Re: [Xen-devel] [PATCH v6 05/11] pvqspinlock, x86: Allow unfair spinlock in a PV guest

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Wed, Mar 12, 2014 at 02:54:52PM -0400, Waiman Long wrote:
> +static inline void arch_spin_lock(struct qspinlock *lock)
> +{
> +     if (static_key_false(&paravirt_unfairlocks_enabled))
> +             queue_spin_lock_unfair(lock);
> +     else
> +             queue_spin_lock(lock);
> +}

So I would have expected something like:

        if (static_key_false(&paravirt_spinlock)) {
                while (!queue_spin_trylock(lock))
                        cpu_relax();
                return;
        }

At the top of queue_spin_lock_slowpath().

> +static inline int arch_spin_trylock(struct qspinlock *lock)
> +{
> +     if (static_key_false(&paravirt_unfairlocks_enabled))
> +             return queue_spin_trylock_unfair(lock);
> +     else
> +             return queue_spin_trylock(lock);
> +}

That just doesn't make any kind of sense; a trylock cannot be fair or
unfair.

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