Re: [PATCH v2 1/2] sched: Add cond_resched_rcu_lock() helper

[Peter Zijlstra]

On Tue, Apr 30, 2013 at 10:52:38AM +0300, Julian Anastasov wrote:
> 
> 	Hello,
> 
> On Tue, 30 Apr 2013, Simon Horman wrote:
> 
> > > > +static void inline cond_resched_rcu_lock(void)
> > > > +{
> > > > +	if (need_resched()) {
> > > 
> > > 	Ops, it should be without above need_resched.
> > 
> > Thanks, to clarify, just this:
> > 
> > static void inline cond_resched_rcu_lock(void)
> > {
> > 	rcu_read_unlock();
> > #ifndef CONFIG_PREEMPT_RCU
> > 	cond_resched();
> > #endif
> > 	rcu_read_lock();
> > }
> 
> 	Yes, thanks!

OK, now I'm confused.. PREEMPT_RCU would preempt in any case, so why bother
dropping rcu_read_lock() at all?

That is; the thing that makes sense to me is:

static void inline cond_resched_rcu_lock(void)
{
#ifdef CONFIG_PREEMPT_RCU
	if (need_resched()) {
		rcu_read_unlock();
		cond_resched();
		rcu_read_lock();
	}
#endif /* CONFIG_PREEMPT_RCU */
}

That would have an rcu_read_lock() break and voluntary preemption point for
non-preemptible RCU and not bother with the stuff for preemptible RCU.
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