linux-kernel - Re: [PATCH v2] MCS spinlock: Use smp_cond_load

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Date:	Thu, 14 Apr 2016 09:53:17 +0100
From:	Will Deacon <will.deacon@....com>
To:	Jason Low <jason.low2@...com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"paulmck@...ux.vnet.ibm.com" <paulmck@...ux.vnet.ibm.com>,
	"Rudd, Terry (HP Cloud Systems Linux R&D)" <terry.rudd@....com>,
	"Long, Wai Man" <waiman.long@....com>,
	"boqun.feng@...il.com" <boqun.feng@...il.com>,
	"dave@...olabs.net" <dave@...olabs.net>,
	kbuild test robot <lkp@...el.com>,
	"kbuild-all@...org" <kbuild-all@...org>, jason.low2@....com
Subject: Re: [PATCH v2] MCS spinlock: Use smp_cond_load_acquire()

On Wed, Apr 13, 2016 at 01:49:04PM -0700, Jason Low wrote:
> On Wed, 2016-04-13 at 10:43 -0700, Will Deacon wrote:
> > On Tue, Apr 12, 2016 at 08:02:17PM -0700, Jason Low wrote:
> > > For qspinlocks on ARM64, we would like to use WFE instead
> > > of purely spinning. Qspinlocks internally have lock
> > > contenders spin on an MCS lock.
> > > 
> > > Update arch_mcs_spin_lock_contended() such that it uses
> > > the new smp_cond_load_acquire() so that ARM64 can also
> > > override this spin loop with its own implementation using WFE.
> > > 
> > > On x86, it can also cheaper to use this than spinning on
> > > smp_load_acquire().
> > > 
> > > Signed-off-by: Jason Low <jason.low2@...com>
> > 
> > FWIW, we just override arch_mcs_spin_lock_contended entirely for arch/arm/
> > and use wfe there so we could do the same for arm64 in mainline already.
> 
> Right, I was also thinking about that, although when we use
> smp_cond_load_acquire() in the generic implementation, would we just end
> up overriding it for the arch/arm64 version with the same thing?  :)

Sure, and we can convert to smp_cond_load_acquire then. I was just thinking
that you can avoid the immediate dependency on Peter's stuff, that's all.

Will