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drm/syncobj: Add a race-free drm_syncobj_fence_get helper (v2)

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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>
This commit is contained in:
Jason Ekstrand 2017-08-25 10:52:20 -07:00 committed by Dave Airlie
parent afaf592378
commit 309a5482fa
2 changed files with 13 additions and 1 deletions
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2
drivers/gpu/drm/drm_syncobj.c
View File

@ -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;
}

12
include/drm/drm_syncobj.h
View File

@ -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,