linux-kernel - Re: [PATCH 3/4] locking: Introduce smp_cond

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Message-ID: <20151203194139.GA5650@linux-uzut.site>
Date:	Thu, 3 Dec 2015 11:41:39 -0800
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	mingo@...nel.org, oleg@...hat.com, 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,
	waiman.long@....com, pjt@...gle.com
Subject: Re: [PATCH 3/4] locking: Introduce smp_cond_acquire()

On Thu, 03 Dec 2015, Peter Zijlstra wrote:

>+/**
>+ * 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_rmb(); /* ctrl + rmb := acquire */	\
>+} while (0)

So this hides the fact that we actually are waiting on the cond, as opposed
to conditional acquiring. Could it be renamed to something like smp_waitcond_acquire()?

Thanks,
Davidlohr
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