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

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Date:	Thu, 10 May 2007 00:47:24 +0200
From:	"Jesper Juhl" <jesper.juhl@...il.com>
To:	"Satyam Sharma" <satyam.sharma@...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 10/05/07, Satyam Sharma <satyam.sharma@...il.com> wrote:
> On 5/10/07, Jesper Juhl <jesper.juhl@...il.com> wrote:
> > On 09/05/07, Jonathan Corbet <corbet@....net> wrote:
<snip>
> > > +"the new C book") has the following to say about the volatile keyword:
> > > +
> > > +       The purpose of volatile is to force an implementation to suppress
> > > +       optimization that could otherwise occur.  For example, for a
> > > +       machine with memory-mapped input/output, a pointer to a device
> > > +       register might be declared as a pointer to volatile, in
> > > +       order to prevent the compiler from removing apparently redundant
> > > +       references through the pointer.
> > > +
> > > +C programmers have often taken volatile to mean that the variable could be
> > > +changed outside of the current thread of execution; as a result, they are
> >
> > 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).

In any case, it's not an uncommon belief, so I just thought it made
sense to also make that little note.


> worst, the mistake someone might make would be to _implement_ locking
> primitives using volatile. "that the variable could be changed outside
> of the current thread of execution" sounds sufficient to me, and after
> all, that is exactly what volatile hints to the compiler.
>


-- 
Jesper Juhl <jesper.juhl@...il.com>
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