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

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Message-Id: <1214241561.19392.21.camel@elijah.suse.cz>
Date:	Mon, 23 Jun 2008 19:19:21 +0200
From:	Petr Tesarik <ptesarik@...e.cz>
To:	Christoph Lameter <clameter@....com>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: Spinlocks: Factor our GENERIC_LOCKBREAK in order to avoid spin
	with irqs disable

On Wed, 2008-05-07 at 11:49 -0700, Christoph Lameter wrote:
> Subject: Spinlocks: Factor our GENERIC_LOCKBREAK in order to avoid spin with irqs disabled
> 
> The nice spinlock functions that enable interrupts while spinning are only
> usable if GENERIC_LOCKBREAK is set (and we do not debug locks but lock
> debugging would not be used for a production configuration).
> 
> Factor out the dependencies on the ->lock_break field from the nice functions
> into a set of BREAKLOCK macros (cannot be functions since the type of the lock
> variable varies).
> 
> The nice spinlock function can then also be used if !GENERIC_LOCKBREAK
> 
> Signed-off-by: Christoph Lameter <clameter@....com>
> 
> ---
>  kernel/spinlock.c |   40 ++++++++++++++++++++++++++++++----------
>  1 file changed, 30 insertions(+), 10 deletions(-)
> 
> Index: linux-2.6/kernel/spinlock.c
> ===================================================================
> --- linux-2.6.orig/kernel/spinlock.c	2008-05-07 11:19:31.000000000 -0700
> +++ linux-2.6/kernel/spinlock.c	2008-05-07 11:40:56.000000000 -0700
> @@ -65,7 +65,7 @@ EXPORT_SYMBOL(_write_trylock);
>   * even on CONFIG_PREEMPT, because lockdep assumes that interrupts are
>   * not re-enabled during lock-acquire (which the preempt-spin-ops do):
>   */
> -#if !defined(CONFIG_GENERIC_LOCKBREAK) || defined(CONFIG_DEBUG_LOCK_ALLOC)
> +#ifdef CONFIG_DEBUG_LOCK_ALLOC

Maybe I'm just blind, but doesn't this change effectively disable any
arch-specific optimized code for _raw_*_lock?

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?

Petr Tesarik


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