linux-kernel - Re: rcu_read_lock lost its compiler barrier

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Date:   Tue, 4 Jun 2019 13:00:03 -0400 (EDT)
From:   Alan Stern <stern@...land.harvard.edu>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
cc:     "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Fengguang Wu <fengguang.wu@...el.com>, LKP <lkp@...org>,
        LKML <linux-kernel@...r.kernel.org>,
        Netdev <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Luc Maranget <luc.maranget@...ia.fr>,
        Jade Alglave <j.alglave@....ac.uk>
Subject: Re: rcu_read_lock lost its compiler barrier

On Tue, 4 Jun 2019, Linus Torvalds wrote:

> So I don't technically disagree with anything you say,

That's good to know!

>  I just wanted
> to point out that as far as the kernel is concerned, we do have higher
> quality expectations from the compiler than just "technically valid
> according to the C standard".

Which suggests asking whether these higher expectations should be
reflected in the Linux Kernel Memory Model.  So far we have largely
avoided doing that sort of thing, although there are a few exceptions.

(For example, we assume the compiler does not destroy address
dependencies from volatile reads -- but we also warn that this
assumption may fail if the programmer does not follow some rules
described in one of Paul's documentation files.)

Alan