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

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Message-ID: <3c0cb8a2-eba2-7bea-8523-b948253a6804@arm.com>
Date:   Fri, 16 Aug 2019 23:27:46 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Joel Fernandes <joel@...lfernandes.org>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Alan Stern <stern@...land.harvard.edu>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        rostedt <rostedt@...dmis.org>,
        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 16/08/2019 21:57, Joel Fernandes wrote:
>> Can we finally put a foot down and tell compiler and standard committee
>> people to stop this insanity?
> 
> Sure, or could the compilers provide flags which prevent such optimization
> similar to -O* flags?
> 

How would you differentiate optimizations you want from those you don't with
just a flag? There's a reason we use volatile casts instead of declaring
everything volatile: we actually *want* those optimizations. It just so
happens that we don't want them *in some places*, and we have tools to tag
them as such.

The alternative is having a compiler that can magically correlate e.g. locked
writes with lock-free reads and properly handle them, but I don't think
there's a foolproof way of doing that.

> thanks,
> 
>  - Joel
>