linux-kernel - Re: [PATCH] Export current_is

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Date:	Wed, 6 Dec 2006 11:02:56 -0800 (PST)
From:	Linus Torvalds <torvalds@...l.org>
To:	David Howells <dhowells@...hat.com>
cc:	Andrew Morton <akpm@...l.org>,
	"Maciej W. Rozycki" <macro@...ux-mips.org>,
	Roland Dreier <rdreier@...co.com>,
	Andy Fleming <afleming@...escale.com>,
	Ben Collins <ben.collins@...ntu.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Jeff Garzik <jeff@...zik.org>, linux-arch@...r.kernel.org
Subject: Re: [PATCH] Export current_is_keventd() for libphy 

On Wed, 6 Dec 2006, David Howells wrote:
>
> Linus Torvalds <torvalds@...l.org> wrote:
> 
> >  (a) "volatile" on kernel data is basically always a bug, and you should 
> >      use locking.
> 
> But what about when you're building a lock?  Actually, I suspect correct usage
> of asm constraints and memory barriers trumps volatile anyway even there.

The word you look for is not "suspect".

You _cannot_ build a lock using "volatile", unless your CPU is strictly 
in-order and has an in-order memory subsystem too (so, for example, while 
all ia64 implementations today are in-order, they do /not/ have an 
in-order memory subsystem). Only then could you do locking with volatile 
and some crazy Peterson's algorithm.

I don't think any such CPU actually exists.

Anyway, we've had this discussion before on linux-kernel, it really boils 
down to that "volatile" is basically never correct with the exception of 
flags that don't have any meaning and that you don't actually _care_ about 
the exact value (the low word of "jiffies" being the canonical example of 
something where "volatile" is actually fine, and where - as long as you 
can load it atomically - "volatile" really does make sense).

		Linus
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