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Subject: Re: [patch] spinlocks: remove 'volatile'
From: Arjan van de Ven <arjan@infradead.org>
To: "linux-os (Dick Johnson)" <linux-os@analogic.com>
Cc: Ingo Molnar <mingo@elte.hu>, Linus Torvalds <torvalds@osdl.org>,
       Andrew Morton <akpm@osdl.org>, linux-kernel@vger.kernel.org
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Date: Thu, 06 Jul 2006 14:39:43 +0200
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On Thu, 2006-07-06 at 08:29 -0400, linux-os (Dick Johnson) wrote:
> On Thu, 6 Jul 2006, Arjan van de Ven wrote:
> 
> > On Thu, 2006-07-06 at 07:59 -0400, linux-os (Dick Johnson) wrote:
> >> On Thu, 6 Jul 2006, Ingo Molnar wrote:
> >>
> >>>
> >>> * Linus Torvalds <torvalds@osdl.org> wrote:
> >>>
> >>>> I wonder if we should remove the "volatile". There really isn't
> >>>> anything _good_ that gcc can do with it, but we've seen gcc code
> >>>> generation do stupid things before just because "volatile" seems to
> >>>> just disable even proper normal working.
> >>
> >> Then GCC must be fixed. The keyword volatile is correct. It should
> >> force the compiler to read the variable every time it's used.
> >
> > this is not really what the C standard says.
> >
> >
> >
> >> This is not pointless. If GCC generates bad code, tell the
> >> GCC people. The volatile keyword is essential.
> >
> > no the "volatile" semantics are vague, trecherous and evil. It's a LOT
> > better to insert the well defined "barrier()" in the right places.
> 
> Look at:
> 
>  	http://en.wikipedia.org/wiki/Volatile_variable
> 
> This is just what is needed to prevent the compiler from making non working
> code during optimization.

and an entry level document at wikipedia is more important than the C
standard ;)

> 
> Also look at:
> 
>  	http://en.wikipedia.org/wiki/Memory_barrier
> 
> This is used to prevent out-of-order execution, not at all what is
> necessary.

I did not talk about memory barriers. In fact, barrier() is NOT a memory
barrier. It's a compiler optimization barrier!