perf/core: Plug potential memory leak in CPU_UP_PREPARE

If CPU_UP_PREPARE is called it is not guaranteed, that a previously allocated
and assigned hash has been freed already, but perf_event_init_cpu()
unconditionally allocates and assignes a new hash if the swhash is referenced.
By overwriting the pointer the existing hash is not longer accessible.

Verify that there is no hash assigned on this cpu before allocating and
assigning a new one.

Signed-off-by: Thomas Gleixner <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Stephane Eranian <[email protected]>
Cc: Vince Weaver <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

kernel/events/core.c

@@ -9206,7 +9206,7 @@ static void perf_event_init_cpu(int cpu)
 	struct swevent_htable *swhash = &per_cpu(swevent_htable, cpu);
 
 	mutex_lock(&swhash->hlist_mutex);
-	if (swhash->hlist_refcount > 0) {
+	if (swhash->hlist_refcount > 0 && !swevent_hlist_deref(swhash)) {
 		struct swevent_hlist *hlist;
 
 		hlist = kzalloc_node(sizeof(*hlist), GFP_KERNEL, cpu_to_node(cpu));