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

[Oleg Nesterov]

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@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.

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().

> So I think this fixes the syzbot data-race report in:
> https://lore.kernel.org/r/0000000000003b1e8005956939f1@xxxxxxxxxx

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.

Oleg.