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

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Date:   Fri, 25 Oct 2019 16:32:25 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Tejun Heo <tj@...nel.org>, dvyukov@...gle.com,
        Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        akpm@...ux-foundation.org, arnd@...db.de, deepa.kernel@...il.com,
        ebiederm@...ssion.com, elver@...gle.com, guro@...com,
        linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com,
        cgroups@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH cgroup/for-5.5] cgroup: remove
 cgroup_enable_task_cg_lists() optimization

On Fri, Oct 25, 2019 at 04:13:25PM +0200, Oleg Nesterov wrote:
> On 10/25, Christian Brauner wrote:
> >
> > [+Dmitry]
> >
> > On Fri, Oct 25, 2019 at 05:56:06AM -0700, Tejun Heo wrote:
> > > On Thu, Oct 24, 2019 at 12:03:51PM -0700, Tejun Heo wrote:
> > > > cgroup_enable_task_cg_lists() is used to lazyily initialize task
> > > > cgroup associations on the first use to reduce fork / exit overheads
> > > > on systems which don't use cgroup.  Unfortunately, locking around it
> > > > has never been actually correct and its value is dubious given how the
> > > > vast majority of systems use cgroup right away from boot.
> > > >
> > > > This patch removes the optimization.  For now, replace the cg_list
> > > > based branches with WARN_ON_ONCE()'s to be on the safe side.  We can
> > > > simplify the logic further in the future.
> > > >
> > > > Signed-off-by: Tejun Heo <tj@...nel.org>
> > > > Reported-by: Oleg Nesterov <oleg@...hat.com>
> > >
> > > Applying to cgroup/for-5.5.
> >
> > The code you removed was the only place where task->flags was set from
> > !current.
> 
> No, that code doesn't modify task->flags. It checks PF_EXITING under siglock
> but this makes no sense and can't avoid the race with cgroup_exit().

Sorry, you are right. I misread
Ah right, sorry I misremembered this from the prior thread where we
discussed where ->flags is set from [1].

> 
> > So I think this fixes the syzbot data-race report in:
> > https://lore.kernel.org/r/0000000000003b1e8005956939f1@google.com
> 
> No.
> 
> 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.

[1]: https://lore.kernel.org/r/20191021134659.GA1339@redhat.com

Anyway, accidental noise on my part.
Christian