lockdep: fix oops in processing workqueue
Under memory load, on x86_64, with lockdep enabled, the workqueue's
process_one_work() has been seen to oops in __lock_acquire(), barfing
on a 0xffffffff00000000 pointer in the lockdep_map's class_cache[].
Because it's permissible to free a work_struct from its callout function,
the map used is an onstack copy of the map given in the work_struct: and
that copy is made without any locking.
Surprisingly, gcc (4.5.1 in Hugh's case) uses "rep movsl" rather than
"rep movsq" for that structure copy: which might race with a workqueue
user's wait_on_work() doing lock_map_acquire() on the source of the
copy, putting a pointer into the class_cache[], but only in time for
the top half of that pointer to be copied to the destination map.
Boom when process_one_work() subsequently does lock_map_acquire()
on its onstack copy of the lockdep_map.
Fix this, and a similar instance in call_timer_fn(), with a
lockdep_copy_map() function which additionally NULLs the class_cache[].
Note: this oops was actually seen on 3.4-next, where flush_work() newly
does the racing lock_map_acquire(); but Tejun points out that 3.4 and
earlier are already vulnerable to the same through wait_on_work().
* Patch orginally from Peter. Hugh modified it a bit and wrote the
description.
Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Reported-by: Hugh Dickins <hughd@google.com>
LKML-Reference: <alpine.LSU.2.00.1205070951170.1544@eggly.anvils>
Signed-off-by: Tejun Heo <tj@kernel.org>
diff --git a/include/linux/lockdep.h b/include/linux/lockdep.h
index d36619e..00e4637 100644
--- a/include/linux/lockdep.h
+++ b/include/linux/lockdep.h
@@ -157,6 +157,24 @@
#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..b123852 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -1102,7 +1102,9 @@
* 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 c36c86c..9a3128d 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1818,7 +1818,9 @@
* 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