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Date: Thu, 29 Mar 2007 22:42:13 +0400
From: Oleg Nesterov <oleg@tv-sign.ru>
To: Nick Piggin <npiggin@suse.de>
Cc: Ravikiran G Thirumalai <kiran@scalex86.org>, Ingo Molnar <mingo@elte.hu>,
       Nikita Danilov <nikita@clusterfs.com>,
       Andrew Morton <akpm@linux-foundation.org>, linux-kernel@vger.kernel.org
Subject: Re: [patch] queued spinlocks (i386)
Message-ID: <20070329184213.GA83@tv-sign.ru>
References: <20070325155407.GA497@tv-sign.ru> <20070328070459.GC12508@wotan.suse.de>
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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,

> +       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?

Oleg.