linux-kernel - Re: [PATCH 1/1] Fix: trace sched switch start/stop racy updates

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Message-ID: <20190820140116.GT2332@hirez.programming.kicks-ass.net>
Date:   Tue, 20 Aug 2019 16:01:16 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alan Stern <stern@...land.harvard.edu>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        rostedt <rostedt@...dmis.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Boqun Feng <boqun.feng@...il.com>,
        Will Deacon <will.deacon@....com>,
        David Howells <dhowells@...hat.com>
Subject: Re: [PATCH 1/1] Fix: trace sched switch start/stop racy updates

On Fri, Aug 16, 2019 at 09:52:17PM -0700, Paul E. McKenney wrote:
> On Fri, Aug 16, 2019 at 03:57:43PM -0700, Linus Torvalds wrote:
> 
> [ . . . ]
> 
> > We add READ_ONCE and WRITE_ONCE annotations when they make sense. Not
> > because of some theoretical "compiler is free to do garbage"
> > arguments. If such garbage happens, we need to fix the compiler, the
> > same way we already do with
> > 
> >   -fno-strict-aliasing
> 
> Yeah, the compete-with-FORTRAN stuff.  :-/
> 
> There is some work going on in the C committee on this, where the
> theorists would like to restrict strict-alias based optimizations to
> enable better analysis tooling.  And no, although the theorists are
> pushing in the direction we would like them to, as far as I can see
> they are not pushing as far as we would like.  But it might be that
> -fno-strict-aliasing needs some upgrades as well.  I expect to learn
> more at the next meeting in a few months.
> 
> http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2364.pdf
> http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2363.pdf
> http://www.open-std.org/jtc1/sc22/wg14/www/docs/n2362.pdf

We really should get the compiler folks to give us a
-fno-pointer-provenance. Waiting on the standards committee to get their
act together seems unlikely, esp. given that some people actually seem
to _want_ this nonsense :/