linux-kernel - Re: [RFC] Deadlock via recursive wakeup via RCU with threadirqs

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Message-Id: <20190628172056.GW26519@linux.ibm.com>
Date:   Fri, 28 Jun 2019 10:20:56 -0700
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Scott Wood <swood@...hat.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        rcu <rcu@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [RFC] Deadlock via recursive wakeup via RCU with threadirqs

On Fri, Jun 28, 2019 at 06:04:08PM +0200, Peter Zijlstra wrote:
> On Fri, Jun 28, 2019 at 08:54:04AM -0700, Paul E. McKenney wrote:
> > Thank you!  Plus it looks like scheduler_ipi() takes an early exit if
> > ->wake_list is empty, regardless of TIF_NEED_RESCHED, right?
> 
> Yes, TIF_NEED_RESCHED is checked in the interrupt return path.

OK, got it.  So the following sequence would be a valid way to get the
scheduler's attention on the current CPU shortly after interrupts
are re-enabled, even if the current CPU is already holding some
rq or pi locks, correct?

	set_tsk_need_resched(current);
	set_preempt_need_resched();
	smp_send_reschedule(smp_processor_id());

							Thanx, Paul