linux-kernel - Re: [PATCH v2 2/3] locking/percpu-rwsem: Rework writer block/wake to not use wait-queues

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Date:   Mon, 5 Dec 2016 12:26:36 +0100
From:   Oleg Nesterov <oleg@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Davidlohr Bueso <dave@...olabs.net>, mingo@...nel.org,
        john.stultz@...aro.org, dimitrysh@...gle.com,
        linux-kernel@...r.kernel.org, Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH v2 2/3] locking/percpu-rwsem: Rework writer block/wake
        to not use wait-queues

Davidlohr, Peter, I'll try to read this patch later, just one note.

On 12/05, Peter Zijlstra wrote:
>
> On Fri, Dec 02, 2016 at 06:18:39PM -0800, Davidlohr Bueso wrote:
> > @@ -102,8 +103,13 @@ void __percpu_up_read(struct percpu_rw_semaphore *sem)
> > 	 */
> > 	__this_cpu_dec(*sem->read_count);
> >
> > +	rcu_read_lock();
> > +	writer = rcu_dereference(sem->writer);
>
> Don't think this is correct, I think Oleg suggested using
> task_rcu_dereference(), which is a giant pile of magic.

Yes, but on a second thought task_rcu_dereference() won't really help,
but we can just use rcu_dereference().

> The problem is that task_struct isn't RCU protected as such.

Yes. But percpu_down_write() should not be used after exit_notify(), so we
can rely on rcu_read_lock(), release_task()->call_rcu(delayed_put_task_struct)
can't be called until an exiting task passes exit_notify().

But then we probably need WARN_ON(current->exit_state) in percpu_down_write().

And personally I think this change should add the new helpers, they can have
more users. Something like

	struct xxx {
		struct task_struct *task;
	};

	xxx_wake_up(struct xxx *xxx)
	{
		rcu_read_lock();
		task = rcu_dereference(xxx->task);
		if (task)
			wake_up_process(task);
		rcu_read_unlock();
	}

	#define xxx_wait_event(xxx, event) {
		// comment to explain why
		WARN_ON(current->exit_state);

		xxx->task = current;

		...
	}

Oleg.