Re: [PATCH cgroup/for-5.5] cgroup: remove cgroup_enable_task_cg_lists() optimization

[Christian Brauner]

[+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@xxxxxxxxxx>
> > Reported-by: Oleg Nesterov <oleg@xxxxxxxxxx>
> 
> 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@xxxxxxxxxx

Link: syzbot+492a4acccd8fc75ddfd0@xxxxxxxxxxxxxxxxxxxxxxxxx

Thanks!
Christian