linux-kernel - Re: [PATCH cgroup/for-5.5] cgroup: remove cgroup_enable_task_cg

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Message-ID: <20191025170523.u43rkulrui22ynix@wittgenstein>
Date:   Fri, 25 Oct 2019 19:05:25 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     dvyukov@...gle.com, ebiederm@...ssion.com, elver@...gle.com,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [PATCH cgroup/for-5.5] cgroup: remove
 cgroup_enable_task_cg_lists() optimization

[Removing a few people from Cc to avoid spamming the whole world]

On Fri, Oct 25, 2019 at 05:52:25PM +0200, Oleg Nesterov wrote:
> On 10/25, Christian Brauner wrote:
> >
> > On Fri, Oct 25, 2019 at 04:13:25PM +0200, Oleg Nesterov wrote:
> > > Almost every usage of task->flags (load or sore) can be reported as "data race".
> > > 
> > > Say, you do
> > > 
> > > 	if (task->flags & PF_KTHREAD)
> > > 
> > > while this task does
> > > 
> > > 	current->flags |= PF_FREEZER_SKIP;
> > > 	schedule().
> > > 
> > > this is data race.
> > 
> > Right, but I thought we agreed on WONTFIX in those scenarios?
> > The alternative is to READ_ONCE()/WRITE_ONCE() all of these.
> 
> Well, in my opinion this is WONTFIX, but I won't argue if someone
> adds _ONCE to all of these. Same for task->state, exit_state, and
> more.

Well, I honestly think that state and exit_state would make sense.
There already were issues that got fixed for example in 3245d6acab98
("exit: fix race between wait_consider_task() and wait_task_zombie()")
and as far as I understand this would also help kcsan to better detect
races.

Christian