linux-kernel - Re: [PATCH -v2 4/6] locking, arch: Update spin_unlock

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Message-ID: <bff5e0b4-c572-cf4c-82fe-a672b8b5fb14@mellanox.com>
Date:	Thu, 26 May 2016 17:10:36 -0400
From:	Chris Metcalf <cmetcalf@...lanox.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	<linux-kernel@...r.kernel.org>, <torvalds@...ux-foundation.org>,
	<manfred@...orfullife.com>, <dave@...olabs.net>,
	<paulmck@...ux.vnet.ibm.com>, <will.deacon@....com>
CC:	<boqun.feng@...il.com>, <Waiman.Long@....com>, <tj@...nel.org>,
	<pablo@...filter.org>, <kaber@...sh.net>, <davem@...emloft.net>,
	<oleg@...hat.com>, <netfilter-devel@...r.kernel.org>,
	<sasha.levin@...cle.com>, <hofrat@...dl.org>, <rth@...ddle.net>,
	<vgupta@...opsys.com>, <linux@...linux.org.uk>,
	<realmz6@...il.com>, <rkuo@...eaurora.org>, <tony.luck@...el.com>,
	<james.hogan@...tec.com>, <ralf@...ux-mips.org>,
	<dhowells@...hat.com>, <jejb@...isc-linux.org>,
	<mpe@...erman.id.au>, <schwidefsky@...ibm.com>,
	<ysato@...rs.sourceforge.jp>, <chris@...kel.net>
Subject: Re: [PATCH -v2 4/6] locking, arch: Update spin_unlock_wait()

On 5/26/2016 10:19 AM, Peter Zijlstra wrote:
> --- a/arch/tile/lib/spinlock_32.c
> +++ b/arch/tile/lib/spinlock_32.c
> @@ -72,10 +72,14 @@ void arch_spin_unlock_wait(arch_spinlock
>   	if (next == curr)
>   		return;
>   
> +	smp_rmb();
> +
>   	/* Wait until the current locker has released the lock. */
>   	do {
>   		delay_backoff(iterations++);
>   	} while (READ_ONCE(lock->current_ticket) == curr);
> +
> +	smp_acquire__after_ctrl_dep();
>   }
>   EXPORT_SYMBOL(arch_spin_unlock_wait);
>   
> --- a/arch/tile/lib/spinlock_64.c
> +++ b/arch/tile/lib/spinlock_64.c
> @@ -72,10 +72,14 @@ void arch_spin_unlock_wait(arch_spinlock
>   	if (arch_spin_next(val) == curr)
>   		return;
>   
> +	smp_rmb();
> +
>   	/* Wait until the current locker has released the lock. */
>   	do {
>   		delay_backoff(iterations++);
>   	} while (arch_spin_current(READ_ONCE(lock->lock)) == curr);
> +
> +	smp_acquire__after_ctrl_dep();
>   }
>   EXPORT_SYMBOL(arch_spin_unlock_wait);

The smp_rmb() are unnecessary for tile.  We READ_ONCE next/curr from the
lock and compare them, so we know the load(s) are complete.  There's no
microarchitectural speculation going on so that's that.  Then we READ_ONCE
the next load on the lock from within the wait loop, so our load/load
ordering is guaranteed.

With that change,

Acked-by: Chris Metcalf <cmetcalf@...lanox.com> [for tile]

-- 
Chris Metcalf, Mellanox Technologies
http://www.mellanox.com