Date: Tue, 25 Nov 2014 15:17:17 -0500 (EST)
From: Nicolas Pitre <nicolas.pitre@linaro.org>
To: Marc Zyngier <marc.zyngier@arm.com>
cc: Daniel Thompson <daniel.thompson@linaro.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Jason Cooper <jason@lakedaemon.net>,
        Russell King <linux@arm.linux.org.uk>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-arm-kernel@lists.infradead.org" 
	<linux-arm-kernel@lists.infradead.org>,
        "patches@linaro.org" <patches@linaro.org>,
        "linaro-kernel@lists.linaro.org" <linaro-kernel@lists.linaro.org>,
        John Stultz <john.stultz@linaro.org>,
        Sumit Semwal <sumit.semwal@linaro.org>,
        Dirk Behme <dirk.behme@de.bosch.com>,
        Daniel Drake <drake@endlessm.com>,
        Dmitry Pervushin <dpervushin@gmail.com>,
        Tim Sander <tim@krieglstein.org>
Subject: Re: [PATCH 3.18-rc3 v9 1/5] irqchip: gic: Finer grain locking for
 gic_raise_softirq
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References: <1415968543-29469-1-git-send-email-daniel.thompson@linaro.org> <1416936401-5147-1-git-send-email-daniel.thompson@linaro.org> <1416936401-5147-2-git-send-email-daniel.thompson@linaro.org> <5474BF19.3040707@arm.com>
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On Tue, 25 Nov 2014, Marc Zyngier wrote:

> Hi Daniel,
> 
> On 25/11/14 17:26, Daniel Thompson wrote:
> > irq_controller_lock is used for multiple purposes within the gic driver.
> > Primarily it is used to make register read-modify-write sequences atomic.
> > It is also used by gic_raise_softirq() in order that the big.LITTLE
> > migration logic can figure out when it is safe to migrate interrupts
> > between physical cores.
> > 
> > The second usage of irq_controller_lock is difficult to discern when
> > reviewing the code because the migration itself takes place outside
> > the lock.
> > 
> > This patch makes the second usage more explicit by splitting it out into
> > a separate lock and providing better comments.
> 
> While we're at it, how about an additional patch that would make this
> lock disappear entirely when the big-little stuff is not compiled in,
> which is likely to be the case on a lot of (dare I say most?) systems?
> That will save expensive barriers that we definitely could do without.

For the record, I reviewed and ACKed a patch doing exactly that a while 
ago:

http://lkml.org/lkml/2014/8/13/486

As far as I can see, no follo-ups happened.


Nicolas
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