Commit 81301a15 authored by Waiman Long's avatar Waiman Long Committed by Greg Kroah-Hartman
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locking/lockdep: Fix debug_locks off performance problem

[ Upstream commit 9506a7425b094d2f1d9c877ed5a78f416669269b ]

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: default avatarWaiman Long <>
Cc: Andrew Morton <>
Cc: Linus Torvalds <>
Cc: Paul E. McKenney <>
Cc: Peter Zijlstra <>
Cc: Thomas Gleixner <>
Cc: Will Deacon <>

Signed-off-by: default avatarIngo Molnar <>
Signed-off-by: default avatarSasha Levin <>
Signed-off-by: default avatarGreg Kroah-Hartman <>
parent 8b110ef8
......@@ -4010,7 +4010,7 @@ 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))
......@@ -4030,7 +4030,7 @@ 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))
......@@ -37,7 +37,7 @@ EXPORT_SYMBOL_GPL(debug_locks_silent);
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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