[PATCH v2 4/9] cgroup: rstat: add WARN_ON_ONCE() if flushing outside task context

[Yosry Ahmed]

rstat flushing is too expensive to perform in irq context.
The previous patch removed the only context that may invoke an rstat
flush from irq context, add a WARN_ON_ONCE() to detect future
violations, or those that we are not aware of.

Ideally, we wouldn't flush with irqs disabled either, but we have one
context today that does so in mem_cgroup_usage(). Forbid callers from
irq context for now, and hopefully we can also forbid callers with irqs
disabled in the future when we can get rid of this callsite.

Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx>
Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
---
 kernel/cgroup/rstat.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/kernel/cgroup/rstat.c b/kernel/cgroup/rstat.c
index d3252b0416b6..c2571939139f 100644
--- a/kernel/cgroup/rstat.c
+++ b/kernel/cgroup/rstat.c
@@ -176,6 +176,8 @@ static void cgroup_rstat_flush_locked(struct cgroup *cgrp, bool may_sleep)
 {
 	int cpu;
 
+	/* rstat flushing is too expensive for irq context */
+	WARN_ON_ONCE(!in_task());
 	lockdep_assert_held(&cgroup_rstat_lock);
 
 	for_each_possible_cpu(cpu) {
-- 
2.40.0.348.gf938b09366-goog