linux-kernel - Re: PATCH? rcu_do_batch: fix a pure theoretical memory ordering race

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Message-ID: <20061203221227.GA468@oleg>
Date:	Mon, 4 Dec 2006 01:12:27 +0300
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Eric Dumazet <dada1@...mosbay.com>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Andrew Morton <akpm@...l.org>,
	Alan Stern <stern@...land.harvard.edu>,
	linux-kernel@...r.kernel.org
Subject: Re: PATCH? rcu_do_batch: fix a pure theoretical memory ordering race

On 12/03, Eric Dumazet wrote:
>
> Oleg Nesterov a ?crit :
> 
> Yes, but how is it related to RCU ?
> I mean, rcu_do_batch() is just a loop like others in kernel.
> The loop itself is not buggy, but can call a buggy function, you are right.

	int start_me_again;

	struct rcu_head rcu_head;

	void rcu_func(struct rcu_head *rcu)
	{
		start_me_again = 1;
	}

	// could be called on arbitrary CPU
	void check_start_me_again(void)
	{
		static spinlock_t lock;

		spin_lock(lock);
		if (start_me_again) {
			start_me_again = 0;
			call_rcu(&rcu_head, rcu_func);
		}
		spin_unlock(lock);
	}

I'd say this code is not buggy.

In case it was not clear. I do not claim we need this patch (I don't know).
And yes, I very much doubt we can hit this problem in practice (even if I am
right).

What I don't agree with is that it is callback which should take care of this
problem.

> A smp_rmb() wont avoid all possible bugs...

For example?

Oleg.

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