UPSTREAM: memcg: Only free spare array when readers are done

A spare array holding mem cgroup threshold events is kept around
to make sure we can always safely deregister an event and have an
array to store the new set of events in.

In the scenario where we're going from 1 to 0 registered events, the
pointer to the primary array containing 1 event is copied to the spare
slot, and then the spare slot is freed because no events are left.
However, it is freed before calling synchronize_rcu(), which means
readers may still be accessing threshold->primary after it is freed.

Fixed by only freeing after synchronize_rcu().

Signed-off-by: Martijn Coenen <maco@google.com>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index e0e486323bf1455da04a4af221fe19dadb855f03..5c714f33f49477f66f70377e60a8e5930dbcb753 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3522,16 +3522,17 @@ static void __mem_cgroup_usage_unregister_event(struct mem_cgroup *memcg,
 swap_buffers:
        /* Swap primary and spare array */
        thresholds->spare = thresholds->primary;
-       /* If all events are unregistered, free the spare array */
-       if (!new) {
-               kfree(thresholds->spare);
-               thresholds->spare = NULL;
-       }
 
        rcu_assign_pointer(thresholds->primary, new);
 
        /* To be sure that nobody uses thresholds */
        synchronize_rcu();
+
+       /* If all events are unregistered, free the spare array */
+       if (!new) {
+               kfree(thresholds->spare);
+               thresholds->spare = NULL;
+       }
 unlock:
        mutex_unlock(&memcg->thresholds_lock);
 }