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Date:	Thu, 19 Jun 2014 15:04:49 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	Christoph Lameter <cl@...two.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	Pekka Enberg <penberg@...nel.org>,
	Matt Mackall <mpm@...enic.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dave Jones <davej@...hat.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: slub/debugobjects: lockup when freeing memory

On Thu, Jun 19, 2014 at 11:32:41PM +0200, Thomas Gleixner wrote:
> 
> 
> On Thu, 19 Jun 2014, Paul E. McKenney wrote:
> 
> > On Thu, Jun 19, 2014 at 10:37:17PM +0200, Thomas Gleixner wrote:
> > > On Thu, 19 Jun 2014, Paul E. McKenney wrote:
> > > > On Thu, Jun 19, 2014 at 09:29:08PM +0200, Thomas Gleixner wrote:
> > > > > On Thu, 19 Jun 2014, Paul E. McKenney wrote:
> > > > > Well, no. Look at the callchain:
> > > > > 
> > > > > __call_rcu
> > > > >     debug_object_activate
> > > > >        rcuhead_fixup_activate
> > > > >           debug_object_init
> > > > >               kmem_cache_alloc
> > > > > 
> > > > > So call rcu activates the object, but the object has no reference in
> > > > > the debug objects code so the fixup code is called which inits the
> > > > > object and allocates a reference ....
> > > > 
> > > > OK, got it.  And you are right, call_rcu() has done this for a very
> > > > long time, so not sure what changed.  But it seems like the right
> > > > approach is to provide a debug-object-free call_rcu_alloc() for use
> > > > by the memory allocators.
> > > > 
> > > > Seem reasonable?  If so, please see the following patch.
> > > 
> > > Not really, you're torpedoing the whole purpose of debugobjects :)
> > > 
> > > So, why can't we just init the rcu head when the stuff is created?
> > 
> > That would allow me to keep my code unchanged, so I am in favor.  ;-)
> 
> Almost unchanged. You need to provide a function to do so, i.e. make
> use of
> 
>     debug_init_rcu_head()

You mean like this?

							Thanx, Paul

------------------------------------------------------------------------

rcu: Export debug_init_rcu_head() and and debug_init_rcu_head()

Currently, call_rcu() relies on implicit allocation and initialization
for the debug-objects handling of RCU callbacks.  If you hammer the
kernel hard enough with Sasha's modified version of trinity, you can end
up with the sl*b allocators recursing into themselves via this implicit
call_rcu() allocation.

This commit therefore exports the debug_init_rcu_head() and
debug_rcu_head_free() functions, which permits the allocators to allocated
and pre-initialize the debug-objects information, so that there no longer
any need for call_rcu() to do that initialization, which in turn prevents
the recursion into the memory allocators.

Reported-by: Sasha Levin <sasha.levin@...cle.com>
Suggested-by: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 063a6bf1a2b6..34ae5c376e35 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -358,9 +358,19 @@ void wait_rcu_gp(call_rcu_func_t crf);
  * initialization.
  */
 #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
+void debug_init_rcu_head(struct rcu_head *head);
+void debug_rcu_head_free(struct rcu_head *head);
 void init_rcu_head_on_stack(struct rcu_head *head);
 void destroy_rcu_head_on_stack(struct rcu_head *head);
 #else /* !CONFIG_DEBUG_OBJECTS_RCU_HEAD */
+static inline void debug_init_rcu_head(struct rcu_head *head)
+{
+}
+
+static inline void debug_rcu_head_free(struct rcu_head *head)
+{
+}
+
 static inline void init_rcu_head_on_stack(struct rcu_head *head)
 {
 }
diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index a2aeb4df0f60..a41c81a26506 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -200,12 +200,12 @@ void wait_rcu_gp(call_rcu_func_t crf)
 EXPORT_SYMBOL_GPL(wait_rcu_gp);
 
 #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
-static inline void debug_init_rcu_head(struct rcu_head *head)
+void debug_init_rcu_head(struct rcu_head *head)
 {
 	debug_object_init(head, &rcuhead_debug_descr);
 }
 
-static inline void debug_rcu_head_free(struct rcu_head *head)
+void debug_rcu_head_free(struct rcu_head *head)
 {
 	debug_object_free(head, &rcuhead_debug_descr);
 }

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