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Subject[PATCH 1/2] locking/lockdep: Fix debug_locks off performance problem
It was found that when debug_locks was turned off because of a problem
found by the lockdep code, the system performance could drop quite
significantly when the lock_stat code was also configured into the
kernel. For instance, parallel kernel build time on a 4-socket x86-64
server nearly doubled.

Further analysis into the cause of the slowdown traced back to the
frequent call to debug_locks_off() from the __lock_acquired() function
probably due to some inconsistent lockdep states with debug_locks
off. The debug_locks_off() function did an unconditional atomic xchg
to write a 0 value into debug_locks which had already been set to 0.
This led to severe cacheline contention in the cacheline that held
debug_locks. As debug_locks is being referenced in quite a few different
places in the kernel, this greatly slow down the system performance.

To prevent that trashing of debug_locks cacheline, lock_acquired()
and lock_contended() now checks the state of debug_locks before
proceeding. The debug_locks_off() function is also modified to check
debug_locks before calling __debug_locks_off().

Signed-off-by: Waiman Long <>
kernel/locking/lockdep.c | 4 ++--
lib/debug_locks.c | 2 +-
2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
index be76f47..1efada2 100644
--- a/kernel/locking/lockdep.c
+++ b/kernel/locking/lockdep.c
@@ -4066,7 +4066,7 @@ void lock_contended(struct lockdep_map *lock, unsigned long ip)
unsigned long flags;

- if (unlikely(!lock_stat))
+ if (unlikely(!lock_stat || !debug_locks))

if (unlikely(current->lockdep_recursion))
@@ -4086,7 +4086,7 @@ void lock_acquired(struct lockdep_map *lock, unsigned long ip)
unsigned long flags;

- if (unlikely(!lock_stat))
+ if (unlikely(!lock_stat || !debug_locks))

if (unlikely(current->lockdep_recursion))
diff --git a/lib/debug_locks.c b/lib/debug_locks.c
index 96c4c63..124fdf2 100644
--- a/lib/debug_locks.c
+++ b/lib/debug_locks.c
@@ -37,7 +37,7 @@
int debug_locks_off(void)
- if (__debug_locks_off()) {
+ if (debug_locks && __debug_locks_off()) {
if (!debug_locks_silent) {
return 1;
 \ /
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