Re: task isolation discussion at Linux Plumbers

[Paul E. McKenney]

On Wed, Nov 09, 2016 at 05:44:02PM -0800, Andy Lutomirski wrote:
> On Wed, Nov 9, 2016 at 9:38 AM, Paul E. McKenney
> <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> 
> Are you planning on changing rcu_nmi_enter()?  It would make it easier
> to figure out how they interact if I could see the code.

It already calls rcu_dynticks_eqs_exit(), courtesy of the earlier
consolidation patches.

> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index dbf20b058f48..342c8ee402d6 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> 
> 
> >  /*
> > @@ -305,17 +318,22 @@ static void rcu_dynticks_eqs_enter(void)
> >  static void rcu_dynticks_eqs_exit(void)
> >  {
> >         struct rcu_dynticks *rdtp = this_cpu_ptr(&rcu_dynticks);
> > +       int seq;
> >
> >         /*
> > -        * CPUs seeing atomic_inc() must see prior idle sojourns,
> > +        * CPUs seeing atomic_inc_return() must see prior idle sojourns,
> >          * and we also must force ordering with the next RCU read-side
> >          * critical section.
> >          */
> > -       smp_mb__before_atomic(); /* See above. */
> > -       atomic_inc(&rdtp->dynticks);
> > -       smp_mb__after_atomic(); /* See above. */
> > +       seq = atomic_inc_return(&rdtp->dynticks);
> >         WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) &&
> > -                    !(atomic_read(&rdtp->dynticks) & 0x1));
> > +                    !(seq & RCU_DYNTICK_CTRL_CTR));
> 
> I think there's still a race here.  Suppose we're running this code on
> cpu n and...
> 
> > +       if (seq & RCU_DYNTICK_CTRL_MASK) {
> > +               rcu_eqs_special_exit();
> > +               /* Prefer duplicate flushes to losing a flush. */
> > +               smp_mb__before_atomic(); /* NMI safety. */
> 
> ... another CPU changes the page tables and calls rcu_eqs_special_set(n) here.

But then rcu_eqs_special_set() will return false because we already
exited the extended quiescent state at the atomic_inc_return() above.

That should tell the caller to send an IPI.

> That CPU expects that we will flush prior to continuing, but we won't.
> Admittedly it's highly unlikely that any stale TLB entries would be
> created yet, but nothing rules it out.

That said, 0day is having some heartburn from this, so I must have broken
something somewhere.  My own tests of course complete just fine...

> > +               atomic_and(~RCU_DYNTICK_CTRL_MASK, &rdtp->dynticks);
> > +       }
> 
> Maybe the way to handle it is something like:
> 
> this_cpu_write(rcu_nmi_needs_eqs_special, 1);
> barrier();
> 
> /* NMI here will call rcu_eqs_special_exit() regardless of the value
> in dynticks */
> 
> atomic_and(...);
> smp_mb__after_atomic();
> rcu_eqs_special_exit();
> 
> barrier();
> this_cpu_write(rcu_nmi_needs_eqs_special, 0);
> 
> 
> Then rcu_nmi_enter() would call rcu_eqs_special_exit() if the dynticks
> bit is set *or* rcu_nmi_needs_eqs_special is set.
> 
> Does that make sense?

I believe that rcu_eqs_special_set() returning false covers this, but
could easily be missing something.

							Thanx, Paul