Re: lockdep splat in CPU hotplug

[Jiri Kosina <jkosina@suse.cz>]

On Wed, 22 Oct 2014, Jiri Kosina wrote:

> Still, the lockdep stacktrace is bogus and didn't really help 
> understanding this. Any idea why it's wrong?
> 
> >  ======================================================
> >  [ INFO: possible circular locking dependency detected ]
> >  3.18.0-rc1-00069-gc2661b8 #1 Not tainted
> >  -------------------------------------------------------
> >  do_s2disk/2367 is trying to acquire lock:
> >   (cpuidle_lock){+.+.+.}, at: [<ffffffff814916c2>] cpuidle_pause_and_lock+0x12/0x20
> >  
> > but task is already holding lock:
> >   (cpu_hotplug.lock#2){+.+.+.}, at: [<ffffffff810522ea>] cpu_hotplug_begin+0x4a/0x80
> >  
> > which lock already depends on the new lock.
> > 
> > the existing dependency chain (in reverse order) is:
> > 
> > -> #1 (cpu_hotplug.lock#2){+.+.+.}:
> >         [<ffffffff81099fac>] lock_acquire+0xac/0x130
> >         [<ffffffff815b9f2c>] mutex_lock_nested+0x5c/0x3b0
> >         [<ffffffff81491892>] cpuidle_pause+0x12/0x30

And the report is clearly bogus here. See:

$ addr2line -a ffffffff81491892 -e vmlinux-3.18.0-rc1-00068-gc2661b8
0xffffffff81491892
/dev/shm/jikos/BUILD/kernel-3.18.0_rc1_00068_gc2661b8/drivers/cpuidle/cpuidle.c:262

where the corresponding lines are

	259	void cpuidle_pause(void)
	260	{
	261		mutex_lock(&cpuidle_lock);
	262		cpuidle_uninstall_idle_handler();
	263		mutex_unlock(&cpuidle_lock);
	264	}

i.e. the RIP saved on stack clearly indicates that we are currently inside 
cpuidle_uninstall_idle_handler().

So it's clearly the case that part of the stack (which would probably 
point to synchronize_rcu_expedited()) is missing for some reason.

-- 
Jiri Kosina
SUSE Labs