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

Andrea Parri
2018-07-12T20:25:11Z

On Thu, Jul 12, 2018 at 09:52:42PM +0200, Andrea Parri wrote:
> On Thu, Jul 12, 2018 at 11:10:58AM -0700, Linus Torvalds wrote:
> > On Thu, Jul 12, 2018 at 11:05 AM Peter Zijlstra <peterz@infradead.org> wrote:
> > >
> > > The locking pattern is fairly simple and shows where RCpc comes apart
> > > from expectation real nice.
> >
> > So who does RCpc right now for the unlock-lock sequence? Somebody
> > mentioned powerpc. Anybody else?
>
> powerpc have RCtso (and RCpc) but not RCsc unlock-lock, according to the
> following indeed original terminology:
>
> - RCsc unlock-lock MUST ORDER:
>
> a) the WRITE and the READ below:
>
> WRITE x=1
> UNLOCK s
> LOCK s
> READ y
>
> as in a store-buffering test;
>
> b) the two WRITEs below:
>
> WRITE x=1
> UNLOCK s
> LOCK s
> WRITE y=1
>
> as in a message-passing test;
>
> c) the two READs below:
>
> READ x
> UNLOCK s
> LOCK s
> READ y
>
> as in a message-passing test;
>
> d) the READ and the WRITE below:
>
> READ x
> UNLOCK s
> LOCK s
> WRITE y
>
> as in a load-buffering test;
>
> - RCtso unlock-lock MUST ORDER b), c), d) above.
>
> - RCpc unlock-lock MUST ORDER none of the above.
>
> AFAICT, all arch _in_ the current implementation have RCtso unlock-lock.
>
>
> >
> > How nasty would be be to make powerpc conform? I will always advocate
> > tighter locking and ordering rules over looser ones..
>
> A simple answer is right above (place a sync somewhere in the sequence);
> for benchmark results, I must defer...

Sorry, not sure why but I did intend "conform to RCsc" here.

>
> Andrea
>
>
> >
> > Linus