Re: [PATCH] mutex: Introduce arch_mutex_cpu_relax()

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

On Mo, 2010-10-18 at 20:54 +0200, Peter Zijlstra wrote:
> On Fri, 2010-10-15 at 13:07 +0200, Gerald Schaefer 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 in
> > include/asm/mutex.h. The default is still cpu_relax(), so this should
> > not affect other architectures than System z for now.
> 
> Ingo's randconfig build found the following, .config attached.
> 
> including "asm/mutex.h" isn't advised.
> 
>   CC      kernel/mutex.o
> In file included from /usr/src/linux-2.6/kernel/mutex.c:33:
> /usr/src/linux-2.6/include/asm-generic/mutex-null.h:13:1: warning: "__mutex_fastpath_lock" redefined
> In file included from /usr/src/linux-2.6/arch/x86/include/asm/mutex.h:4,
>                  from /usr/src/linux-2.6/include/linux/mutex.h:19,
>                  from /usr/src/linux-2.6/kernel/mutex.c:20:
> /usr/src/linux-2.6/arch/x86/include/asm/mutex_64.h:19:1: warning: this is the location of the previous definition
> In file included from /usr/src/linux-2.6/kernel/mutex.c:33:
> /usr/src/linux-2.6/include/asm-generic/mutex-null.h:15:1: warning: "__mutex_fastpath_unlock" redefined
> In file included from /usr/src/linux-2.6/arch/x86/include/asm/mutex.h:4,
>                  from /usr/src/linux-2.6/include/linux/mutex.h:19,
>                  from /usr/src/linux-2.6/kernel/mutex.c:20:
> /usr/src/linux-2.6/arch/x86/include/asm/mutex_64.h:62:1: warning: this is the location of the previous definition
> In file included from /usr/src/linux-2.6/kernel/mutex.c:33:
> /usr/src/linux-2.6/include/asm-generic/mutex-null.h:13:1: warning: "__mutex_fastpath_lock" redefined
> In file included from /usr/src/linux-2.6/arch/x86/include/asm/mutex.h:4,
>                  from /usr/src/linux-2.6/include/linux/mutex.h:19,
>                  from /usr/src/linux-2.6/kernel/mutex.c:20:
> /usr/src/linux-2.6/arch/x86/include/asm/mutex_64.h:19:1: warning: this is the location of the previous definition
> In file included from /usr/src/linux-2.6/kernel/mutex.c:33:
> /usr/src/linux-2.6/include/asm-generic/mutex-null.h:15:1: warning: "__mutex_fastpath_unlock" redefined
> In file included from /usr/src/linux-2.6/arch/x86/include/asm/mutex.h:4,
>                  from /usr/src/linux-2.6/include/linux/mutex.h:19,
>                  from /usr/src/linux-2.6/kernel/mutex.c:20:
> /usr/src/linux-2.6/arch/x86/include/asm/mutex_64.h:62:1: warning: this is the location of the previous definition

Ok, I see now that including <asm/mutex.h> from include/linux/mutex.h is
not a good idea, because of this code in kernel/mutex.c (and the conflict
with CONFIG_DEBUG_MUTEXES set):

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

Putting the architecture specific details of arch_mutex_cpu_relax()
somewhere else than <asm/mutex.h> doesn't seem like a good idea either.
Also, putting an "#ifndef CONFIG_DEBUG_MUTEXES" around my
"#include <asm/mutex.h>" in include/linux/mutex.h would fix the conflict,
but that also looks rather ugly.

So I guess I'll just go back to the original Kconfig approach, which
at least avoids all this header file mess. I'll send a new patch.