lkml.org 
[lkml]   [2012]   [Oct]   [2]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: Lockdep complains about commit 1331e7a1bb ("rcu: Remove _rcu_barrier() dependency on __stop_machine()")
On Tue, 2 Oct 2012, Jiri Kosina wrote:

> > > 1331e7a1bbe1f11b19c4327ba0853bee2a606543 is the first bad commit
> > > commit 1331e7a1bbe1f11b19c4327ba0853bee2a606543
> > > Author: Paul E. McKenney <paul.mckenney@linaro.org>
> > > 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@linaro.org>
> > > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > > Reviewed-by: Josh Triplett <josh@joshtriplett.org>
> > > ==
> > >
> > > 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


\
 
 \ /
  Last update: 2012-10-03 00:21    [W:0.074 / U:2.756 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site