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

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Date:   Fri, 28 Jun 2019 16:15:22 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
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 Thu, Jun 27, 2019 at 01:36:12PM -0700, Paul E. McKenney wrote:
> On Thu, Jun 27, 2019 at 03:17:27PM -0500, Scott Wood wrote:
> > On Thu, 2019-06-27 at 11:41 -0700, Paul E. McKenney wrote:
> > > On Thu, Jun 27, 2019 at 02:16:38PM -0400, Joel Fernandes wrote:
> > > > 
> > > > I think the fix should be to prevent the wake-up not based on whether we
> > > > are
> > > > in hard/soft-interrupt mode but that we are doing the rcu_read_unlock()
> > > > from
> > > > a scheduler path (if we can detect that)
> > > 
> > > Or just don't do the wakeup at all, if it comes to that.  I don't know
> > > of any way to determine whether rcu_read_unlock() is being called from
> > > the scheduler, but it has been some time since I asked Peter Zijlstra
> > > about that.
> > > 
> > > Of course, unconditionally refusing to do the wakeup might not be happy
> > > thing for NO_HZ_FULL kernels that don't implement IRQ work.
> > 
> > Couldn't smp_send_reschedule() be used instead?
> 
> Good point.  If current -rcu doesn't fix things for Sebastian's case,
> that would be well worth looking at.  But there must be some reason
> why Peter Zijlstra didn't suggest it when he instead suggested using
> the IRQ work approach.
> 
> Peter, thoughts?

I've not exactly kept up with the thread; but irq_work allows you to run
some actual code on the remote CPU which is often useful and it is only
a little more expensive than smp_send_reschedule().

Also, just smp_send_reschedule() doesn't really do anything without
first poking TIF_NEED_RESCHED (or other scheduler state) and if you want
to do both, there's other helpers you should use, like resched_cpu().