linux-kernel - Re: KCSAN: data-race in cgroup_rstat_flush_locked / cgroup_rstat

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Message-ID: <20210920180956.GA15043@blackbody.suse.cz>
Date:   Mon, 20 Sep 2021 20:09:56 +0200
From:   Michal Koutný <mkoutny@...e.com>
To:     Hao Sun <sunhao.th@...il.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        cgroups@...r.kernel.org, hannes@...xchg.org,
        lizefan.x@...edance.com, tj@...nel.org
Subject: Re: KCSAN: data-race in cgroup_rstat_flush_locked /
 cgroup_rstat_updated

On Sat, Sep 18, 2021 at 09:27:08AM +0800, Hao Sun <sunhao.th@...il.com> wrote:
> Would it be better to add a `data_race` macro to the corresponding
> location so that the false report can be disabled?

Something like this

--- a/kernel/cgroup/rstat.c
+++ b/kernel/cgroup/rstat.c
@@ -35,7 +35,7 @@ void cgroup_rstat_updated(struct cgroup *cgrp, int cpu)
         * instead of NULL, we can tell whether @cgrp is on the list by
         * testing the next pointer for NULL.
         */
-       if (cgroup_rstat_cpu(cgrp, cpu)->updated_next)
+       if (data_race(cgroup_rstat_cpu(cgrp, cpu)->updated_next))
                return;

        raw_spin_lock_irqsave(cpu_lock, flags);
?

Makes sense to me. Will you send a patch (if this resolves your KCSAN
noise)?

(IIUC, this becase more visible after commit aa48e47e3906 ("memcg:
infrastructure to flush memcg stats") v5.15-rc1 but it was present since
d8ef4b38cb69 ("Revert "cgroup: Add memory barriers to plug
cgroup_rstat_updated() race window"") v5.7.)

> See https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/tools/memory-model/Documentation/access-marking.txt#n58
> for more details.

(Interesting, learning...)

Thanks,
Michal