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

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Date:   Mon, 19 Aug 2019 11:34:47 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     paulmck@...ux.ibm.com
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "Joel Fernandes, Google" <joel@...lfernandes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alan Stern <stern@...land.harvard.edu>,
        rostedt <rostedt@...dmis.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.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 18/08/2019 00:00, Paul E. McKenney wrote:
[...]
> Linus noted that he believes that compilers for architectures supporting
> Linux can be trusted to avoid store-to-load transformations, invented
> stores, and unnecessary store tearing.  Should these appear, Linus would
> report a bug against the compiler and expect it to be fixed.
> 
>> I'll be honest, it's not 100% clear to me when those optimizations can
>> actually be done (maybe the branch thingy but the others are dubious), and
>> it's even less clear when compilers *actually* do it - only that they have
>> been reported to do it (so it's not made up).
> 
> There is significant unclarity inherent in the situation.  The standard
> says one thing, different compilers do other things, and developers
> often expect yet a third thing.  And sometimes things change over time,
> for example, the ca. 2011 dictim against compilers inventing data races.
> 
> Hey, they didn't teach me this aspect of software development in school,
> either.  ;-)
> 

Gotta keeps things "interesting" somehow, eh...

Thanks for the clarifications.

> 							Thanx, Paul
>