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

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Message-ID: <20191025133358.pxpzxkhqc3mboi5x@wittgenstein>
Date:   Fri, 25 Oct 2019 15:33:59 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Tejun Heo <tj@...nel.org>, dvyukov@...gle.com
Cc:     Oleg Nesterov <oleg@...hat.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

[+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. So I think this fixes the syzbot data-race report in:
https://lore.kernel.org/r/0000000000003b1e8005956939f1@google.com

Link: syzbot+492a4acccd8fc75ddfd0@...kaller.appspotmail.com

Thanks!
Christian