linux-kernel - Re: [PATCH] "volatile considered harmful" document

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Date:	Thu, 10 May 2007 04:50:31 +0530
From:	"Satyam Sharma" <satyam.sharma@...il.com>
To:	"Jesper Juhl" <jesper.juhl@...il.com>
Cc:	"Jonathan Corbet" <corbet@....net>, linux-kernel@...r.kernel.org,
	akpm@...ux-foundation.org, "Randy Dunlap" <randy.dunlap@...cle.com>
Subject: Re: [PATCH] "volatile considered harmful" document

On 5/10/07, Jesper Juhl <jesper.juhl@...il.com> wrote:
> On 10/05/07, Satyam Sharma <satyam.sharma@...il.com> wrote:
> > On 5/10/07, Jesper Juhl <jesper.juhl@...il.com> wrote:
> > > [snip]
> > > you write: "... that the variable could be changed outside of the
> > > current thread of execution ..."
> > >
> > > I suggest: "... that the variable could be changed outside of the
> > > current thread of execution - a sort of simple atomic variable ..."
> >
> > I'm not so sure here. Why would any C programmer (worth his weight in
> > salt) think that volatile objects are automatically _atomic_? At
>
> I honestly don't really know, but I've encountered that confusion a
> few times. Both from friends who (for some reason) believed that and
> from documents on the web that implied it, aparently it's a common
> confusion - a few examples:
>
>     http://lists.freebsd.org/pipermail/freebsd-perl/2004-June/000124.html
>         "... volatile (atomic) fixes the problem. ..."
>
>     http://blogs.msdn.com/ricom/archive/2006/04/28/586406.aspx
>         "That's the point of the volatile keyword. It makes sure that
> the line "dict = d;" is atomic."
>
>     http://forum.java.sun.com/thread.jspa?threadID=5126877&start=0
>         "A volatile variable is also guaranteed to be read or written
> as an atomic operation ..."  (yes, this link talks about Java, which I
> don't know, but if java volatile means atomic, that might explain why
> some people assume the same for C).

Perl / Microsoft / Java programmers are probably not worth their
weight in salt anyway :-)

I'm not an expert in any of the above platforms either, so don't know
if the semantics of "volatile" in those other languages are different
from that in C -- and this document clearly applies to only the kernel
(and thus C). But if this volatile == atomic disease is indeed common
among _C_ programmers too, then your suggested addition would make
sense.
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