linux-kernel - Re: [PATCH v2 RFC 1/3] documentation: Add needed ACCESS

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Message-ID: <20131122181313.GU4138@linux.vnet.ibm.com>
Date:	Fri, 22 Nov 2013 10:13:13 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...nel.org,
	laijs@...fujitsu.com, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
	darren@...art.com, fweisbec@...il.com, sbw@....edu
Subject: Re: [PATCH v2 RFC 1/3] documentation: Add needed ACCESS_ONCE() calls
 to memory-barriers.txt

On Fri, Nov 22, 2013 at 04:39:08PM +0100, Peter Zijlstra wrote:
> On Thu, Nov 21, 2013 at 01:31:27PM -0800, Paul E. McKenney wrote:
> > From: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> > 
> > The Documentation/memory-barriers.txt file was written before the need
> > for ACCESS_ONCE() was fully appreciated.  It therefore contains no
> > ACCESS_ONCE() calls, which can be a problem when people lift examples
> > from it.  This commit therefore adds ACCESS_ONCE() calls.
> > 
> 
> Under the 'COMPILER BARRIER' section we state that:
> 
> "This is a general barrier - lesser varieties of compiler barrier do not
> exist."
> 
> One could argue ACCESS_ONCE() is such a lesser barrier.

Fair point -- I should have updated this section when adding ACCESS_ONCE().

How about the following?

							Thanx, Paul

------------------------------------------------------------------------

COMPILER BARRIER
----------------

The Linux kernel has an explicit compiler barrier function that prevents the
compiler from moving the memory accesses either side of it to the other side:

        barrier();

This is a general barrier -- there are no read-read or write-write variants
of barrier().  Howevever, ACCESS_ONCE() can be thought of as a weak form
for barrier() that affects only the specific accesses flagged by the
ACCESS_ONCE().

The compiler barrier has no direct effect on the CPU, which may then reorder
things however it wishes.

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