linux-kernel - Re: [PATCH 3.18-rc3 v9 1/5] irqchip: gic: Finer grain locking for gic_raise

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Message-ID: <alpine.LFD.2.11.1411251510231.11690@knanqh.ubzr>
Date:	Tue, 25 Nov 2014 15:17:17 -0500 (EST)
From:	Nicolas Pitre <nicolas.pitre@...aro.org>
To:	Marc Zyngier <marc.zyngier@....com>
cc:	Daniel Thompson <daniel.thompson@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jason Cooper <jason@...edaemon.net>,
	Russell King <linux@....linux.org.uk>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"patches@...aro.org" <patches@...aro.org>,
	"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
	John Stultz <john.stultz@...aro.org>,
	Sumit Semwal <sumit.semwal@...aro.org>,
	Dirk Behme <dirk.behme@...bosch.com>,
	Daniel Drake <drake@...lessm.com>,
	Dmitry Pervushin <dpervushin@...il.com>,
	Tim Sander <tim@...eglstein.org>
Subject: Re: [PATCH 3.18-rc3 v9 1/5] irqchip: gic: Finer grain locking for
 gic_raise_softirq

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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