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

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Message-ID: <20180625163828.GV2458@hirez.programming.kicks-ass.net>
Date:   Mon, 25 Jun 2018 18:38:28 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Daniel Lustig <dlustig@...dia.com>
Cc:     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>,
        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>,
        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 08:44:14AM -0700, Daniel Lustig wrote:
> RISC-V is (other-)multi-copy-atomic, so I don't think transitivity
> should be an issue here.

Ah, ok.

> We do have a "fence w,r", but we decided to warn against actually using
> it for a few reasons: 1) lack of known common use cases :), 2) IIRC
> there was some corner case discrepancy between the axiomatic and
> operational models if we allowed it, and 3) in practice, it's already
> both expensive enough and obscure enough that many or most
> implementations will simply just treat it as "fence rw,rw" anyway.

Because the majority of the cost is flushing the store-buffer in either
case?

> So, in theory, "fence w,r" should be enough to prevent SB-like patterns.
> It's just not yet clear that it's a big enough win that it's worth
> creating a new fence macro for it, or pulling the current RISC-V
> recommendation against its use.  What do you all think?

It was mostly a theoretical argument for why smp_mb() is too strong, not
a real practical desire to have w,t.