linux-kernel - Re: [PATCH] mutex: Introduce arch_mutex_cpu

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Date:	Tue, 23 Nov 2010 16:03:32 +0100
From:	Gerald Schaefer <gerald.schaefer@...ibm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-s390 <linux-s390@...r.kernel.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>
Subject: Re: [PATCH] mutex: Introduce arch_mutex_cpu_relax()

On Di, 2010-11-23 at 15:20 +0100, Peter Zijlstra wrote:
> On Tue, 2010-11-23 at 15:12 +0100, Gerald Schaefer wrote:
> > 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@...ibm.com> wrote:
> > > 
> > > > From: Gerald Schaefer <gerald.schaefer@...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)?
> 
> Yeah, that all cause massive grief.. I've applied your patch as is,
> assuming s390 already has the needed arch_mutex_cpu_relax()
> implementation (otherwise I've just broken my s390 build).

Thanks, my patch already includes the s390 implementation for
arch_mutex_cpu_relax() in arch/s390/include/asm/mutex.h (we use a
simple barrier() in this case). We still have HAVE_DEFAULT_NO_SPIN_MUTEXES
selected though, this will be removed with one of Martins next patches,
once this preparation patch is upstream.


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