linux-kernel - Re: [patch] queued spinlocks (i386)

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Date:	Thu, 29 Mar 2007 22:42:13 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Ravikiran G Thirumalai <kiran@...lex86.org>,
	Ingo Molnar <mingo@...e.hu>,
	Nikita Danilov <nikita@...sterfs.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [patch] queued spinlocks (i386)

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.

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