Re: [patch] spinlocks: remove 'volatile'

Chase Venters
2006-07-07T21:49:03Z

On Fri, 7 Jul 2006, linux-os \(Dick Johnson\) wrote:

> Again, I didn't propose to do that. In fact, your spin-lock
> code already inserts "rep nops" and I never implied that they
> should be removed. I said only that "volatile" still needs to
> be used, not some macro that tells the compiler that everything
> in memory probably got trashed. Read what I said, not what you
> think some idiot might have said.
>

Dude, are you even paying attention? "volatile" very much does not need to
be used (and as Linus points out, it is _wrong_). Since we're using GCC's
inline asm syntax _already_, it is perfectly sufficient to use the same
syntax to tell GCC that memory should be considered invalid.

Locks are supposed to be syncronization points, which is why they ALREADY
HAVE "memory" on the clobber list! "memory" IS NECESSARY. The fact
that "=m" is changing to "+m" in Linus's patches is because "=m" is in
fact insufficient, because it would let the compiler believe we're just
going to over-write the value. "volatile" merely hides that bug -- once
that bug is _fixed_ (by going to "+m"), "volatile" is no longer useful.
(It wasn't useful before, it just _papered over_ a problem).

If "volatile" is in use elsewhere (other than locks), it's still
probably wrong. In these cases, you can use a barrier, a volatile cast, or
an inline asm with a specific clobber.

Thanks,
Chase