Re: [PATCH] mutex: Introduce arch_mutex_cpu_relax()

[Gerald Schaefer <gerald.schaefer@de.ibm.com>]

On Mo, 2010-11-22 at 12:10 -0800, Andrew Morton wrote:
> 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.

Well, I've tried that with my last approach, defining arch_mutex_cpu_relax()
in <asm/mutex.h> and including that from <linux/mutex.h>. This didn't work
well because of ugly header file dependencies, and Peter also commented
that "including "asm/mutex.h" isn't advised". The problem is the following
code in kernel/mutex.c (after including <linux/mutex.h>) when
CONFIG_DEBUG_MUTEXES is set:

#ifdef CONFIG_DEBUG_MUTEXES
# include "mutex-debug.h"
# include <asm-generic/mutex-null.h>
#else
# include "mutex.h"
# include <asm/mutex.h>
#endif

So I can only include <asm/mutex.h> from <linux/mutex.h> with an ugly
"#ifndef CONFIG_DEBUG_MUTEXES" around it, or use a completely different
or new arch header file (but <asm/mutex.h> seems like the right place
for this). The CONFIG_ approach avoids all this header file dependency
mess, or did I miss something (or maybe it's just me and it is not ugly
at all)?