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Re: [PATCH] Linux: Implement membarrier function
On Thu, Nov 29, 2018 at 09:44:22AM -0500, Mathieu Desnoyers wrote:
> ----- On Nov 29, 2018, at 8:50 AM, Florian Weimer fweimer@redhat.com wrote:
>
> > * Torvald Riegel:
> >
> >> On Wed, 2018-11-28 at 16:05 +0100, Florian Weimer wrote:
> >>> This is essentially a repost of last year's patch, rebased to the glibc
> >>> 2.29 symbol version and reflecting the introduction of
> >>> MEMBARRIER_CMD_GLOBAL.
> >>>
> >>> I'm not including any changes to manual/ here because the set of
> >>> supported operations is evolving rapidly, we could not get consensus for
> >>> the language I proposed the last time, and I do not want to contribute
> >>> to the manual for the time being.
> >>
> >> Fair enough. Nonetheless, can you summarize how far you're along with
> >> properly defining the semantics (eg, based on the C/C++ memory model)?
> >
> > I wrote down what you could, but no one liked it.
> >
> > <https://sourceware.org/ml/libc-alpha/2017-12/msg00796.html>
> >
> > I expect that a formalization would interact in non-trivial ways with
> > any potential formalization of usable relaxed memory order semantics,
> > and I'm not sure if anyone knows how to do the latter today.
>
> Adding Paul E. McKenney in CC.
There is some prototype C++ memory model wording from David Goldblatt (CCed)
here (search for "Standarese"):
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1202r0.pdf
David's key insight is that (in Linuxese) light fences cannot pair with
each other.
Thanx, Paul