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

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Message-ID: <20180625095031.GX2494@hirez.programming.kicks-ass.net>
Date:   Mon, 25 Jun 2018 11:50:31 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andrea Parri <andrea.parri@...rulasolutions.com>
Cc:     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>,
        David Howells <dhowells@...hat.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

On Mon, Jun 25, 2018 at 11:17:38AM +0200, Andrea Parri wrote:
> Both the implementation and the users' expectation [1] for the various
> wakeup primitives have evolved over time, but the documentation has not
> kept up with these changes: brings it into 2018.

I wanted to reply to this saying that I'm not aware of anything relying
on this actually being a smp_mb() and that I've been treating it as an
RELEASE.

But then I found my own comment that goes with smp_mb__after_spinlock(),
which explains why we do in fact need the transitive thing if I'm not
mistaken.

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?