linux-kernel - Re: Query regarding synchronize_sched_expedited and resched

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Message-Id: <20170921153358.GH3521@linux.vnet.ibm.com>
Date:   Thu, 21 Sep 2017 08:33:58 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        josh@...htriplett.org, mathieu.desnoyers@...icios.com,
        jiangshanlai@...il.com, linux-kernel@...r.kernel.org,
        sramana@...eaurora.org, prsood@...eaurora.org,
        pkondeti@...eaurora.org, markivx@...eaurora.org
Subject: Re: Query regarding synchronize_sched_expedited and resched_cpu

On Thu, Sep 21, 2017 at 03:57:49PM +0200, Peter Zijlstra wrote:
> On Mon, Sep 18, 2017 at 09:55:27AM -0700, Paul E. McKenney wrote:
> > On Mon, Sep 18, 2017 at 12:29:31PM -0400, Steven Rostedt wrote:
> > > On Mon, 18 Sep 2017 09:24:12 -0700
> > > "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> > > 
> > > 
> > > > As soon as I work through the backlog of lockdep complaints that
> > > > appeared in the last merge window...  :-(
> > > > 
> > > > sparse_irq_lock, I am looking at you!!!  ;-)
> 
> That one is a false positive and I have send patches to address.

I did try them out, and they work fine for me when lockdep is enabled.
I get build failures if lockdep is not enabled.  Things are a bit crazy
here, so I have not had a chance to try to fix them, though it should
not be a big deal (easy for me to say!).

> > > I just hit one too, and decided to write a patch to show a chain of 3
> > > when applicable.
> > > 
> > > For example:
> > > 
> > >  Chain exists of:
> > >    cpu_hotplug_lock.rw_sem --> smpboot_threads_lock --> (complete)&self->parked
> > > 
> > >   Possible unsafe locking scenario by crosslock:
> > > 
> > >         CPU0                    CPU1                    CPU2
> > >         ----                    ----                    ----
> > >    lock(smpboot_threads_lock);
> > >    lock((complete)&self->parked);
> > >                                 lock(cpu_hotplug_lock.rw_sem);
> > >                                 lock(smpboot_threads_lock);
> > >                                                        lock(cpu_hotplug_lock.rw_sem);
> > >                                                        unlock((complete)&self->parked);
> > > 
> > >   *** DEADLOCK ***
> > > 
> > > :-)
> > 
> > Nice!!!
> 
> That one looks like the watchdog thing, and Thomas was poking at that.

For whatever it is worth, I am still chasing lost-timer bugs.  I now know
of a large number of things that are not the cause of the problem.  :-/

							Thanx, Paul