sync: signal pt before sync_timeline object gets destroyed

There is a race condition

Assume we have *one* sync_fence object, with *one* sync_pt
which belongs to *one* sync_timeline, given this condition,
sync_timeline->kref will have two counts, one for sync_timeline
(implicit) and another for sync_pt.

Assume following is the situation on CPU

Theead-1 : (Thread which calls sync_timeline_destroy())
  -> (some function calls)
   -> sync_timeline_destory()
    -> sync_timeline_signal() (CPU is inside this
function after putting reference to sync_timeline)

At this time Thread-2 comes and does following

Thread-2 : (fclose on fence fd)
> sync_fence_release() -> because of fclose() on fence object
 -> sync_fence_free()
  -> sync_pt_free()
   -> kref_put(&pt->parent->kref, sync_timeline_free);
    -> sync_timeline_free() (CPU is inside this because
this time kref will be zero after _put)

Thread-2 will free sync_timeline object before Thread-1
has finished its work inside sync_timeline_signal.

With this change we signals all sync_pt before putting
reference to sync_timeline object.

Change-Id: Ic680e4d0bbef1c46bcb7cfba693395645241d203
Signed-off-by: Prakash Kamliya <pkamliya@codeaurora.org>

diff --git a/drivers/staging/android/sync.c b/drivers/staging/android/sync.c
index 3893a35747698603cce0fe62ee7dcc1d1c333352..c5a43934e855a84b13d83c29bb6ecb96da99dedd 100644 (file)
--- a/drivers/staging/android/sync.c
+++ b/drivers/staging/android/sync.c
@@ -92,14 +92,14 @@ static void sync_timeline_free(struct kref *kref)
 void sync_timeline_destroy(struct sync_timeline *obj)
 {
        obj->destroyed = true;
+       smp_wmb();
 
        /*
-        * If this is not the last reference, signal any children
-        * that their parent is going away.
+        * signal any children that their parent is going away.
         */
+       sync_timeline_signal(obj);
 
-       if (!kref_put(&obj->kref, sync_timeline_free))
-               sync_timeline_signal(obj);
+       kref_put(&obj->kref, sync_timeline_free);
 }
 EXPORT_SYMBOL(sync_timeline_destroy);