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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: Jason Ekstrand <jason@jlekstrand.net> Acked-by: Christian König <christian.koenig@amd.com> (v1) Signed-off-by: Dave Airlie <airlied@redhat.com>
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parent
afaf592378
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2 changed files with 13 additions and 1 deletions
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@ -105,7 +105,7 @@ int drm_syncobj_find_fence(struct drm_file *file_private,
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if (!syncobj)
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if (!syncobj)
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return -ENOENT;
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return -ENOENT;
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*fence = dma_fence_get(syncobj->fence);
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*fence = drm_syncobj_fence_get(syncobj);
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if (!*fence) {
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if (!*fence) {
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ret = -EINVAL;
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ret = -EINVAL;
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}
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}
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@ -77,6 +77,18 @@ drm_syncobj_put(struct drm_syncobj *obj)
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kref_put(&obj->refcount, drm_syncobj_free);
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kref_put(&obj->refcount, drm_syncobj_free);
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}
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}
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static inline struct dma_fence *
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drm_syncobj_fence_get(struct drm_syncobj *syncobj)
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{
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struct dma_fence *fence;
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rcu_read_lock();
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fence = dma_fence_get_rcu_safe(&syncobj->fence);
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rcu_read_unlock();
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return fence;
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}
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struct drm_syncobj *drm_syncobj_find(struct drm_file *file_private,
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struct drm_syncobj *drm_syncobj_find(struct drm_file *file_private,
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u32 handle);
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u32 handle);
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void drm_syncobj_replace_fence(struct drm_syncobj *syncobj,
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void drm_syncobj_replace_fence(struct drm_syncobj *syncobj,
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