linux-kernel - Re: [PATCH] list: Prevent compiler reloads inside 'safe' list iteration

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Message-ID: <20200310154749.GZ2935@paulmck-ThinkPad-P72>
Date:   Tue, 10 Mar 2020 08:47:49 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     David Laight <David.Laight@...LAB.COM>
Cc:     'Marco Elver' <elver@...gle.com>,
        Chris Wilson <chris@...is-wilson.co.uk>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "intel-gfx@...ts.freedesktop.org" <intel-gfx@...ts.freedesktop.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH] list: Prevent compiler reloads inside 'safe' list
 iteration

On Tue, Mar 10, 2020 at 03:05:57PM +0000, David Laight wrote:
> From: Marco Elver
> > Sent: 10 March 2020 14:10
> ...
> > FWIW, for writes we're already being quite generous, in that plain
> > aligned writes up to word-size are assumed to be "atomic" with the
> > default (conservative) config, i.e. marking such writes is optional.
> > Although, that's a generous assumption that is not always guaranteed
> > to hold (https://lore.kernel.org/lkml/20190821103200.kpufwtviqhpbuv2n@willie-the-truck/).
> 
> Remind me to start writing everything in assembler.

Been there, done that.  :-/

> That and to mark all structure members 'volatile'.

Indeed.  READ_ONCE() and WRITE_ONCE() get this same effect, but without
pessimizing non-concurrent accesses to those same members.  Plus KCSAN
knows about READ_ONCE(), WRITE_ONCE(), and also volatile members.

							Thanx, Paul