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

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Date:   Fri, 16 Aug 2019 16:44:10 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Alan Stern <stern@...land.harvard.edu>
Cc:     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>,
        Thomas Gleixner <tglx@...utronix.de>,
        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, 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.

Perhaps the commit message can be reworded to mention that the _ONCE
is an additional clean up for safe access.

thanks,

 - Joel