Re: [PATCH tip/core/urgent] rcu: protect fork-time cgroup access

[Peter Zijlstra]

On Mon, 2010-03-29 at 23:19 +0200, Peter Zijlstra wrote:
> On Mon, 2010-03-29 at 14:15 -0700, Paul E. McKenney wrote:
> > Add an rcu_read_lock() / rcu_read_unlock() pair to protect a fork-time
> > cgroup access.  This seems likely to be a false positive.
> > 
> > Located by: Alessio Igor Bogani <abogani@xxxxxxxxxx>
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > ---
> > 
> >  sched.c |    2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/kernel/sched.c b/kernel/sched.c
> > index 9ab3cd7..d4bb5e0 100644
> > --- a/kernel/sched.c
> > +++ b/kernel/sched.c
> > @@ -2621,7 +2621,9 @@ void sched_fork(struct task_struct *p, int clone_flags)
> >  	if (p->sched_class->task_fork)
> >  		p->sched_class->task_fork(p);
> >  
> > +	rcu_read_lock();
> >  	set_task_cpu(p, cpu);
> > +	rcu_read_unlock();
> >  
> >  #if defined(CONFIG_SCHEDSTATS) || defined(CONFIG_TASK_DELAY_ACCT)
> >  	if (likely(sched_info_on()))
> 
> What got accessed? This patch just looks wrong.

So the only cgroup thing I can find is set_task_rq()'s task_group()
usage, which does indeed look like it wants RCU, but then, why would
only the sched_fork() usage of set_task_cpu() need this.

If it's needed (possible) then set_task_rq() needs it unconditionally.

That said, the whole task_group stuff is tied to the cgroup muck, so it
shouldn't be possible for the task_group to disappear on us, but then, I
always sorta glaze over when I get near the cgroup core.

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/