Hiroyuki Machida <machida@sm.sony.co.jp> writes: |> 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. There is no way to find out anything about intermediate values of *p when compare_and_swap returns zero. The value of *p can change anytime, even if it only was different from oldval just at the time compare_and_swap did the comparison. So there is zero chance that a spurious failure of compare_and_swap breaks anything. Andreas. -- Andreas Schwab, SuSE Labs, schwab@suse.de SuSE GmbH, Deutschherrnstr. 15-19, D-90429 Nürnberg Key fingerprint = 58CA 54C7 6D53 942B 1756 01D3 44D5 214B 8276 4ED5 "And now for something completely different."
