When running workloads that have high contention in mutexes on an 8 socket
machine, mutex spinners would often spin for a long time with no lock owner.
The main reason why this is occuring is in __mutex_unlock_common_slowpath(),
if __mutex_slowpath_needs_to_unlock(), then the owner needs to acquire the
mutex->wait_lock before releasing the mutex (setting lock->count to 1). When
the wait_lock is contended, this delays the mutex from being released.
We should be able to release the mutex without holding the wait_lock.
Signed-off-by: Jason Low <jason.l...@hp.com>
---
kernel/locking/mutex.c | 8 ++++----
1 files changed, 4 insertions(+), 4 deletions(-)
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 7519d27..6d85b08 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -718,10 +718,6 @@ __mutex_unlock_common_slowpath(atomic_t *lock_count, int
nested)
struct mutex *lock = container_of(lock_count, struct mutex, count);
unsigned long flags;
- spin_lock_mutex(&lock->wait_lock, flags);
- mutex_release(&lock->dep_map, nested, _RET_IP_);
- debug_mutex_unlock(lock);
-
/*
* some architectures leave the lock unlocked in the fastpath failure
* case, others need to leave it locked. In the later case we have to
@@ -730,6 +726,10 @@ __mutex_unlock_common_slowpath(atomic_t *lock_count, int
nested)
if (__mutex_slowpath_needs_to_unlock())
atomic_set(&lock->count, 1);
+ spin_lock_mutex(&lock->wait_lock, flags);
+ mutex_release(&lock->dep_map, nested, _RET_IP_);
+ debug_mutex_unlock(lock);
+
if (!list_empty(&lock->wait_list)) {
/* get the first entry from the wait-list: */
struct mutex_waiter *waiter =
--
1.7.1
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