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

Alan Stern
2018-07-17T19:47:16Z

On Tue, 17 Jul 2018, Peter Zijlstra wrote:

> Isn't ISYNC the instruction-sync pipeline flush instruction? That is
> used as an smp_rmb() here to, together with the control dependency from the
> LL/SC, to form a LOAD->{LOAD,STORE} (aka LOAD-ACQUIRE) ordering?

That's right.

> Where LWSYNC provides a TSO like ordering and SYNC provides a full
> transitive barrier aka. smp_mb() (althgouh I think it is strictly
> stronger than smp_mb() since it also implies completion, which smp_mb()
> does not).

What is the difference? That is, can you give a litmus test for
"completion" in this sense?

> And since both LL/SC-CTRL + ISYNC / LWSYNC are strictly CPU local, they
> cannot be used to create RCsc ordering.

Yes, sort of -- it depends on whether you consider the store buffer to
be part of the CPU. But in any case, you are right that none of these
things can create RCsc ordering.

Alan