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Date: Thu, 29 Mar 2007 15:16:49 -0700 (PDT)
From: Davide Libenzi <davidel@xmailserver.org>
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To: Oleg Nesterov <oleg@tv-sign.ru>
cc: Nick Piggin <npiggin@suse.de>, Ravikiran G Thirumalai <kiran@scalex86.org>,
       Ingo Molnar <mingo@elte.hu>, Nikita Danilov <nikita@clusterfs.com>,
       Andrew Morton <akpm@linux-foundation.org>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [patch] queued spinlocks (i386)
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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