Re: [PATCH v8 5/9] MCS Lock: Restructure the MCS lock defines and locking code into its own file

From: Rafael Aquini <aquini@redhat.com>
To: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Ingo Molnar <mingo@elte.hu>,
	Andrew Morton <akpm@linux-foundation.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Alex Shi <alex.shi@linaro.org>, Andi Kleen <andi@firstfloor.org>,
	Michel Lespinasse <walken@google.com>,
	Davidlohr Bueso <davidlohr.bueso@hp.com>,
	Matthew R Wilcox <matthew.r.wilcox@intel.com>,
	Dave Hansen <dave.hansen@intel.com>,
	Peter Zijlstra <a.p.zijlstra@chello.nl>,
	Rik van Riel <riel@redhat.com>,
	Peter Hurley <peter@hurleysoftware.com>,
	"Paul E.McKenney" <paulmck@linux.vnet.ibm.com>,
	Jason Low <jason.low2@hp.com>, Waiman Long <Waiman.Long@hp.com>,
	linux-kernel@vger.kernel.org, linux-mm <linux-mm@kvack.org>
Subject: Re: [PATCH v8 5/9] MCS Lock: Restructure the MCS lock defines and locking code into its own file
Date: Tue, 8 Oct 2013 18:31:27 -0300	[thread overview]
Message-ID: <20131008213126.GB21046@localhost.localdomain> (raw)
In-Reply-To: <1381264495.11046.110.camel@schen9-DESK>

On Tue, Oct 08, 2013 at 01:34:55PM -0700, Tim Chen wrote:
> On Tue, 2013-10-08 at 16:51 -0300, Rafael Aquini wrote:
> > On Wed, Oct 02, 2013 at 03:38:32PM -0700, Tim Chen wrote:
> > > We will need the MCS lock code for doing optimistic spinning for rwsem.
> > > Extracting the MCS code from mutex.c and put into its own file allow us
> > > to reuse this code easily for rwsem.
> > > 
> > > Reviewed-by: Ingo Molnar <mingo@elte.hu>
> > > Reviewed-by: Peter Zijlstra <peterz@infradead.org>
> > > Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
> > > Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
> > > ---
> > >  include/linux/mcs_spinlock.h |   64 ++++++++++++++++++++++++++++++++++++++++++
> > >  include/linux/mutex.h        |    5 ++-
> > >  kernel/mutex.c               |   60 ++++----------------------------------
> > >  3 files changed, 74 insertions(+), 55 deletions(-)
> > >  create mode 100644 include/linux/mcs_spinlock.h
> > > 
> > > diff --git a/include/linux/mcs_spinlock.h b/include/linux/mcs_spinlock.h
> > > new file mode 100644
> > > index 0000000..b5de3b0
> > > --- /dev/null
> > > +++ b/include/linux/mcs_spinlock.h
> > > @@ -0,0 +1,64 @@
> > > +/*
> > > + * MCS lock defines
> > > + *
> > > + * This file contains the main data structure and API definitions of MCS lock.
> > > + *
> > > + * The MCS lock (proposed by Mellor-Crummey and Scott) is a simple spin-lock
> > > + * with the desirable properties of being fair, and with each cpu trying
> > > + * to acquire the lock spinning on a local variable.
> > > + * It avoids expensive cache bouncings that common test-and-set spin-lock
> > > + * implementations incur.
> > > + */
> > 
> > nitpick:
> > 
> > I believe you need 
> > 
> > +#include <asm/processor.h>
> > 
> > here, to avoid breaking the build when arch_mutex_cpu_relax() is not defined
> > (arch/s390 is one case)
> 

Humm... sorry by my noise as I was looking into an old tree, before this commit:
commit 083986e8248d978b6c961d3da6beb0c921c68220
Author: Heiko Carstens <heiko.carstens@de.ibm.com>
Date:   Sat Sep 28 11:23:59 2013 +0200

    mutex: replace CONFIG_HAVE_ARCH_MUTEX_CPU_RELAX with simple ifdef

> Probably 
> 
> +#include <linux/mutex.h> 
>

Yeah, but I guess right now you're ok without it, as the only place this 
header is included is in kernel/mutex.c and it linux/mutex.h get in before us.

If the plan is to extend usage for other places where mutex.h doesn't go, then
perhaps the better thing would be just copycat the same #ifdef here.

Cheers! (and sorry again for the noise)

> should be added instead?
> It defines arch_mutex_cpu_relax when there's no 
> architecture specific version.
> 
> Thanks.
> Tim
> 
> 