linux-kernel - Re: [PATCH v2 5/5] rwsem: do optimistic spinning for writer lock acquisition

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Message-ID: <20130626125338.GK28407@twins.programming.kicks-ass.net>
Date:	Wed, 26 Jun 2013 14:53:38 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Tim Chen <tim.c.chen@...ux.intel.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Alex Shi <alex.shi@...el.com>,
	Andi Kleen <andi@...stfloor.org>,
	Michel Lespinasse <walken@...gle.com>,
	Davidlohr Bueso <davidlohr.bueso@...com>,
	Matthew R Wilcox <matthew.r.wilcox@...el.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Rik van Riel <riel@...hat.com>,
	Peter Hurley <peter@...leysoftware.com>,
	linux-kernel@...r.kernel.org, linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH v2 5/5] rwsem: do optimistic spinning for writer lock
 acquisition

On Mon, Jun 24, 2013 at 04:20:51PM -0700, Tim Chen wrote:
> +#ifdef CONFIG_RWSEM_SPIN_ON_WRITE_OWNER
> +static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
> +{
> +	int retval;
> +	task_struct *owner;
> +
> +	rcu_read_lock();
> +	owner = sem->owner;

That wants to be: owner = ACCESS_ONCE(sem->owner);

> +
> +	/* Spin only if active writer running */
> +	if (owner)
> +		retval = owner->on_cpu;
> +	else
> +		retval = false;
> +
> +	rcu_read_unlock();
> +	/*
> +	 * if lock->owner is not set, the sem owner may have just acquired
> +	 * it and not set the owner yet, or the sem has been released, or
> +	 * reader active.
> +	 */
> +	return retval;
> +}
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