Re: [RFC PATCH 1/3] x86,smp: move waiting on contended lock out ofline

[Rik van Riel]

On 12/21/2012 11:40 PM, Michel Lespinasse wrote:
> On Fri, Dec 21, 2012 at 3:50 PM, Rik van Riel <riel@xxxxxxxxxx> wrote:

> > @@ -53,12 +55,11 @@ static __always_inline void __ticket_spin_lock(arch_spinlock_t *lock)
> >
> >          inc = xadd(&lock->tickets, inc);

> > +       if (inc.head == inc.tail)
> > +               goto out;
> > +
> > +       ticket_spin_lock_wait(lock, inc);
> > + out:
>
> why not just:
>
> if (inc.head != inc.tail)
>    ticket_spin_lock_wait(lock, inc)

That makes the code nicer, thank you. Applied.

> > +++ b/arch/x86/kernel/smp.c
> > @@ -113,6 +113,20 @@ static atomic_t stopping_cpu = ATOMIC_INIT(-1);
> >   static bool smp_no_nmi_ipi = false;
> >
> >   /*
> > + * Wait on a congested ticket spinlock.
> > + */
> > +void ticket_spin_lock_wait(arch_spinlock_t *lock, struct __raw_tickets inc)
> > +{
> > +       for (;;) {
> > +               cpu_relax();
> > +               inc.head = ACCESS_ONCE(lock->tickets.head);
> > +
> > +               if (inc.head == inc.tail)
> > +                       break;
> > +       }
>
> Why not just:
>
> do {
>    cpu_relax()
>    inc.head = ...
> } while (inc.head != inc.tail);
>
>
> Other than that, no problems with the principle of it.

In patch #3 I do something else inside the head == tail
conditional block, so this one is best left alone.

Thank you for the comments.

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