linux-kernel - Re: [PATCH 1/7] atomic: Export fetch

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Message-ID: <20151125091349.GW17308@twins.programming.kicks-ass.net>
Date:	Wed, 25 Nov 2015 10:13:49 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Chris Metcalf <cmetcalf@...hip.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Luiz Capitulino <lcapitulino@...hat.com>,
	Christoph Lameter <cl@...ux.com>,
	Ingo Molnar <mingo@...nel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 1/7] atomic: Export fetch_or()

On Tue, Nov 24, 2015 at 10:19:52PM +0100, Frederic Weisbecker wrote:
> Also note that or_return() means that you first do OR and then return the new value.

Yes, that's useless. OR is an irreversible operator, which means
or_return() looses data. You can never say if a bit included in the
mask was set before.

You really must have fetch_or().
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