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

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Date:   Sat, 12 Mar 2022 23:54:28 +0800
From:   Chengming Zhou <zhouchengming@...edance.com>
To:     Peter Zijlstra <peterz@...radead.org>
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: [External] Re: [PATCH linux-next] cgroup: fix suspicious
 rcu_dereference_check() usage warning

On 2022/3/12 9:34 下午, Peter Zijlstra wrote:
> 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.

Oh, sorry for this.. I should've add "cpuacct" in the subject. The
"linux-next" prefix was added because I thought any patch based on
the linux-next branch should add this prefix.

> 
> 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.

Yes, I think so too. This patch is workaround to fix the warning to
follow the usage in RCU Documentation.

Maybe changes should be made in RCU code to make rcu_dereference_check()
more flexible as you expressed in the conversation with Paul.

Thanks.