Re: [patch] queued spinlocks (i386)

[Davide Libenzi]

On Thu, 29 Mar 2007, Oleg Nesterov wrote:

> On 03/28, Nick Piggin wrote:
> >
> > Well with my queued spinlocks, all that lockbreak stuff can just come out
> > of the spin_lock, break_lock out of the spinlock structure, and
> > need_lockbreak just becomes (lock->qhead - lock->qtail > 1).
> 
> Q: queued spinlocks are not CONFIG_PREEMPT friendly,

Why? Is CONFIG_PREEMPT friendly to anyone? :)



> > +       asm volatile(LOCK_PREFIX "xaddw %0, %1\n\t"
> > +                    : "+r" (pos), "+m" (lock->qhead) : : "memory");
> > +       while (unlikely(pos != lock->qtail))
> > +               cpu_relax();
> 
> once we incremented lock->qhead, we have no optiion but should spin with
> preemption disabled until pos == lock->qtail, yes?

Yes, preemption and deterministic spinlock policies are not friends.



- Davide


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