linux-kernel - Re: [Bug] race condition at rebind

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Date:   Fri, 15 Jul 2022 06:47:59 -1000
From:   Tejun Heo <tj@...nel.org>
To:     Michal Koutný <mkoutny@...e.com>
Cc:     Jing-Ting Wu <jing-ting.wu@...iatek.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Zefan Li <lizefan.x@...edance.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        linux-mediatek@...ts.infradead.org,
        Shakeel Butt <shakeelb@...gle.com>, wsd_upstream@...iatek.com,
        lixiong.liu@...iatek.com, wenju.xu@...iatek.com,
        jonathan.jmchen@...iatek.com
Subject: Re: [Bug] race condition at rebind_subsystems()

(resending, I messed up the message header, sorry)

Hello,

On Fri, Jul 15, 2022 at 01:59:38PM +0200, Michal Koutný wrote:
> The css->rstat_css_node should not be modified if there are possible RCU
> readers elsewhere.
> One way to fix this would be to insert synchronize_rcu() after
> list_del_rcu() and before list_add_rcu().
> (A further alternative (I've heard about) would be to utilize 'nulls'
> RCU lists [1] to make the move between lists detectable.)
> 
> But as I'm looking at it from distance, it may be simpler and sufficient
> to just take cgroup_rstat_lock around the list migration (the nesting
> under cgroup_mutex that's held with rebind_subsystems() is fine).

synchronize_rcu() prolly is the better fit here given how that list_node's
usage, but yeah, great find.

Thanks.

-- 
tejun