On Wed, Apr 16, 2014 at 07:21:48AM +0200, Peter Zijlstra wrote:
> On Tue, Apr 15, 2014 at 08:54:19PM -0700, Paul E. McKenney wrote:
> > But falling back on the old ways of doing this at least looks a bit
> > nicer:
> >
> > static inline bool rcu_should_resched(void)
> > {
> > int t;
> > int *tp = &per_cpu(rcu_cond_resched_count,
> > raw_smp_processor_id());
> >
> > t = ACCESS_ONCE(*tp) + 1;
> > if (t < RCU_COND_RESCHED_LIM) {
>
> <here>
>
> > ACCESS_ONCE(*tp) = t;
> > return false;
> > }
> > return true;
> > }
> >
> > Other thoughts?
>
> Still broken, if A starts out on CPU1, gets migrated to CPU0 at <here>,
> then B starts the same on CPU1. It is possible for both CPU0 and CPU1 to
> write a different value into your rcu_cond_resched_count.
That is actually OK. The values written are guaranteed to be between
zero and RCU_COND_RESCHED_LIM-1. In theory, yes, rcu_should_resched()
could end up failing due to a horribly unlucky sequence of preemptions,
but the probability is -way- lower than that of hardware failure.
However...
> You really want to disable preemption around there. The proper old way
> would've been get_cpu_var()/put_cpu_var().
If you are OK with unconditional disabling of preemption at this point,
that would avoid worrying about probabilities and would be quite a bit
simpler.
So unconditional preempt_disable()/preempt_enable() it is.
Thanx, Paul
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