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

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Date:	Wed, 20 Apr 2016 12:30:59 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jason Low <jason.low2@...com>
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>
Subject: Re: [RFC] arm64: Implement WFE based spin wait for MCS spinlocks

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.