On Tue, 2012-05-08 at 11:11 -0700, Hugh Dickins wrote:
> Please send me the version of patch you'd like to put in
> (lest I make it up myself and you don't like the result).

something like so?

---
 include/linux/lockdep.h |   17 +++++++++++++++++
 kernel/timer.c          |    4 +++-
 kernel/workqueue.c      |    4 +++-
 3 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index d36619e..968c3e2 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -157,6 +157,23 @@ struct lockdep_map {
 #endif
 };
 
+static inline void lockdep_copy_map(struct lockdep_map *to struct lockdep_map *from)
+{
+	int i;
+
+	*to = *from;
+	/*
+	 * Since the class cache can be modified concurrently we could observe
+	 * half pointers (64bit arch using 32bit copy insns). Therefore clear
+	 * the caches and take the performance hit.
+	 *
+	 * XXX it doesn't work well with lockdep_set_class_and_subclass(), since
+	 *     that relies on cache abuse.
+	 */
+	for (i = 0; i < NR_LOCKDEP_CACHING_CLASSES; i++)
+		to->class_cache[i] = NULL;
+}
+
 /*
  * Every lock has a list of other locks that were taken after it.
  * We only grow the list, never remove from it:
diff --git a/kernel/timer.c b/kernel/timer.c
index a297ffc..6aa7ad8 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -1102,7 +1102,9 @@ static void call_timer_fn(struct timer_list *timer, void (*fn)(unsigned long),
 	 * warnings as well as problems when looking into
 	 * timer->lockdep_map, make a copy and use that here.
 	 */
-	struct lockdep_map lockdep_map = timer->lockdep_map;
+	struct lockdep_map lockdep_map;
+       
+	lockdep_copy_map(&lockdep_map, &timer->lockdep_map);
 #endif
 	/*
 	 * Couple the lock chain with the lock chain at
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 5abf42f..7d77d1f 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1810,7 +1810,9 @@ __acquires(&gcwq->lock)
 	 * lock freed" warnings as well as problems when looking into
 	 * work->lockdep_map, make a copy and use that here.
 	 */
-	struct lockdep_map lockdep_map = work->lockdep_map;
+	struct lockdep_map lockdep_map;
+       
+	lockdep_copy_map(&lockdep_map, &work->lockdep_map);
 #endif
 	/*
 	 * A single work shouldn't be executed concurrently by