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

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Date:	Sat, 23 Nov 2013 10:04:06 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
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 10:13:13AM -0800, Paul E. McKenney wrote:
> 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.
> 

Seems ok, however this also seems like the natural spot to put that
chunk about how a compiler can mis-transform stuff without either
barrier or ACCESS_ONC(); that currently seems spread out over the
document in some notes.

The biggest of which seems to have ended up in the GUARANTEES chapter.
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