linux-kernel - Re: Spinlocks: Factor our GENERIC_LOCKBREAK in order to avoid spin with irqs disable

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Date:	Mon, 23 Jun 2008 22:39:53 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Christoph Lameter <clameter@....com>
Cc:	Petr Tesarik <ptesarik@...e.cz>, Ingo Molnar <mingo@...e.hu>,
	linux-kernel@...r.kernel.org
Subject: Re: Spinlocks: Factor our GENERIC_LOCKBREAK in order to avoid spin
	with irqs disable

On Mon, 2008-06-23 at 11:20 -0700, Christoph Lameter wrote:
> On Mon, 23 Jun 2008, Petr Tesarik wrote:
> 
> > Maybe I'm just blind, but doesn't this change effectively disable any
> > arch-specific optimized code for _raw_*_lock?
> 
> True.  Only the __raw_xxx_trylock is still used.
> 
> > If CONFIG_DEBUG_LOCK_ALLOC is set, then CONFIG_DEBUG_SPINLOCK must also
> > be set, so in that case the debugging versions of _raw_*_lock are used.
> > But if CONFIG_DEBUG_LOCK_ALLOC is _not_ set, then the locks are built
> > with _trylock and _can_lock primitives.
> > 
> > What am I missing here?
> 
> It is good that the locks are build with _trylock and _can_lock because 
> then we can reenable interrupts while spinning.

Well, good and bad, the turn side is that fairness schemes like ticket
locks are utterly defeated.

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