[PATCH v2 2/2] perf/core: Fix WARN_ON_ONCE(cpuctx->ctx.nr_cgroups == 0) in perf_cgroup_switch

[Luo Gengkun <luogengkun@huaweicloud.com>]

There may be concurrency between perf_cgroup_switch and
perf_cgroup_event_disable. Consider the following scenario: after a new
perf cgroup event is created on CPU0, the new event may not trigger
a reprogramming, causing ctx->is_active to be 0. In this case, when CPU1
disables this perf event, it executes __perf_remove_from_context->
list _del_event->perf_cgroup_event_disable on CPU1, which causes a race
with perf_cgroup_switch running on CPU0.

The following describes the details of this concurrency scenario:

CPU0						CPU1

perf_cgroup_switch:
   ...
   # cpuctx->cgrp is not NULL here
   if (READ_ONCE(cpuctx->cgrp) == NULL)
   	return;

						perf_remove_from_context:
						   ...
						   raw_spin_lock_irq(&ctx->lock);
						   ...
						   # ctx->is_active == 0 because reprogramm is not
						   # tigger, so CPU1 can do __perf_remove_from_context
						   # for CPU0
						   __perf_remove_from_context:
						         perf_cgroup_event_disable:
							    ...
							    if (--ctx->nr_cgroups)
							    ...

   # this warning will happened because CPU1 changed
   # ctx.nr_cgroups to 0.
   WARN_ON_ONCE(cpuctx->ctx.nr_cgroups == 0);

To fix this problem, expand the lock-holding critical section in
perf_cgroup_switch.

Fixes: db4a835601b7 ("perf/core: Set cgroup in CPU contexts for new cgroup events")
Signed-off-by: Luo Gengkun <luogengkun@huaweicloud.com>
---
 kernel/events/core.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/kernel/events/core.c b/kernel/events/core.c
index 280d42b40b34..1e442897ebde 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -931,20 +931,20 @@ static void perf_cgroup_switch(struct task_struct *task)
 	struct perf_cpu_context *cpuctx = this_cpu_ptr(&perf_cpu_context);
 	struct perf_cgroup *cgrp;
 
+	cgrp = perf_cgroup_from_task(task, NULL);
+	perf_ctx_lock(cpuctx, cpuctx->task_ctx);
 	/*
 	 * cpuctx->cgrp is set when the first cgroup event enabled,
 	 * and is cleared when the last cgroup event disabled.
 	 */
 	if (READ_ONCE(cpuctx->cgrp) == NULL)
-		return;
+		goto unlock;
 
 	WARN_ON_ONCE(cpuctx->ctx.nr_cgroups == 0);
 
-	cgrp = perf_cgroup_from_task(task, NULL);
 	if (READ_ONCE(cpuctx->cgrp) == cgrp)
-		return;
+		goto unlock;
 
-	perf_ctx_lock(cpuctx, cpuctx->task_ctx);
 	perf_ctx_disable(&cpuctx->ctx, true);
 
 	ctx_sched_out(&cpuctx->ctx, NULL, EVENT_ALL|EVENT_CGROUP);
@@ -962,6 +962,7 @@ static void perf_cgroup_switch(struct task_struct *task)
 	ctx_sched_in(&cpuctx->ctx, NULL, EVENT_ALL|EVENT_CGROUP);
 
 	perf_ctx_enable(&cpuctx->ctx, true);
+unlock:
 	perf_ctx_unlock(cpuctx, cpuctx->task_ctx);
 }
 
-- 
2.34.1