linux-kernel - Re: [RFC PATCH] timekeeping: introduce timekeeping_is

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Message-ID: <20130912012531.GS31370@twins.programming.kicks-ass.net>
Date:	Thu, 12 Sep 2013 03:25:31 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	John Stultz <john.stultz@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Richard Cochran <richardcochran@...il.com>,
	Prarit Bhargava <prarit@...hat.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	lttng-dev@...ts.lttng.org
Subject: Re: [RFC PATCH] timekeeping: introduce timekeeping_is_busy()

On Wed, Sep 11, 2013 at 08:48:11PM -0400, Mathieu Desnoyers wrote:
> Thoughts ?


struct foo {
	...
};

spinlock_t foo_lock;
unsigned int foo_head = 0;
unsigned int foo_tail = 0;
struct foo foo_array[2];

void foo_assign(struct foo f)
{
	spin_lock(&foo_lock);
	foo_head++;
	smp_wmb();
	foo_array[foo_head & 1] = f;
	smp_wmb();
	foo_tail++;
	spin_unlock(&foo_lock);
}

struct foo foo_get(void)
{
	unsigned int tail, head;
	struct foo ret;

again:
	tail = ACCESS_ONCE(foo_tail);
	smp_rmb();
	ret = foo_array[tail & 1];
	smp_rmb();
	head = ACCESS_ONCE(foo_head);
	if (head - tail >= 2)
		goto again;

	return ret;
}

Should work and get you the most recent 'complete' foo even when
foo_get() is called nested inside foo_assign().



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