Re: [PATCH v2] tools/memory-model: Add extra ordering for locks and remove it for ordinary release/acquire

Andrea Parri
2018-07-17T19:40:21Z

> > That said, I don't understand the powerpc memory ordering. I thought
> > the rules were "isync on lock, lwsync on unlock".
> >
> > That's what the AIX docs imply, at least.
> >
> > In particular, I find:
> >
> > "isync is not a memory barrier instruction, but the
> > load-compare-conditional branch-isync sequence can provide this
> > ordering property"
> >
> > so why are you doing "sync/lwsync", when it sounds like "isync/lwsync"
> > (for lock/unlock) is the right thing and would already give memory
> > barrier semantics?
>
> The PowerPC guys will correct me if I miss something here...

[Same here.]

>
> The isync provides ordering roughly similar to lwsync, but nowhere near
> as strong as sync, and it is sync that would be needed to cause lock
> acquisition to provide full ordering.

IIRC, ctrl+isync is even *weaker* than lwsync in certain respects, e.g.,
the former doesn't provide A-cumulativity according to the architectural
intent.

>The reason for using lwsync instead
> of isync is that the former proved to be faster on recent hardware.

Interesting; can you add some references about this?

Andrea

> The reason that the kernel still has the ability to instead generate
> isync instructions is that some older PowerPC hardware does not provide
> the lwsync instruction. If the hardware does support lwsync, the isync
> instructions are overwritten with lwsync at boot time.
>
> Thanx, Paul
>