linux-kernel - Re: [PATCH] doc: Update wake_up() & co. memory-barrier guarantees

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Message-ID: <2779.1529928765@warthog.procyon.org.uk>
Date:   Mon, 25 Jun 2018 13:12:45 +0100
From:   David Howells <dhowells@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     dhowells@...hat.com,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org,
        Alan Stern <stern@...land.harvard.edu>,
        Will Deacon <will.deacon@....com>,
        Boqun Feng <boqun.feng@...il.com>,
        Nicholas Piggin <npiggin@...il.com>,
        Jade Alglave <j.alglave@....ac.uk>,
        Luc Maranget <luc.maranget@...ia.fr>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Akira Yokosawa <akiyks@...il.com>,
        Daniel Lustig <dlustig@...dia.com>,
        Jonathan Corbet <corbet@....net>,
        Ingo Molnar <mingo@...hat.com>,
        Randy Dunlap <rdunlap@...radead.org>
Subject: Re: [PATCH] doc: Update wake_up() & co. memory-barrier guarantees

Peter Zijlstra <peterz@...radead.org> wrote:

> So yes, I suppose we're entirely suck with the full memory barrier
> semantics like that. But I still find it easier to think of it like a
> RELEASE that pairs with the ACQUIRE of waking up, such that the task
> is guaranteed to observe it's own wake condition.
> 
> And maybe that is the thing I'm missing here. These comments only state
> that it does in fact imply a full memory barrier, but do not explain
> why, should it?

I think because RELEASE and ACQUIRE concepts didn't really exist in Linux at
the time I wrote the doc, so the choices were read/readdep, write or full.

Since this document defines the *minimum* you can expect rather than what the
kernel actually gives you, I think it probably makes sense to switch to
RELEASE and ACQUIRE here.

David