linux-kernel - Re: [PATCH memory-barriers.txt 1/7] documentation: Clarify relationship of barrier() to control dependencies

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Message-ID: <20160414152815.GF3755@linux.vnet.ibm.com>
Date:	Thu, 14 Apr 2016 08:28:15 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	jiangshanlai@...il.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
	dhowells@...hat.com, edumazet@...gle.com, dvhart@...ux.intel.com,
	fweisbec@...il.com, oleg@...hat.com, bobby.prani@...il.com
Subject: Re: [PATCH memory-barriers.txt 1/7] documentation: Clarify
 relationship of barrier() to control dependencies

On Wed, Apr 13, 2016 at 11:56:14PM -0400, Steven Rostedt wrote:
> On Tue, 12 Apr 2016 08:52:49 -0700
> "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > The current documentation claims that the compiler ignores barrier(),
> > which is not the case.  Instead, the compiler carefully pays attention
> > to barrier(), but in a creative way that still manages to destroy
> > the control dependency.  This commit sets the story straight.
> > 
> > Reported-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> > Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > ---
> >  Documentation/memory-barriers.txt | 7 ++++---
> >  1 file changed, 4 insertions(+), 3 deletions(-)
> > 
> > diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt
> > index 3729cbe60e41..ec1289042396 100644
> > --- a/Documentation/memory-barriers.txt
> > +++ b/Documentation/memory-barriers.txt
> > @@ -813,9 +813,10 @@ In summary:
> >        the same variable, then those stores must be ordered, either by
> >        preceding both of them with smp_mb() or by using smp_store_release()
> >        to carry out the stores.  Please note that it is -not- sufficient
> > -      to use barrier() at beginning of each leg of the "if" statement,
> > -      as optimizing compilers do not necessarily respect barrier()
> > -      in this case.
> > +      to use barrier() at beginning of each leg of the "if" statement
> > +      because, as shown by the example above, optimizing compilers can
> > +      destroy the control dependency while respecting the letter of the
> > +      barrier() law.
> 
> Which country has the jurisdiction over this barrier() law?
> 
> What about "the letter of the barrier() rules"?

>From https://en.wikipedia.org/wiki/Letter_and_spirit_of_the_law:

	"Law" originally referred to legislative statute, but in the
	idiom may refer to any kind of rule.

So I believe that the current wording respects the spirit of that idiom.  ;-)

							Thanx, Paul