perf/core: Avoid removing shared pmu_context on unregister

In commit 1fd7e41 ("perf/core: Remove perf_cpu_context::unique_pmu"),
the search for another user of the pmu_cpu_context was removed, and so
we unconditionally free it during perf_pmu_unregister. This leads to
random corruption later and a BUG at mm/percpu.c:689.

v2: Check for shared pmu_contexts under the mutex.

Fixes: 1fd7e41 ("perf/core: Remove perf_cpu_context::unique_pmu")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: David Carrillo-Cisneros <davidcc@google.com>
Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: <stable@vger.kernel.org> # v4.11+
Link: http://patchwork.freedesktop.org/patch/msgid/20170512114525.17575-1-chris@chris-wilson.co.uk

kernel/events/core.c

@@ -10423,7 +10423,8 @@ void perf_pmu_unregister(struct pmu *pmu)
 		device_del(pmu->dev);
 		put_device(pmu->dev);
 	}
-	free_pmu_context(pmu);
+	if (!find_pmu_context(pmu->task_ctx_nr))
+		free_pmu_context(pmu);
 	mutex_unlock(&pmus_lock);
 }
 EXPORT_SYMBOL_GPL(perf_pmu_unregister);