linux-kernel - Re: [PATCH] Documentation: memory-barriers: Fix typo in the first example

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Message-ID: <20141202215006.GT25340@linux.vnet.ibm.com>
Date:	Tue, 2 Dec 2014 13:50:06 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Jonathan Corbet <corbet@....net>
Cc:	Srikanth Thokala <sriku.linux@...il.com>,
	linux-doc@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Documentation: memory-barriers: Fix typo in the first
 example

On Tue, Dec 02, 2014 at 04:15:19PM -0500, Jonathan Corbet wrote:
> On Thu, 27 Nov 2014 12:19:26 +0530
> Srikanth Thokala <sriku.linux@...il.com> wrote:
> 
> > Hi,
> > 
> > Kindly review the patch.
> 
> To me it looks right.  Something like this, though, needs an ack from
> Paul (cc'd) before I can be really confident.  Paul...?

I am guessing that this patch is against an old version of this file
(there have been two patches applied to this example in the last six
months).  I believe that the current version is correct, in other words,
that Alexey Dobriyan and Pranith Kumar beat you to this one.  ;-)

Please see below for the current version, which is in -tip as of
November 20th.

							Thanx, Paul

> jon
> 
> > Thanks
> > Srikanth
> > 
> > On Tue, Nov 18, 2014 at 10:09 AM, Srikanth Thokala
> > <sriku.linux@...il.com> wrote:
> > > In the first example, the loads into 'x' and 'y' on CPU 2 doesn't
> > > match the sequence of events described below it.  To match the
> > > sequence of events, the values of 'A' and 'B' should be loaded
> > > into 'x' and 'y' respectively.
> > >
> > > Signed-off-by: Srikanth Thokala <sriku.linux@...il.com>
> > > ---
> > >  Documentation/memory-barriers.txt | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> > > index 22a969c..2770bce 100644
> > > --- a/Documentation/memory-barriers.txt
> > > +++ b/Documentation/memory-barriers.txt
> > > @@ -115,8 +115,8 @@ For example, consider the following sequence of events:
> > >         CPU 1           CPU 2
> > >         =============== ===============
> > >         { A == 1; B == 2 }
> > > -       A = 3;          x = B;
> > > -       B = 4;          y = A;
> > > +       A = 3;          x = A;
> > > +       B = 4;          y = B;
> > >
> > >  The set of accesses as seen by the memory system in the middle can be arranged
> > >  in 24 different combinations:

For example, consider the following sequence of events:

	CPU 1		CPU 2
	===============	===============
	{ A == 1; B == 2 }
	A = 3;		x = B;
	B = 4;		y = A;

The set of accesses as seen by the memory system in the middle can be arranged
in 24 different combinations:

	STORE A=3,	STORE B=4,	y=LOAD A->3,	x=LOAD B->4
	STORE A=3,	STORE B=4,	x=LOAD B->4,	y=LOAD A->3
	STORE A=3,	y=LOAD A->3,	STORE B=4,	x=LOAD B->4
	STORE A=3,	y=LOAD A->3,	x=LOAD B->2,	STORE B=4
	STORE A=3,	x=LOAD B->2,	STORE B=4,	y=LOAD A->3
	STORE A=3,	x=LOAD B->2,	y=LOAD A->3,	STORE B=4
	STORE B=4,	STORE A=3,	y=LOAD A->3,	x=LOAD B->4
	STORE B=4, ...
	...

and can thus result in four different combinations of values:

	x == 2, y == 1
	x == 2, y == 3
	x == 4, y == 1
	x == 4, y == 3

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