Messages in this thread | | | Date | Tue, 24 Sep 2013 10:47:18 -0700 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH] hotplug: Optimize {get,put}_online_cpus() |
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On Tue, Sep 24, 2013 at 07:06:31PM +0200, Oleg Nesterov wrote: > On 09/24, Steven Rostedt wrote: > > > > On Tue, 24 Sep 2013 18:03:59 +0200 > > Oleg Nesterov <oleg@redhat.com> wrote: > > > > > On 09/24, Peter Zijlstra wrote: > > > > > > > > +static inline void get_online_cpus(void) > > > > +{ > > > > + might_sleep(); > > > > + > > > > + if (current->cpuhp_ref++) { > > > > + barrier(); > > > > + return; > > > > > > I don't undestand this barrier()... we are going to return if we already > > > hold the lock, do we really need it? > > > > I'm confused too. Unless gcc moves this after the release, but the > > release uses preempt_disable() which is its own barrier. > > > > If anything, it requires a comment. > > And I am still confused even after emails from Paul and Peter... > > If gcc can actually do something wrong, then I suspect this barrier() > should be unconditional.
If you are saying that there should be a barrier() on all return paths from get_online_cpus(), I agree.
Thanx, Paul
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