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Commit 309a5482 authored by Jason Ekstrand's avatar Jason Ekstrand Committed by Dave Airlie
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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: default avatarJason Ekstrand <jason@jlekstrand.net>
Acked-by: Christian König <christian.koenig@amd.com> (v1)
Signed-off-by: default avatarDave Airlie <airlied@redhat.com>
parent afaf5923

drivers/gpu/drm/drm_syncobj.c

+1 −1
Original line number Diff line number Diff line
@@ -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;

	}

include/drm/drm_syncobj.h

+12 −0
Original line number Diff line number Diff line
@@ -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,