Re: [Xen-devel] [PATCH v10 10/19] qspinlock, x86: Allow unfair spinlock in a virtual guest

[Waiman Long <waiman.long@xxxxxx>]

On 05/08/2014 03:12 PM, Peter Zijlstra wrote:
> On Wed, May 07, 2014 at 11:01:38AM -0400, Waiman Long wrote:
>
>
> No, we want the unfair thing for VIRT, not PARAVIRT.
Yes, you are right. I will change that to VIRT.

> > diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
> > index 9e7659e..10e87e1 100644
> > --- a/kernel/locking/qspinlock.c
> > +++ b/kernel/locking/qspinlock.c
> > @@ -227,6 +227,14 @@ static __always_inline int get_qlock(struct qspinlock 
> > *lock)
> >   {
> >         struct __qspinlock *l = (void *)lock;
> >
> > +#ifdef CONFIG_PARAVIRT_UNFAIR_LOCKS
> > +       if (static_key_false(&paravirt_unfairlocks_enabled))
> > +               /*
> > +                * Need to use atomic operation to get the lock when
> > +                * lock stealing can happen.
> > +                */
> > +               return cmpxchg(&l->locked, 0, _Q_LOCKED_VAL) == 0;
> That's missing {}.
It is a single statement which doesn't need braces according to kernel 
coding style. I could move the comments up a bit to make it easier to read.
> > +#endif
> >         barrier();
> >         ACCESS_ONCE(l->locked) = _Q_LOCKED_VAL;
> >         barrier();
> But no, what you want is:
>
> static __always_inline bool virt_lock(struct qspinlock *lock)
> {
> #ifdef CONFIG_VIRT_MUCK
>         if (static_key_false(&virt_unfairlocks_enabled)) {
>                 while (!queue_spin_trylock(lock))
>                         cpu_relax();
>
>                 return true;
>         }
> #else
>         return false;
> }
>
>
> void queue_spin_lock_slowpath(struct qspinlock *lock, u32 val)
> {
>         if (virt_lock(lock))
>                 return;
>
>         ...
> }
This is a possible way of doing it. I can do that in the patch series to 
simplify it. Hopefully that will speed up the review process and get it 
done quicker.
-Longman

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