Re: Weird rcu lockdep warning

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Tue, Apr 13, 2010 at 05:13:06PM -0700, David Miller wrote:
> From: Frederic Weisbecker <fweisbec@gmail.com>
> Date: Wed, 14 Apr 2010 02:02:27 +0200
> 
> > I just have a guess though....
> > This seems to always happen from NMI path, and lockdep is disabled on NMI.
> > I suspect the lock_acquire() performed by rcu_read_lock() is just ignored
> > and then the rcu_read_lock_held() check has the wrong result...
> 
> Yeah, I bet that's it too.
> 
> lock_is_held() can't return anything meaningful while lockdep is
> disabled, which it is during NMIs.

Ah!  So I just need to add a "current->lockdep_recursion"
check to debug_lockdep_rcu_enabled().  And move the function to
kernel/rcutree_plugin.h to avoid #include hell.

See below for (untested) patch.

						Thanx, Paul

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

 include/linux/rcupdate.h |    5 +----
 kernel/rcutree_plugin.h  |   11 +++++++++++
 2 files changed, 12 insertions(+), 4 deletions(-)

commit 304d8da6cd791a81ce3164f867e1b3ef4f9af1d1
Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Date:   Tue Apr 13 18:45:51 2010 -0700

    rcu: Make RCU lockdep check the lockdep_recursion variable
    
    The lockdep facility temporarily disables lockdep checking by incrementing
    the current->lockdep_recursion variable.  Such disabling happens in NMIs
    and in other situations where lockdep might expect to recurse on itself.
    This patch therefore checks current->lockdep_recursion, disabling RCU
    lockdep splats when this variable is non-zero.
    
    Reported-by: Frederic Weisbecker <fweisbec@gmail.com>
    Reported-by: David Miller <davem@davemloft.net>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 9f1ddfe..07db2fe 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -101,10 +101,7 @@ extern struct lockdep_map rcu_sched_lock_map;
 # define rcu_read_release_sched() \
 		lock_release(&rcu_sched_lock_map, 1, _THIS_IP_)
 
-static inline int debug_lockdep_rcu_enabled(void)
-{
-	return likely(rcu_scheduler_active && debug_locks);
-}
+extern int debug_lockdep_rcu_enabled(void);
 
 /**
  * rcu_read_lock_held - might we be in RCU read-side critical section?
diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
index 79b53bd..2169abe 100644
--- a/kernel/rcutree_plugin.h
+++ b/kernel/rcutree_plugin.h
@@ -1067,3 +1067,14 @@ static void rcu_needs_cpu_flush(void)
 }
 
 #endif /* #else #if !defined(CONFIG_RCU_FAST_NO_HZ) */
+
+#ifdef CONFIG_DEBUG_LOCK_ALLOC
+
+int debug_lockdep_rcu_enabled(void)
+{
+	return likely(rcu_scheduler_active &&
+		      debug_locks &&
+		      current->lockdep_recursion == 0);
+}
+
+#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */