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

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Message-ID: <20191028164852.GA17900@redhat.com>
Date:   Mon, 28 Oct 2019 17:48:52 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Christian Brauner <christian.brauner@...ntu.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

On 10/25, Christian Brauner wrote:
>
> 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.

Heh. Again, I am not arguing, but...

OK, lets suppose we blindly add READ_ONCE() to every access of
task->state/exit_state.

Yes, this won't hurt and possibly can fix some bugs we are not aware of.

However,

> There already were issues that got fixed for example in 3245d6acab98
> ("exit: fix race between wait_consider_task() and wait_task_zombie()")

The change above can't fix the problem like this.

It is not that this code lacked READ_ONCE(). I am sure me and others
understood that this code can read ->exit_state more than once, just
nobody noticed that in this case this is really wrong.

IOW, if we simply change the code before 3245d6acab98 to use READ_ONCE()
the code will be equally wrong, and

> and as far as I understand this would also help kcsan to better detect
> races.

this change will simply hide the problem from kcsan.

Oleg.