[PATCH] perf/core: do not set cpuctx->cgrp for unscheduled cgroups

[David Carrillo-Cisneros <davidcc@google.com>]

commit db4a835601b7
("perf/core: Set cgroup in CPU contexts for new cgroup events")

fails to verify that event->cgrp is actually the scheduled cgroup
in a CPU before setting cpuctx->cgrp. This patch fixes that.
The bug only impacts the first cgroup that is created in a CPU.

Now that there is a different path for scheduled and unscheduled
cgroups, add a warning to catch when cpuctx->cgrp is still set after
the last cgroup event has been unsheduled.

Reviewed-by: Stephane Eranian <eranian@google.com>
Signed-off-by: David Carrillo-Cisneros <davidcc@google.com>
---
 kernel/events/core.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 7c0d263..62f70a3 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -902,6 +902,17 @@ list_update_cgroup_event(struct perf_event *event,
 	 * this will always be called from the right CPU.
 	 */
 	cpuctx = __get_cpu_context(ctx);
+
+	/* Only set/clear cpuctx->cgrp if current task uses event->cgrp. */
+	if (perf_cgroup_from_task(current, ctx) != event->cgrp) {
+		/*
+		 * We are removing the last cpu event in this context.
+		 * If that event is not active in this cpu, cpuctx->cgrp
+		 * should've been cleared by perf_cgroup_switch.
+		 */
+		WARN_ON_ONCE(!add && cpuctx->cgrp);
+		return;
+	}
 	cpuctx->cgrp = add ? event->cgrp : NULL;
 }
 
-- 
2.8.0.rc3.226.g39d4020