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

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Message-ID: <20190909062111.GA14572@gondor.apana.org.au>
Date:   Mon, 9 Sep 2019 16:21:11 +1000
From:   Herbert Xu <herbert@...dor.apana.org.au>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     torvalds@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        tglx@...utronix.de, joel@...lfernandes.org,
        stern@...land.harvard.edu, rostedt@...dmis.org,
        valentin.schneider@....com, linux-kernel@...r.kernel.org,
        paulmck@...ux.ibm.com, boqun.feng@...il.com, will.deacon@....com,
        dhowells@...hat.com
Subject: Re: [PATCH 1/1] Fix: trace sched switch start/stop racy updates

Peter Zijlstra <peterz@...radead.org> wrote:
>
> Paulmck actually has an example of that somewhere; ISTR that particular
> case actually got fixed by GCC, but I'd really _love_ for some compiler
> people (both GCC and LLVM) to state that their respective compilers will
> not do load/store tearing for machine word sized load/stores.
> 
> Without this written guarantee (which supposedly was in older GCC
> manuals but has since gone missing), I'm loathe to rely on it.

IIRC in that case gcc actually broke atomic writes even with a
volatile keyword.  So even WRITE_ONCE wouldn't have saved us.

Cheers,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt