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

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Message-ID: <20230328221644.803272-5-yosryahmed@google.com>
Date:   Tue, 28 Mar 2023 22:16:39 +0000
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Tejun Heo <tj@...nel.org>, Josef Bacik <josef@...icpanda.com>,
        Jens Axboe <axboe@...nel.dk>,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Shakeel Butt <shakeelb@...gle.com>,
        Muchun Song <muchun.song@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Michal Koutný" <mkoutny@...e.com>
Cc:     Vasily Averin <vasily.averin@...ux.dev>, cgroups@...r.kernel.org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, bpf@...r.kernel.org,
        Yosry Ahmed <yosryahmed@...gle.com>
Subject: [PATCH v2 4/9] cgroup: rstat: add WARN_ON_ONCE() if flushing outside
 task context

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@...gle.com>
Reviewed-by: Shakeel Butt <shakeelb@...gle.com>
---
 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