Re: [PATCH] Export current_is_keventd() for libphy

[Linus Torvalds <torvalds@osdl.org>]

On Wed, 6 Dec 2006, David Howells wrote:
>
> Linus Torvalds <torvalds@osdl.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
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/