Re: [PATCH] mutex: Introduce arch_mutex_cpu_relax()

[Andrew Morton <akpm@linux-foundation.org>]

On Mon, 22 Nov 2010 15:47:36 +0100
Gerald Schaefer <gerald.schaefer@de.ibm.com> wrote:

> From: Gerald Schaefer <gerald.schaefer@de.ibm.com>
> 
> The spinning mutex implementation uses cpu_relax() in busy loops as a
> compiler barrier. Depending on the architecture, cpu_relax() may do more
> than needed in this specific mutex spin loops. On System z we also give
> up the time slice of the virtual cpu in cpu_relax(), which prevents
> effective spinning on the mutex.
> 
> This patch replaces cpu_relax() in the spinning mutex code with
> arch_mutex_cpu_relax(), which can be defined by each architecture that
> selects HAVE_ARCH_MUTEX_CPU_RELAX. The default is still cpu_relax(), so
> this patch should not affect other architectures than System z for now.
> 
> ...
>
> --- a/include/linux/mutex.h
> +++ b/include/linux/mutex.h
> @@ -160,4 +160,8 @@ extern int mutex_trylock(struct mutex *l
>  extern void mutex_unlock(struct mutex *lock);
>  extern int atomic_dec_and_mutex_lock(atomic_t *cnt, struct mutex *lock);
>  
> +#ifndef CONFIG_HAVE_ARCH_MUTEX_CPU_RELAX
> +#define arch_mutex_cpu_relax()	cpu_relax()
> +#endif

A simpler way of doing this is to remove the CONFIG_ variable
altogether and do 

#ifndef arch_mutex_cpu_relax
#define arch_mutex_cpu_relax()	cpu_relax()
#endif

When doing this, one should be clear about _which_ arch file has the
responsibility of defining arch_mutex_cpu_relax, and make sure that
this arch file is reliably included in the .c file.