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

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Message-ID: <20180625105618.GA12676@andrea>
Date:   Mon, 25 Jun 2018 12:56:18 +0200
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 11:50:31AM +0200, Peter Zijlstra wrote:
> 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.

A concrete example being the store-buffering pattern reported in [1].


> 
> 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?

"code (people) is relying on it" is really the only "why" I can think
of.  With this patch, that same/SB pattern is also reported in memory
-barriers.txt.  Other ideas?

  Andrea