linux-kernel - Re: [PATCH linux-next] cgroup: fix suspicious rcu_dereference

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Message-ID: <20220312133445.GA28086@worktop.programming.kicks-ass.net>
Date:   Sat, 12 Mar 2022 14:34:45 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Chengming Zhou <zhouchengming@...edance.com>
Cc:     tj@...nel.org, lizefan.x@...edance.com, hannes@...xchg.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        songmuchun@...edance.com,
        Linux Kernel Functional Testing <lkft@...aro.org>,
        syzbot+16e3f2c77e7c5a0113f9@...kaller.appspotmail.com,
        Zhouyi Zhou <zhouzhouyi@...il.com>
Subject: Re: [PATCH linux-next] cgroup: fix suspicious
 rcu_dereference_check() usage warning

On Sat, Mar 12, 2022 at 01:19:13PM +0100, Peter Zijlstra wrote:
> On Sat, Mar 05, 2022 at 11:41:03AM +0800, Chengming Zhou wrote:
> > task_css_set_check() will use rcu_dereference_check() to check for
> > rcu_read_lock_held() on the read-side, which is not true after commit
> > dc6e0818bc9a ("sched/cpuacct: Optimize away RCU read lock"). This
> > commit drop explicit rcu_read_lock(), change to RCU-sched read-side
> > critical section. So fix the RCU warning by adding check for
> > rcu_read_lock_sched_held().
> > 
> > Fixes: dc6e0818bc9a ("sched/cpuacct: Optimize away RCU read lock")
> > Reported-by: Linux Kernel Functional Testing <lkft@...aro.org>
> > Reported-by: syzbot+16e3f2c77e7c5a0113f9@...kaller.appspotmail.com
> > Tested-by: Zhouyi Zhou <zhouzhouyi@...il.com>
> > Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
> 
> Thanks, I'll go stick this in sched/core so it's in the same branch that
> caused the problem.

FWIW I never saw this patch because it doesn't instantly look like a
patch I should be interested in. It's classified as 'for-next' and I
don't run -next, sfr does that. Then it's tagged as cgroup, which I also
don't do.

Nowhere does that look like a patch that wants to go in sched/core and
fixes a cpuacct issue.

On top of that, I still don't agree with this, I really think
rcu_dereference_check() itself should be changed.