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