perf: Fix hotplug splat

Drew Richardson reported that he could make the kernel go *boom* when hotplugging
while having perf events active.

It turned out that when you have a group event, the code in
__perf_event_exit_context() fails to remove the group siblings from
the context.

We then proceed with destroying and freeing the event, and when you
re-plug the CPU and try and add another event to that CPU, things go
*boom* because you've still got dead entries there.

Reported-by: Drew Richardson <drew.richardson@arm.com>
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Cc: Will Deacon <will.deacon@arm.com>
Cc: <stable@vger.kernel.org>
Link: http://lkml.kernel.org/n/tip-k6v5wundvusvcseqj1si0oz0@git.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 6 insertions, 6 deletions

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 56003c6edfd3..fa0b2d4ad83c 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -7856,14 +7856,14 @@ static void perf_pmu_rotate_stop(struct pmu *pmu)
 static void __perf_event_exit_context(void *__info)
 {
         struct perf_event_context *ctx = __info;
-        struct perf_event *event, *tmp;
+        struct perf_event *event;
 
         perf_pmu_rotate_stop(ctx->pmu);
 
-        list_for_each_entry_safe(event, tmp, &ctx->pinned_groups, group_entry)
-                __perf_remove_from_context(event);
-        list_for_each_entry_safe(event, tmp, &ctx->flexible_groups, group_entry)
+        rcu_read_lock();
+        list_for_each_entry_rcu(event, &ctx->event_list, event_entry)
                 __perf_remove_from_context(event);
+        rcu_read_unlock();
 }
 
 static void perf_event_exit_cpu_context(int cpu)
@@ -7887,11 +7887,11 @@ static void perf_event_exit_cpu(int cpu)
 {
         struct swevent_htable *swhash = &per_cpu(swevent_htable, cpu);
 
+        perf_event_exit_cpu_context(cpu);
+
         mutex_lock(&swhash->hlist_mutex);
         swevent_hlist_release(swhash);
         mutex_unlock(&swhash->hlist_mutex);
-
-        perf_event_exit_cpu_context(cpu);
 }
 #else
 static inline void perf_event_exit_cpu(int cpu) { }