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

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Date:	Tue, 25 Nov 2014 21:10:54 +0000
From:	Daniel Thompson <daniel.thompson@...aro.org>
To:	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Marc Zyngier <marc.zyngier@....com>
CC:	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 25/11/14 20:17, Nicolas Pitre wrote:
> 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

Well remembered! That patch had a different motivation but is very
similar to mine... so much so I might steal bit of it.

I'll make sure I put Stephen on Cc: when I respin with the changes Marc
requested.

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