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

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Date:   Fri, 16 Aug 2019 22:49:04 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Joel Fernandes <joel@...lfernandes.org>
cc:     Alan Stern <stern@...land.harvard.edu>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        rostedt <rostedt@...dmis.org>,
        Valentin Schneider <valentin.schneider@....com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        paulmck <paulmck@...ux.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Will Deacon <will.deacon@....com>,
        David Howells <dhowells@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 1/1] Fix: trace sched switch start/stop racy updates

On Fri, 16 Aug 2019, Joel Fernandes wrote:
> On Fri, Aug 16, 2019 at 3:19 PM Alan Stern <stern@...land.harvard.edu> wrote:
> > On Fri, 16 Aug 2019, Mathieu Desnoyers wrote:
> >
> > > If you choose not to use READ_ONCE(), then the "load tearing" issue can
> > > cause similar spurious 1 -> 0 -> 1 transitions near 16-bit counter
> > > overflow as described above. The "Invented load" also becomes an issue,
> > > because the compiler could use the loaded value for a branch, and re-load
> > > that value between two branches which are expected to use the same value,
> > > effectively generating a corrupted state.
> > >
> > > I think we need a statement about whether READ_ONCE/WRITE_ONCE should
> > > be used in this kind of situation, or if we are fine dealing with the
> > > awkward compiler side-effects when they will occur.
> >
> > The only real downside (apart from readability) of READ_ONCE and
> > WRITE_ONCE is that they prevent the compiler from optimizing accesses
> > to the location being read or written.  But if you're just doing a
> > single access in each place, not multiple accesses, then there's
> > nothing to optimize anyway.  So there's no real reason not to use
> > READ_ONCE or WRITE_ONCE.
> 
> I am also more on the side of using *_ONCE. To me, by principal, I
> would be willing to convert any concurrent plain access using _ONCE,
> just so we don't have to worry about it now or in the future and also
> documents the access.

By that argumentation we need to plaster half of the kernel with _ONCE()
and I'm so not looking forward to the insane amount of script kiddies
patches to do that.

Can we finally put a foot down and tell compiler and standard committee
people to stop this insanity?

Thanks,

	tglx