perf/x86/intel/uncore: Fix minor race in box set up

I was looking for the trinity oops cause in the uncore driver.
(so far didn't found it)

However I found this tiny race: when a box is set up two threads on the
same CPU, they may be setting up the box in parallel (e.g. with kernel
preemption). This could lead to the reference count being increasing
too much. Always recheck there is no existing cpu reference inside the lock.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: eranian@google.com
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Link: http://lkml.kernel.org/r/1411424826-15629-1-git-send-email-andi@firstfloor.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/cpu/perf_event_intel_uncore.c

@@ -42,13 +42,17 @@ struct intel_uncore_box *uncore_pmu_to_box(struct intel_uncore_pmu *pmu, int cpu
 		return box;
 
 	raw_spin_lock(&uncore_box_lock);
+	/* Recheck in lock to handle races. */
+	if (*per_cpu_ptr(pmu->box, cpu))
+		goto out;
 	list_for_each_entry(box, &pmu->box_list, list) {
 		if (box->phys_id == topology_physical_package_id(cpu)) {
 			atomic_inc(&box->refcnt);
 			*per_cpu_ptr(pmu->box, cpu) = box;
 			break;
 		}
 	}
+out:
 	raw_spin_unlock(&uncore_box_lock);
 
 	return *per_cpu_ptr(pmu->box, cpu);