Re: [PATCH tip/core/rcu 4/6] rcu: Fix bug in rcu_barrier() torture test

Paul E. McKenney
2012-06-15T20:17:01Z

On Fri, Jun 15, 2012 at 03:44:54PM -0400, Mathieu Desnoyers wrote:
> * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote:
> [...]
> > @@ -1642,6 +1643,7 @@ void rcu_torture_barrier_cbf(struct rcu_head *rcu)
> > static int rcu_torture_barrier_cbs(void *arg)
> > {
> > long myid = (long)arg;
> > + bool lastphase = 0;
> > struct rcu_head rcu;
> >
> > init_rcu_head_on_stack(&rcu);
> > @@ -1649,9 +1651,11 @@ static int rcu_torture_barrier_cbs(void *arg)
> > set_user_nice(current, 19);
> > do {
> > wait_event(barrier_cbs_wq[myid],
> > - atomic_read(&barrier_cbs_count) == n_barrier_cbs ||
> > + barrier_phase != lastphase ||
> > kthread_should_stop() ||
> > fullstop != FULLSTOP_DONTSTOP);
> > + lastphase = barrier_phase;
> > + smp_mb();
>
> Hi Paul,
>
> Documenting this barrier, and the barrier below, along with the variable
> accesses they order, would appear to be a good idea, especially since
> the bug this is correcting was caused by the lack of appropriate
> smp_mb().

Fair point, even in rcutorture. Or maybe especially in rcutorture...

Will fix!

Thanx, Paul

> Thanks,
>
> Mathieu
>
> > if (kthread_should_stop() || fullstop != FULLSTOP_DONTSTOP)
> > break;
> > cur_ops->call(&rcu, rcu_torture_barrier_cbf);
> > @@ -1676,7 +1680,8 @@ static int rcu_torture_barrier(void *arg)
> > do {
> > atomic_set(&barrier_cbs_invoked, 0);
> > atomic_set(&barrier_cbs_count, n_barrier_cbs);
> > - /* wake_up() path contains the required barriers. */
> > + smp_mb();
> > + barrier_phase = !barrier_phase;
> > for (i = 0; i < n_barrier_cbs; i++)
> > wake_up(&barrier_cbs_wq[i]);
> > wait_event(barrier_wq,
> > --
> > 1.7.8
> >
>
> --
> Mathieu Desnoyers
> Operating System Efficiency R&D Consultant
> EfficiOS Inc.
> http://www.efficios.com
>