Re: [Ksummit-discuss] [TECH TOPIC] asm-generic implementations of low-level synchronisation constructs

[Will Deacon <will.deacon@arm.com>]

On Thu, May 08, 2014 at 03:27:34PM +0100, Peter Zijlstra wrote:
> On Thu, May 08, 2014 at 11:13:12AM +0200, Peter Zijlstra wrote:
> > ATOMIC_RET(ptr, __ret, stmt)
> > ({
> > 	typeof(*ptr) __new, __val;
> > 
> > 	smp_mb__before_llsc();
> > 
> > 	do {
> > 		__val = load_locked(ptr);
> > 		stmt;
> > 	} while (!store_conditional(ptr, __new));
> > 
> > 	smp_mb__after_llsc();
> > 
> > 	__ret;
> > })
> 
> So the most common constraint (which you've confirmed is true for ARM as
> well) is that we should not have memory accesses in between an LL/SC.

Yup.

> Making sure GCC doesn't do any is tricky, the best I can come up with is
> tagging all variables with the register qualifier, like:
> 
> ATOMIC_RET(ptr, __ret, stmt)
> ({
> 	register typeof(*ptr) __new, __val;
> 
> 	smp_mb__before_llsc();
> 
> 	do {
> 		__val = load_locked(ptr);
> 		stmt;
> 	} while (!store_conditional(ptr, __new));
> 
> 	smp_mb__after_llsc();
> 
> 	__ret;
> })
> 
> Now, I'm not at all sure if register still means anything to GCC, but in
> the faint hope that it still sees it as a hint this might just work.

I just ran this past our compiler guys and they threw rocks at me. Even if
it happens to work today, I don't think we can rely on it tomorrow,
unfortunately.

I think that makes the case for extended the fine-grained atomics
implemented by each architecture, but it would still be great to have a way
to compose them.

Will