On Wed, May 01, 2013 at 11:10:12AM +0200, 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:
> >
> > > > > +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?
Good point, I was assuming that the goal was to let grace periods end
as well as to allow preemption. The momentary dropping out of the
RCU read-side critical section allows the grace periods to end.
> 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.
If the only goal is to allow preemption, and if long grace periods are
not a concern, then this alternate approach would work fine as well.
Of course, both approaches assume that the caller is in a place
where having all RCU-protected data disappear is OK!
Thanx, Paul
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