[PATCH RESEND v7 4/6] arm64: futex: refactor futex atomic operation

[Catalin Marinas]

On Mon, Sep 15, 2025 at 11:32:39AM +0100, Yeoreum Yun wrote:
> > On Sat, Aug 16, 2025 at 04:19:27PM +0100, Yeoreum Yun wrote:
> > > diff --git a/arch/arm64/include/asm/futex.h b/arch/arm64/include/asm/futex.h
> > > index bc06691d2062..ab7003cb4724 100644
> > > --- a/arch/arm64/include/asm/futex.h
> > > +++ b/arch/arm64/include/asm/futex.h
> > > @@ -7,17 +7,21 @@
> > >
> > >  #include <linux/futex.h>
> > >  #include <linux/uaccess.h>
> > > +#include <linux/stringify.h>
> > >
> > >  #include <asm/errno.h>
> > >
> > > -#define FUTEX_MAX_LOOPS	128 /* What's the largest number you can think of? */
> > > +#define LLSC_MAX_LOOPS	128 /* What's the largest number you can think of? */
> >
> > I just noticed - you might as well leave the name as is here, especially
> > if in patch 6 you align down address and use CAS on a 64-bit value as
> > per Will's comment (and it's no longer LLSC). I think renaming this is
> > unnecessary.
> 
> Okay. I'll restore to use origin name.
> But I think LSUI wouldn't be used with CAS according to patch 6's
> comments from you and additionally i think
> chaning the CAS would make a failure because of
> change of unrelated field. i.e)
> 
> struct user_structure{
>   uint32 futex;
>   uint32 some_value;
> };
> 
> In this case, the change of some_value from user side could make a
> failure of futex atomic operation.

Yes but the loop would read 'some_value' again, fold in 'futex' and
retry. It would eventually succeed or fail after 128 iterations if the
user keeps changing that location. Note that's also the case with LL/SC,
the exclusive monitor would be cleared by some store in the same cache
line (well, depending on the hardware implementation) and the STXR fail.
>From this perspective, CAS has better chance of succeeding.

> So I think it would be better to keep the current LLSC implementation
> in LSUI.

I think the code would look simpler with LL/SC but you can give it a try
and post the code sample here (not in a new series).

BTW, is there a test suite for all the futex operations? The cover
letter did not mention any.

-- 
Catalin