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

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

* 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.
http://www.efficios.com