linux-kernel - Re: [PATCH v2] context_tracking: remove local_irq_save from __acct_update

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Message-ID: <20150427111842.GA4491@osiris>
Date:	Mon, 27 Apr 2015 13:18:42 +0200
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Rik van Riel <riel@...riel.com>
Cc:	linux-kernel@...r.kernel.org,
	Andy Lutomirsky <amluto@...capital.com>,
	Frederic Weisbecker <fweisbec@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>, williams@...hat.com
Subject: Re: [PATCH v2] context_tracking: remove local_irq_save from
 __acct_update_integrals

On Sat, Apr 25, 2015 at 08:50:49AM -0400, Rik van Riel wrote:
> On 04/25/2015 05:43 AM, Heiko Carstens wrote:
> > ...the READ_ONCE() doesn't give you any guarantees about reading
> > tsk->acct_timexpd in an atomic way.
> > Well, actually you don't need atomic semantics, but only to make sure that
> > the read access happens with a single instruction, since you want to protect
> > against interrupts.
> > But still: if the size of acct_timexpd is 64 bit READ_ONCE() may still result
> > in two instructions on 32 bit architectures.
> > (or isn't there currently no 32 bit architecture with 64 bit cputime_t left?)
> 
> Even if there is (maybe some ARM system?), can we even guarantee
> that a single instruction to read 64 bits exists on such a system?

I wouldn't bet on it. I can only talk for s390 and there is an instruction
available which would do that. But since s390 is now a 64 bit only architecture
it doesn't matter anyway.
For other architectures I'd say: no, you can't rely on that.

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