linux-kernel - Re: [PATCH 4/4] locking: Introduce smp_cond

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Message-ID: <20151102135726.GR17308@twins.programming.kicks-ass.net>
Date:	Mon, 2 Nov 2015 14:57:26 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	mingo@...nel.org, oleg@...hat.com
Cc:	linux-kernel@...r.kernel.org, paulmck@...ux.vnet.ibm.com,
	boqun.feng@...il.com, corbet@....net, mhocko@...nel.org,
	dhowells@...hat.com, torvalds@...ux-foundation.org,
	will.deacon@....com
Subject: Re: [PATCH 4/4] locking: Introduce smp_cond_acquire()

On Mon, Nov 02, 2015 at 02:29:05PM +0100, Peter Zijlstra wrote:

> Note that while smp_cond_acquire() has an explicit
> smp_read_barrier_depends() for Alpha, neither sites it gets used in
> were actually buggy on Alpha for their lack of it. The first uses
> smp_rmb(), which on Alpha is a full barrier too and therefore serves
> its purpose. The second had an explicit full barrier.

> +/**
> + * smp_cond_acquire() - Spin wait for cond with ACQUIRE ordering
> + * @cond: boolean expression to wait for
> + *
> + * Equivalent to using smp_load_acquire() on the condition variable but employs
> + * the control dependency of the wait to reduce the barrier on many platforms.
> + *
> + * The control dependency provides a LOAD->STORE order, the additional RMB
> + * provides LOAD->LOAD order, together they provide LOAD->{LOAD,STORE} order,
> + * aka. ACQUIRE.
> + */
> +#define smp_cond_acquire(cond)	do {		\
> +	while (!(cond))				\
> +		cpu_relax();			\
> +	smp_read_barrier_depends(); /* ctrl */	\
> +	smp_rmb(); /* ctrl + rmb := acquire */	\
> +} while (0)

So per the above argument we could leave out the
smp_read_barrier_depends() for Alpha, although that would break
consistency with all the other control dependency primitives we have. It
would avoid issuing a double barrier.

Thoughts?
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