percpu_counter: fix CPU unplug race in percpu_counter_destroy()

We should first delete the counter from percpu_counters list
before freeing memory, or a percpu_counter_hotcpu_callback()
could dereference a NULL pointer.
Signed-off-by: NEric Dumazet <dada1@cosmosbay.com>
Acked-by: NDavid S. Miller <davem@davemloft.net>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

lib/percpu_counter.c

@@ -104,13 +104,13 @@ void percpu_counter_destroy(struct percpu_counter *fbc)
 	if (!fbc->counters)
 		return;
 
-	free_percpu(fbc->counters);
-	fbc->counters = NULL;
 #ifdef CONFIG_HOTPLUG_CPU
 	mutex_lock(&percpu_counters_lock);
 	list_del(&fbc->list);
 	mutex_unlock(&percpu_counters_lock);
 #endif
+	free_percpu(fbc->counters);
+	fbc->counters = NULL;
 }
 EXPORT_SYMBOL(percpu_counter_destroy);