[PATCH v2] perf/core: Avoid removing shared pmu_context on unregister

[Chris Wilson]

In commit 1fd7e4169954 ("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: 1fd7e4169954 ("perf/core: Remove perf_cpu_context::unique_pmu")
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Cc: David Carrillo-Cisneros <davidcc@xxxxxxxxxx>
Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # v4.11+
---
 kernel/events/core.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index aaefaa27e1a6..4f60f66b35ad 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -8983,10 +8983,12 @@ EXPORT_SYMBOL_GPL(perf_pmu_register);
 void perf_pmu_unregister(struct pmu *pmu)
 {
 	int remove_device;
+	int remove_context;
 
 	mutex_lock(&pmus_lock);
 	remove_device = pmu_bus_running;
 	list_del_rcu(&pmu->entry);
+	remove_context = !find_pmu_context(pmu->task_ctx_nr);
 	mutex_unlock(&pmus_lock);
 
 	/*
@@ -9005,7 +9007,8 @@ void perf_pmu_unregister(struct pmu *pmu)
 		device_del(pmu->dev);
 		put_device(pmu->dev);
 	}
-	free_pmu_context(pmu);
+	if (remove_context)
+		free_pmu_context(pmu);
 }
 EXPORT_SYMBOL_GPL(perf_pmu_unregister);
 
-- 
2.11.0