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Subject[PATCH 3/5] locking/rtmutex: Use acquire/release semantics
Date
As such, weakly ordered archs can benefit from more relaxed use
of barriers when locking/unlocking.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
kernel/locking/rtmutex.c | 30 +++++++++++++++++++++---------
1 file changed, 21 insertions(+), 9 deletions(-)

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 7781d80..226a629 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -74,14 +74,23 @@ static void fixup_rt_mutex_waiters(struct rt_mutex *lock)
* set up.
*/
#ifndef CONFIG_DEBUG_RT_MUTEXES
-# define rt_mutex_cmpxchg(l,c,n) (cmpxchg(&l->owner, c, n) == c)
+# define rt_mutex_cmpxchg_relaxed(l,c,n) (cmpxchg_relaxed(&l->owner, c, n) == c)
+# define rt_mutex_cmpxchg_acquire(l,c,n) (cmpxchg_acquire(&l->owner, c, n) == c)
+# define rt_mutex_cmpxchg_release(l,c,n) (cmpxchg_release(&l->owner, c, n) == c)
+
+/*
+ * Callers must hold the ->wait_lock -- which is the whole purpose as we force
+ * all future threads that attempt to [Rmw] the lock to the slowpath. As such
+ * relaxed semantics suffice.
+ */
static inline void mark_rt_mutex_waiters(struct rt_mutex *lock)
{
unsigned long owner, *p = (unsigned long *) &lock->owner;

do {
owner = *p;
- } while (cmpxchg(p, owner, owner | RT_MUTEX_HAS_WAITERS) != owner);
+ } while (cmpxchg_relaxed(p, owner,
+ owner | RT_MUTEX_HAS_WAITERS) != owner);
}

/*
@@ -121,11 +130,14 @@ static inline bool unlock_rt_mutex_safe(struct rt_mutex *lock)
* lock(wait_lock);
* acquire(lock);
*/
- return rt_mutex_cmpxchg(lock, owner, NULL);
+ return rt_mutex_cmpxchg_acquire(lock, owner, NULL);
}

#else
-# define rt_mutex_cmpxchg(l,c,n) (0)
+# define rt_mutex_cmpxchg_relaxed(l,c,n) (0)
+# define rt_mutex_cmpxchg_acquire(l,c,n) (0)
+# define rt_mutex_cmpxchg_release(l,c,n) (0)
+
static inline void mark_rt_mutex_waiters(struct rt_mutex *lock)
{
lock->owner = (struct task_struct *)
@@ -1321,7 +1333,7 @@ rt_mutex_fastlock(struct rt_mutex *lock, int state,
struct hrtimer_sleeper *timeout,
enum rtmutex_chainwalk chwalk))
{
- if (likely(rt_mutex_cmpxchg(lock, NULL, current))) {
+ if (likely(rt_mutex_cmpxchg_acquire(lock, NULL, current))) {
rt_mutex_deadlock_account_lock(lock, current);
return 0;
} else
@@ -1337,7 +1349,7 @@ rt_mutex_timed_fastlock(struct rt_mutex *lock, int state,
enum rtmutex_chainwalk chwalk))
{
if (chwalk == RT_MUTEX_MIN_CHAINWALK &&
- likely(rt_mutex_cmpxchg(lock, NULL, current))) {
+ likely(rt_mutex_cmpxchg_acquire(lock, NULL, current))) {
rt_mutex_deadlock_account_lock(lock, current);
return 0;
} else
@@ -1348,7 +1360,7 @@ static inline int
rt_mutex_fasttrylock(struct rt_mutex *lock,
int (*slowfn)(struct rt_mutex *lock))
{
- if (likely(rt_mutex_cmpxchg(lock, NULL, current))) {
+ if (likely(rt_mutex_cmpxchg_acquire(lock, NULL, current))) {
rt_mutex_deadlock_account_lock(lock, current);
return 1;
}
@@ -1362,7 +1374,7 @@ rt_mutex_fastunlock(struct rt_mutex *lock,
{
WAKE_Q(wake_q);

- if (likely(rt_mutex_cmpxchg(lock, current, NULL))) {
+ if (likely(rt_mutex_cmpxchg_release(lock, current, NULL))) {
rt_mutex_deadlock_account_unlock(current);

} else {
@@ -1484,7 +1496,7 @@ EXPORT_SYMBOL_GPL(rt_mutex_unlock);
bool __sched rt_mutex_futex_unlock(struct rt_mutex *lock,
struct wake_q_head *wqh)
{
- if (likely(rt_mutex_cmpxchg(lock, current, NULL))) {
+ if (likely(rt_mutex_cmpxchg_release(lock, current, NULL))) {
rt_mutex_deadlock_account_unlock(current);
return false;
}
--
2.1.4


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