linux-kernel - Re: [RFC] arm64: Implement WFE based spin wait for MCS spinlocks

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Date:	Wed, 20 Apr 2016 12:36:51 -0700
From:	Jason Low <jason.low2@...com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Will Deacon <will.deacon@....com>,
	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>,
	terry.rudd@....com, "Long, Wai Man" <waiman.long@....com>,
	"boqun.feng@...il.com" <boqun.feng@...il.com>,
	"dave@...olabs.net" <dave@...olabs.net>, jason.low2@...com
Subject: Re: [RFC] arm64: Implement WFE based spin wait for MCS spinlocks

On Wed, 2016-04-20 at 12:30 +0200, Peter Zijlstra wrote:
> On Thu, Apr 14, 2016 at 12:13:38AM -0700, Jason Low wrote:
> > Use WFE to avoid most spinning with MCS spinlocks. This is implemented
> > with the new cmpwait() mechanism for comparing and waiting for the MCS
> > locked value to change using LDXR + WFE.
> > 
> > Signed-off-by: Jason Low <jason.low2@...com>
> > ---
> >  arch/arm64/include/asm/mcs_spinlock.h | 21 +++++++++++++++++++++
> >  1 file changed, 21 insertions(+)
> >  create mode 100644 arch/arm64/include/asm/mcs_spinlock.h
> > 
> > diff --git a/arch/arm64/include/asm/mcs_spinlock.h b/arch/arm64/include/asm/mcs_spinlock.h
> > new file mode 100644
> > index 0000000..d295d9d
> > --- /dev/null
> > +++ b/arch/arm64/include/asm/mcs_spinlock.h
> > @@ -0,0 +1,21 @@
> > +#ifndef __ASM_MCS_SPINLOCK_H
> > +#define __ASM_MCS_SPINLOCK_H
> > +
> > +#define arch_mcs_spin_lock_contended(l)					\
> > +do {									\
> > +	int locked_val;							\
> > +	for (;;) {							\
> > +		locked_val = READ_ONCE(*l);				\
> > +		if (locked_val)						\
> > +			break;						\
> > +		cmpwait(l, locked_val);					\
> > +	}								\
> > +	smp_rmb();							\
> > +} while (0)
> 
> If you make the generic version use smp_cond_load_acquire() this isn't
> needed.

Yup, in the email thread about modifying the generic version to use
smp_cond_load_acquire(), I mentioned that overriding it in arch/arm64
would not be needed anymore.

Will made a suggestion about overriding it on arm64, but it turns out he
was just referring to avoiding the immediate dependency on
smp_cond_load_acquire().

Thanks,
Jason