gpu/drm/drm_syncobj.c | 12 +++++++++---
 include/drm/drm_syncobj.h     |  6 +++++-
 2 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
index 0412b0b..0190ec2 100644
--- a/drivers/gpu/drm/drm_syncobj.c
+++ b/drivers/gpu/drm/drm_syncobj.c
@@ -75,6 +75,12 @@ struct drm_syncobj *drm_syncobj_find(struct drm_file *file_private,
 }
 EXPORT_SYMBOL(drm_syncobj_find);
 
+struct dma_fence *drm_syncobj_fence_get(struct drm_syncobj *syncobj)
+{
+	return dma_fence_get_rcu_safe(&syncobj->_fence);
+}
+EXPORT_SYMBOL(drm_syncobj_fence_get);
+
 /**
  * drm_syncobj_replace_fence - replace fence in a sync object.
  * @syncobj: Sync object to replace fence in
@@ -89,7 +95,7 @@ void drm_syncobj_replace_fence(struct drm_syncobj *syncobj,
 
 	if (fence)
 		dma_fence_get(fence);
-	old_fence = xchg(&syncobj->fence, fence);
+	old_fence = xchg(&syncobj->_fence, fence);
 
 	dma_fence_put(old_fence);
 }
@@ -105,7 +111,7 @@ 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;
 	}
@@ -125,7 +131,7 @@ void drm_syncobj_free(struct kref *kref)
 	struct drm_syncobj *syncobj = container_of(kref,
 						   struct drm_syncobj,
 						   refcount);
-	dma_fence_put(syncobj->fence);
+	dma_fence_put(syncobj->_fence);
 	kfree(syncobj);
 }
 EXPORT_SYMBOL(drm_syncobj_free);
diff --git a/include/drm/drm_syncobj.h b/include/drm/drm_syncobj.h
index 7d4ad77..c06a441 100644
--- a/include/drm/drm_syncobj.h
+++ b/include/drm/drm_syncobj.h
@@ -43,8 +43,11 @@ struct drm_syncobj {
 	/**
 	 * @fence:
 	 * NULL or a pointer to the fence bound to this object.
+	 *
+	 * This pointer should not be accessed directly.  Instead, use
+	 * drm_syncobj_fence_get or drm_syncobj_replace_fence.
 	 */
-	struct dma_fence *fence;
+	struct dma_fence *_fence;
 	/**
 	 * @file:
 	 * a file backing for this syncobj.
@@ -79,6 +82,7 @@ drm_syncobj_put(struct drm_syncobj *obj)
 
 struct drm_syncobj *drm_syncobj_find(struct drm_file *file_private,
 				     u32 handle);
+struct dma_fence *drm_syncobj_fence_get(struct drm_syncobj *syncobj);
 void drm_syncobj_replace_fence(struct drm_syncobj *syncobj,
 			       struct dma_fence *fence);
 int drm_syncobj_find_fence(struct drm_file *file_private,
-- 
2.5.0.400.gff86faf

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