Hello,
On Wed, 1 May 2013, Peter Zijlstra wrote:
> On Tue, Apr 30, 2013 at 10:52:38AM +0300, Julian Anastasov wrote:
> >
> > Hello,
> >
> > On Tue, 30 Apr 2013, Simon Horman wrote:
> >
> > > 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
You mean '#ifndef' here, right? But in the non-preempt
case is using the need_resched() needed? rcu_read_unlock
and rcu_read_lock do not generate code.
> 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.
I see. So, can we choose one of both variants:
1. Your variant but with ifndef:
static void inline cond_resched_rcu_lock(void)
{
#ifndef CONFIG_PREEMPT_RCU
if (need_resched()) {
rcu_read_unlock();
cond_resched();
rcu_read_lock();
}
#endif
}
2. Same without need_resched because cond_resched already
performs the same checks:
static void inline cond_resched_rcu_lock(void)
{
#ifndef CONFIG_PREEMPT_RCU
rcu_read_unlock();
cond_resched();
rcu_read_lock();
#endif
}
Regards
--
Julian Anastasov <ja@xxxxxx>
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