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

[Christian Brauner]

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@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().

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@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.

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@xxxxxxxxxx

Anyway, accidental noise on my part.
Christian