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

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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().

Change-Id: Iee3ad8eb400612ec24898832eb19ff34eb2aecb4
Signed-off-by: Martijn Coenen <maco@google.com>
This commit is contained in:
Martijn Coenen 2016-01-14 14:33:52 +01:00 committed by Srinivasarao P
parent fb0ef85108
commit ba88acf8c2
1 changed files with 6 additions and 5 deletions
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11
mm/memcontrol.c
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@ -5790,16 +5790,17 @@ static void mem_cgroup_usage_unregister_event(struct cgroup *cgrp,
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);
}