Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync

From: Oleg Nesterov <oleg@tv-sign.ru>
To: "Paul E. McKenney" <paulmck@us.ibm.com>,
	Alan Stern <stern@rowland.harvard.edu>
Cc: linux-kernel@vger.kernel.org
Subject: Re: [patch] cpufreq: mark cpufreq_tsc() as core_initcall_sync
Date: Thu, 23 Nov 2006 17:59:10 +0300	[thread overview]
Message-ID: <20061123145910.GA145@oleg> (raw)
In-Reply-To: <20061119190027.GA3676@oleg>

(Sorry, responding to the wrong message)

Paul E. McKenney wrote:
>
> I am concerned about this as well, and am beginning to suspect that I
> need to make a special-purpose primitive specifically for Jens that he
> can include with his code.

How about this?

	struct xxx_struct {
		int completed;
		atomic_t ctr[2];
		struct mutex mutex;
		wait_queue_head_t wq;
	};

	void init_xxx_struct(struct xxx_struct *sp)
	{
		sp->completed = 0;
		atomic_set(sp->ctr + 0, 1);	// active
		atomic_set(sp->ctr + 1, 0);	// inactive
		mutex_init(&sp->mutex);
		init_waitqueue_head(&sp->wq);
	}

	int xxx_read_lock(struct xxx_struct *sp)
	{
		for (;;) {
			int idx = sp->completed & 0x1;
			if (likely(atomic_inc_not_zero(sp->ctr + idx)))
				return idx;
		}
	}

	void xxx_read_unlock(struct xxx_struct *sp, int idx)
	{
		if (unlikely(atomic_dec_and_test(sp->ctr + idx)))
			wake_up(&sp->wq);
	}

	void synchronize_xxx(struct xxx_struct *sp)
	{
		int idx;

		mutex_lock(&sp->mutex);

		idx = ++sp->completed & 0x1;
		smp_mb__before_atomic_inc();
		atomic_inc(&sp->ctr + idx);

		idx = !idx;
		if (!atomic_dec_and_test(&sp->ctr + idx))
			__wait_event(&sp->wq, !atomic_read(&sp->ctr + idx));

		mutex_unlock(&sp->mutex);
	}

Yes, cache thrashing... But I think this is hard to avoid if we want writer
to be fast.

I do not claim this is the best solution, but for some reason I'd like to
suggest something that doesn't need synchronize_sched(). What do you think
about correctness at least?

Oleg.