Re: [libc-alpha] Re: PATCH: Fix ll/sc for mips

["H . J . Lu" <hjl@lucon.org>]

On Fri, Feb 01, 2002 at 01:59:03PM +0900, Hiroyuki Machida wrote:
> 
> From: Kaz Kylheku <kaz@ashi.footprints.net>
> Subject: Re: [libc-alpha] Re: PATCH: Fix ll/sc for mips
> Date: Thu, 31 Jan 2002 20:02:25 -0800 (PST)
> 
> > On Fri, 1 Feb 2002, Hiroyuki Machida wrote:
> > > Please note that "sc" may fail even if nobody write the
> > > variable. (See P.211 "8.4.2 Load-Linked/Sotre-Conditional" of "See 
> > > MIPS RUN" for more detail.) 
> > > So, after your patch applied, compare_and_swap() may fail, even if
> > > *p is equal to oldval.
> > 
> > I can't think of anything that will break because of this, as long
> > as the compare_and_swap eventually succeeds on some subsequent trial.
> > If the atomic operation has to abort for some reason other than *p being
> > unequal to oldval, that should be cool.
> 
> I mean that this patch breaks the spec of compare_and_swap().
> In most case, this patch may works as Kaz said. If this patch have
> no side-effect to any application, it's ok to apply the patch. But
> we can't know how to use compare_and_swap() in all aplications in a
> whole world. So we have to follow the spec.  
> 

Please note that the old compare_and_swap is broken. If you use
compare_and_swap to check if *p == oldval, my patch doesn't help
you. But if you use it to swap old/new, my patch works fine. But I
don't think you can use it check if *p == oldval since *p can change
at any time. It is the same as simply using "*p == oldval". I don't
see my patch should break any sane applications.


H.J.