linux-kernel - Re: [PATCH] rcu/nocb: Fix the WARN_ON_ONCE() in rcu_nocb_rdp

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Message-ID: <ZxY-jRKBf8672czT@localhost.localdomain>
Date: Mon, 21 Oct 2024 13:44:13 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Z qiang <qiang.zhang1211@...il.com>
Cc: paulmck@...nel.org, neeraj.upadhyay@...nel.org, joel@...lfernandes.org,
	urezki@...il.com, boqun.feng@...il.com, rcu@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] rcu/nocb: Fix the WARN_ON_ONCE() in
 rcu_nocb_rdp_deoffload()

Le Mon, Oct 21, 2024 at 07:01:02PM +0800, Z qiang a écrit :
> > For the non-nocb cpus set during boot, the corresponding
> > rcuop kthread, we should park directly, otherwise
> > WARN_ON_ONCE(!rcu_rdp_is_offloaded(rdp)) will be triggered.

Ah but this case is different. kthread_park() is called on
the kthread that is freshly created. In that case it is
parked before the kthread even had a chance to call its handler
(which is rcu_nocb_cb_kthread()).

See these lines in kthread():

	/* OK, tell user we're spawned, wait for stop or wakeup */
	__set_current_state(TASK_UNINTERRUPTIBLE);
	create->result = current;
	/*
	 * Thread is going to call schedule(), do not preempt it,
	 * or the creator may spend more time in wait_task_inactive().
	 */
	preempt_disable();
	complete(done);
	schedule_preempt_disabled();
	preempt_enable();

	ret = -EINTR;
	if (!test_bit(KTHREAD_SHOULD_STOP, &self->flags)) {
		cgroup_kthread_ready();
		__kthread_parkme(self);
		ret = threadfn(data);
	}



So really rcu_rdp_is_offloaded() has to be true (but we can
warn if it's not. Though we already have a test for this in
nocb_cb_wait()).

Thanks.