Re: Lockdep complains about commit 1331e7a1bb ("rcu: Remove_rcu_barrier() dependency on __stop_machine()")

[Jiri Kosina]

On Tue, 2 Oct 2012, Jiri Kosina wrote:

> > > 1331e7a1bbe1f11b19c4327ba0853bee2a606543 is the first bad commit
> > > commit 1331e7a1bbe1f11b19c4327ba0853bee2a606543
> > > Author: Paul E. McKenney <paul.mckenney@xxxxxxxxxx>
> > > Date:   Thu Aug 2 17:43:50 2012 -0700
> > > 
> > >     rcu: Remove _rcu_barrier() dependency on __stop_machine()
> > >     
> > >     Currently, _rcu_barrier() relies on preempt_disable() to prevent
> > >     any CPU from going offline, which in turn depends on CPU hotplug's
> > >     use of __stop_machine().
> > >     
> > >     This patch therefore makes _rcu_barrier() use get_online_cpus() to
> > >     block CPU-hotplug operations.  This has the added benefit of removing
> > >     the need for _rcu_barrier() to adopt callbacks:  Because CPU-hotplug
> > >     operations are excluded, there can be no callbacks to adopt.  This
> > >     commit simplifies the code accordingly.
> > >     
> > >     Signed-off-by: Paul E. McKenney <paul.mckenney@xxxxxxxxxx>
> > >     Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> > >     Reviewed-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> > > ==
> > > 
> > > is causing lockdep to complain (see the full trace below). I haven't yet 
> > > had time to analyze what exactly is happening, and probably will not have 
> > > time to do so until tomorrow, so just sending this as a heads-up in case 
> > > anyone sees the culprit immediately.
> > 
> > Hmmm...  Does the following patch help?  It swaps the order in which
> > rcu_barrier() acquires the hotplug and rcu_barrier locks.
> 
> It changed the report slightly (see for example the change in possible 
> unsafe locking scenario, rcu_sched_state.barrier_mutex vanished and it's 
> now directly about cpu_hotplug.lock). With the patch applied I get
> 
> 
> 
> ======================================================
> [ INFO: possible circular locking dependency detected ]
> 3.6.0-03888-g3f99f3b #145 Not tainted

And it really seems valid. 

kmem_cache_destroy() calls rcu_barrier() with slab_mutex locked, which 
introduces slab_mutex -> cpu_hotplug.lock dependency (through 
rcu_barrier() -> _rcu_barrier() -> get_online_cpus()).

On the other hand, _cpu_up() acquires cpu_hotplug.lock through 
cpu_hotplug_begin(), and with this lock held cpuup_callback() notifier 
gets called, which acquires slab_mutex. This gives the reverse dependency, 
i.e. deadlock scenario is valid one.

1331e7a1bbe1f11b19c4327ba0853bee2a606543 is triggering this, because 
before that, there was no slab_mutex -> cpu_hotplug.lock dependency.

Simply put, the commit causes get_online_cpus() to be called with 
slab_mutex held, which is invalid.

-- 
Jiri Kosina
SUSE Labs
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