linux-kernel - Re: lockdep splat in CPU hotplug

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Tue, 21 Oct 2014 09:23:30 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Jiri Kosina <jkosina@...e.cz>
Cc:	Dave Jones <davej@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Pavel Machek <pavel@....cz>, linux-kernel@...r.kernel.org,
	linux-pm@...r.kernel.org
Subject: Re: lockdep splat in CPU hotplug

On Tue, Oct 21, 2014 at 06:04:52PM +0200, Jiri Kosina wrote:
> On Tue, 21 Oct 2014, Paul E. McKenney wrote:
> 
> > > Looks like this indeed is something that lockdep *should* report (*), 
> > > although I would be suprised that stack unwinder would be so confused 
> > > by this -- there is no way for synchronize_sched_expedited() to be 
> > > inlined all the way to cpuidle_pause().
> > 
> > I think that if synchronize_sched_expedited() was in fact called, it
> > had already returned by the time we hit this problem.  But I must confess
> > that I am not seeing how cpuidle_uninstall_idle_handler() gets to
> > synchronize_rcu().
> 
> Umm, it directly calls it? :-)
> 
> 	void cpuidle_uninstall_idle_handler(void)
> 	{
> 		if (enabled_devices) {
> 			initialized = 0;
> 			wake_up_all_idle_cpus();
> 		}
> 
> 		/*
> 		 * Make sure external observers (such as the scheduler)
> 		 * are done looking at pointed idle states.
> 		 */
> 		synchronize_rcu();
> 	}

Ah, it would help if I did "git checkout linus/master" after updating,
wouldn't it now?

> > > (*) there are multiple places where cpu_hotplug.lock -> cpuidle_lock lock 
> > >     dependency is assumed. The patch that Dave pointed out adds 
> > >     cpuidle_lock -> cpu_hotplug.lock dependency.
> > > 
> > > Still not clear whether this is what's happening here ... anyway, adding 
> > > Paul to CC.
> > 
> > Hmmm...
> > 
> > Both cpuidle_pause() and cpuidle_pause_and_lock() acquire cpuidle_lock,
> > and are at the top of both stacks.  Which was the original confusion.  ;-)
> 
> Yup, they are, but lockdep is complaining about cpuidle_pause() acquiring 
> cpu_hotplug.lock ...

If it was attempting to acquire it via synchronize_sched_expedited(),
the attempt would fail and synchronize_sched_expedited() would fall
back to synchronize_sched()'s normal grace-period mechanism.  (Not to
synchronize_sched() itself, of course, as that would be infinite
recursion.)

So I believe that something else is going on here.

							Thanx, Paul

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/