Re: [PATCH] Export current_is_keventd() for libphy

Linus Torvalds
2006-12-06T19:03:38Z

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