Subject: Re: [PATCH] RFC: powerpc: expose the multi-bit ops that underlie single-bit ops.

From: Benjamin Herrenschmidt <[email-masked]>
Date: Fri, 19 Jun 2009 08:22:31 +1000

On Thu, 2009-06-18 at 16:30 -0400, Geoff Thorpe wrote:
> I've left the volatile qualifier in the generated API because I didn't
> feel so comfortable changing APIs, but I also added the "memory" clobber
> for all cases - whereas it seems the existing set_bits(), clear_bits(),
> [...] functions didn't declare this... Do you see any issue with having
> the 'volatile' in the prototype as well as the clobber in the asm?
> 
> Actually, might as well just respond to the new patch instead... :-) Thx.

I think the story with the memory clobber is that it depends whether
we consider the functions as ordering accesses or not (ie, can
potentially be used with lock/unlock semantics).

The general rule is that those who don't return anything don't need
to have those semantics, and thus could only be advertised as clobbering
p[word] -but- there are issues there. For example, despite the
(relatively new) official _lock/_unlock variants, there's still code
that abuses constructs like test_and_set_bit/clear_bit as locks and in
that case, clear bits needs a clobber.

So I would say at this stage better safe than having to track down
incredibly hard to find bugs, and let's make them all take that clobber.

Cheers,
Ben.