From: "H . J . Lu" <hjl@lucon.org>
Subject: Re: [libc-alpha] Re: PATCH: Fix ll/sc for mips
Date: Thu, 31 Jan 2002 23:00:50 -0800
> 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.
>
I know the orinal compare_and_swap() is bad, and I believe the
spec of compare_and_swap() as below;
compare_and_swap(p, oldval, newval)
{
retval = 0;
begin_atomic
if (*p==oldval) {
*p = newval;
retval = 1;
}
end_atomic
return retval;
}
So, compare_and_swap() should be ...
__compare_and_swap (a0 long int oldval, long int newval)
a0: *p
a1: oldval
a2: newval
v0: return value
.set noreorder
retry:
ll v0, (a0)
bne v0, a1
move v0, zero
move v0, a2
sc v0, (a0)
beqz v0, retry
nop
j ra
But, with your patch ...
.set noreorder
ll t0, (a0)
bne t0, a1
move v0, zero
move v0, a
sc v0, (a0)
j ra
In this way, compare_and_swap() was changed as
compare_and_swap(p, oldval, newval)
{
retval = 0;
begin_atomic
if (*p==oldval) {
if "sc" was failed goto out;
*p = newval;
retval = 1;
}
out:
end_atomic
return retval;
}
---
Hiroyuki Machida
Sony Corp.
