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

Mathieu Desnoyers
2012-06-15T19:54:41Z

* 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().

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.