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Subject[PATCH 2/5] locking/mutex: Relax to atomic/release semantics
Date
... by using acquire semantics where appropriate As such, weakly
ordered archs can benefit from more relaxed use of barriers when
issuing atomics.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
include/asm-generic/mutex-dec.h | 8 ++++----
include/asm-generic/mutex-xchg.h | 10 +++++-----
kernel/locking/mutex.c | 9 +++++----
3 files changed, 14 insertions(+), 13 deletions(-)

diff --git a/include/asm-generic/mutex-dec.h b/include/asm-generic/mutex-dec.h
index d4f9fb4..fd694cf 100644
--- a/include/asm-generic/mutex-dec.h
+++ b/include/asm-generic/mutex-dec.h
@@ -20,7 +20,7 @@
static inline void
__mutex_fastpath_lock(atomic_t *count, void (*fail_fn)(atomic_t *))
{
- if (unlikely(atomic_dec_return(count) < 0))
+ if (unlikely(atomic_dec_return_acquire(count) < 0))
fail_fn(count);
}

@@ -35,7 +35,7 @@ __mutex_fastpath_lock(atomic_t *count, void (*fail_fn)(atomic_t *))
static inline int
__mutex_fastpath_lock_retval(atomic_t *count)
{
- if (unlikely(atomic_dec_return(count) < 0))
+ if (unlikely(atomic_dec_return_acquire(count) < 0))
return -1;
return 0;
}
@@ -56,7 +56,7 @@ __mutex_fastpath_lock_retval(atomic_t *count)
static inline void
__mutex_fastpath_unlock(atomic_t *count, void (*fail_fn)(atomic_t *))
{
- if (unlikely(atomic_inc_return(count) <= 0))
+ if (unlikely(atomic_inc_return_release(count) <= 0))
fail_fn(count);
}

@@ -80,7 +80,7 @@ __mutex_fastpath_unlock(atomic_t *count, void (*fail_fn)(atomic_t *))
static inline int
__mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
{
- if (likely(atomic_cmpxchg(count, 1, 0) == 1))
+ if (likely(atomic_cmpxchg_acquire(count, 1, 0) == 1))
return 1;
return 0;
}
diff --git a/include/asm-generic/mutex-xchg.h b/include/asm-generic/mutex-xchg.h
index f169ec0..a6b4a7b 100644
--- a/include/asm-generic/mutex-xchg.h
+++ b/include/asm-generic/mutex-xchg.h
@@ -31,7 +31,7 @@ __mutex_fastpath_lock(atomic_t *count, void (*fail_fn)(atomic_t *))
* to ensure that any waiting tasks are woken up by the
* unlock slow path.
*/
- if (likely(atomic_xchg(count, -1) != 1))
+ if (likely(atomic_xchg_acquire(count, -1) != 1))
fail_fn(count);
}

@@ -46,7 +46,7 @@ __mutex_fastpath_lock(atomic_t *count, void (*fail_fn)(atomic_t *))
static inline int
__mutex_fastpath_lock_retval(atomic_t *count)
{
- if (unlikely(atomic_xchg(count, 0) != 1))
+ if (unlikely(atomic_xchg_acquire(count, 0) != 1))
if (likely(atomic_xchg(count, -1) != 1))
return -1;
return 0;
@@ -67,7 +67,7 @@ __mutex_fastpath_lock_retval(atomic_t *count)
static inline void
__mutex_fastpath_unlock(atomic_t *count, void (*fail_fn)(atomic_t *))
{
- if (unlikely(atomic_xchg(count, 1) != 0))
+ if (unlikely(atomic_xchg_release(count, 1) != 0))
fail_fn(count);
}

@@ -91,7 +91,7 @@ __mutex_fastpath_unlock(atomic_t *count, void (*fail_fn)(atomic_t *))
static inline int
__mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
{
- int prev = atomic_xchg(count, 0);
+ int prev = atomic_xchg_acquire(count, 0);

if (unlikely(prev < 0)) {
/*
@@ -105,7 +105,7 @@ __mutex_fastpath_trylock(atomic_t *count, int (*fail_fn)(atomic_t *))
* owner's unlock path needlessly, but that's not a problem
* in practice. ]
*/
- prev = atomic_xchg(count, prev);
+ prev = atomic_xchg_acquire(count, prev);
if (prev < 0)
prev = 0;
}
diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
index 4cccea6..0551c21 100644
--- a/kernel/locking/mutex.c
+++ b/kernel/locking/mutex.c
@@ -277,7 +277,7 @@ static inline int mutex_can_spin_on_owner(struct mutex *lock)
static inline bool mutex_try_to_acquire(struct mutex *lock)
{
return !mutex_is_locked(lock) &&
- (atomic_cmpxchg(&lock->count, 1, 0) == 1);
+ (atomic_cmpxchg_acquire(&lock->count, 1, 0) == 1);
}

/*
@@ -529,7 +529,8 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
* Once more, try to acquire the lock. Only try-lock the mutex if
* it is unlocked to reduce unnecessary xchg() operations.
*/
- if (!mutex_is_locked(lock) && (atomic_xchg(&lock->count, 0) == 1))
+ if (!mutex_is_locked(lock) &&
+ (atomic_xchg_acquire(&lock->count, 0) == 1))
goto skip_wait;

debug_mutex_lock_common(lock, &waiter);
@@ -553,7 +554,7 @@ __mutex_lock_common(struct mutex *lock, long state, unsigned int subclass,
* non-negative in order to avoid unnecessary xchg operations:
*/
if (atomic_read(&lock->count) >= 0 &&
- (atomic_xchg(&lock->count, -1) == 1))
+ (atomic_xchg_acquire(&lock->count, -1) == 1))
break;

/*
@@ -867,7 +868,7 @@ static inline int __mutex_trylock_slowpath(atomic_t *lock_count)

spin_lock_mutex(&lock->wait_lock, flags);

- prev = atomic_xchg(&lock->count, -1);
+ prev = atomic_xchg_acquire(&lock->count, -1);
if (likely(prev == 1)) {
mutex_set_owner(lock);
mutex_acquire(&lock->dep_map, 0, 1, _RET_IP_);
--
2.1.4


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