From 309a5482fa9eb7bc754bf95a2cd89091b01c33d2 Mon Sep 17 00:00:00 2001 From: Jason Ekstrand Date: Fri, 25 Aug 2017 10:52:20 -0700 Subject: [PATCH] drm/syncobj: Add a race-free drm_syncobj_fence_get helper (v2) MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit 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 Acked-by: Christian König (v1) Signed-off-by: Dave Airlie --- drivers/gpu/drm/drm_syncobj.c | 2 +- include/drm/drm_syncobj.h | 12 ++++++++++++ 2 files changed, 13 insertions(+), 1 deletion(-) 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, -- 2.20.1