[PATCH 0/5] locking: Adopt relaxed barrier semantics

[PATCH 0/5] locking: Adopt relaxed barrier semantics

[Davidlohr Bueso]

Hi,

This series continues porting users to Will's new _{acquire|release|relaxed}
optimizations for weakly ordered archs -- and in practice, arm64 being the only
actual user we have, thus could use a _lot_ more testing. While we have osq in
-tip recently, along with Waiman's one for native qspinlock, the following
patches pretty much complete the move over for all things kernel/locking/*.
Of course, particular primitives can be found in the obvious patch titles. Note
that patch 1 adds the necessary machinery to fully support some locks, such as
rwsems.

Compiles on x86 and x86-64.

Thanks!

Davidlohr Bueso (5):
  asm-generic: Add _{relaxed|acquire|release}() variants for inc/dec
    atomics
  locking/mutex: Relax to atomic/release semantics
  locking/rtmutex: Relax to acquire/release semantics
  locking/rwsem: Relax to acquire/release semantics
  locking/mcs: Relax to atomic/release semantics

 include/asm-generic/atomic-long.h | 29 +++++++-----
 include/asm-generic/mutex-dec.h   |  8 ++--
 include/asm-generic/mutex-xchg.h  | 10 ++--
 include/asm-generic/rwsem.h       | 14 +++---
 include/linux/atomic.h            | 97 +++++++++++++++++++++++++++++++++++++++
 kernel/locking/mcs_spinlock.h     |  4 +-
 kernel/locking/mutex.c            |  9 ++--
 kernel/locking/rtmutex.c          | 30 ++++++++----
 kernel/locking/rwsem-xadd.c       |  5 +-
 9 files changed, 161 insertions(+), 45 deletions(-)

-- 
2.1.4

[PATCH 1/5] asm-generic: Add _{relaxed|acquire|release}() variants for inc/dec atomics

[Davidlohr Bueso]

Similar to what we have for regular add/sub calls. For now, no actual arch
implements them, so everyone falls back to the default atomics... iow,
nothing changes. These will be used in future primitives.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
 include/asm-generic/atomic-long.h | 29 +++++++-----
 include/linux/atomic.h            | 97 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 114 insertions(+), 12 deletions(-)

diff --git a/include/asm-generic/atomic-long.h b/include/asm-generic/atomic-long.h
index a94cbeb..c3c7183 100644
--- a/include/asm-generic/atomic-long.h
+++ b/include/asm-generic/atomic-long.h
@@ -154,19 +154,24 @@ static inline int atomic_long_add_negative(long i, atomic_long_t *l)
 	return ATOMIC_LONG_PFX(_add_negative)(i, v);
 }
 
-static inline long atomic_long_inc_return(atomic_long_t *l)
-{
-	ATOMIC_LONG_PFX(_t) *v = (ATOMIC_LONG_PFX(_t) *)l;
-
-	return (long)ATOMIC_LONG_PFX(_inc_return)(v);
-}
-
-static inline long atomic_long_dec_return(atomic_long_t *l)
-{
-	ATOMIC_LONG_PFX(_t) *v = (ATOMIC_LONG_PFX(_t) *)l;
-
-	return (long)ATOMIC_LONG_PFX(_dec_return)(v);
+#define ATOMIC_LONG_INC_DEC_OP(op, mo)					\
+static inline long							\
+atomic_long_##op##_return##mo(atomic_long_t *l)				\
+{									\
+	ATOMIC_LONG_PFX(_t) *v = (ATOMIC_LONG_PFX(_t) *)l;		\
+									\
+	return (long)ATOMIC_LONG_PFX(_##op##_return##mo)(v);		\
 }
+ATOMIC_LONG_INC_DEC_OP(inc,)
+ATOMIC_LONG_INC_DEC_OP(inc, _relaxed)
+ATOMIC_LONG_INC_DEC_OP(inc, _acquire)
+ATOMIC_LONG_INC_DEC_OP(inc, _release)
+ATOMIC_LONG_INC_DEC_OP(dec,)
+ATOMIC_LONG_INC_DEC_OP(dec, _relaxed)
+ATOMIC_LONG_INC_DEC_OP(dec, _acquire)
+ATOMIC_LONG_INC_DEC_OP(dec, _release)
+
+#undef ATOMIC_LONG_INC_DEC_OP
 
 static inline long atomic_long_add_unless(atomic_long_t *l, long a, long u)
 {
diff --git a/include/linux/atomic.h b/include/linux/atomic.h
index 00a5763..fb44d3b 100644
--- a/include/linux/atomic.h
+++ b/include/linux/atomic.h
@@ -81,6 +81,30 @@
 #endif
 #endif /* atomic_add_return_relaxed */
 
+/* atomic_inc_return_relaxed */
+#ifndef atomic_inc_return_relaxed
+#define  atomic_inc_return_relaxed	atomic_inc_return
+#define  atomic_inc_return_acquire	atomic_inc_return
+#define  atomic_inc_return_release	atomic_inc_return
+
+#else /* atomic_inc_return_relaxed */
+
+#ifndef atomic_inc_return_acquire
+#define  atomic_inc_return_acquire(...)					\
+	__atomic_op_acquire(atomic_inc_return, __VA_ARGS__)
+#endif
+
+#ifndef atomic_inc_return_release
+#define  atomic_inc_return_release(...)					\
+	__atomic_op_release(atomic_inc_return, __VA_ARGS__)
+#endif
+
+#ifndef atomic_inc_return
+#define  atomic_inc_return(...)						\
+	__atomic_op_fence(atomic_inc_return, __VA_ARGS__)
+#endif
+#endif /* atomic_inc_return_relaxed */
+
 /* atomic_sub_return_relaxed */
 #ifndef atomic_sub_return_relaxed
 #define  atomic_sub_return_relaxed	atomic_sub_return
@@ -105,6 +129,30 @@
 #endif
 #endif /* atomic_sub_return_relaxed */
 
+/* atomic_dec_return_relaxed */
+#ifndef atomic_dec_return_relaxed
+#define  atomic_dec_return_relaxed	atomic_dec_return
+#define  atomic_dec_return_acquire	atomic_dec_return
+#define  atomic_dec_return_release	atomic_dec_return
+
+#else /* atomic_dec_return_relaxed */
+
+#ifndef atomic_dec_return_acquire
+#define  atomic_dec_return_acquire(...)					\
+	__atomic_op_acquire(atomic_dec_return, __VA_ARGS__)
+#endif
+
+#ifndef atomic_dec_return_release
+#define  atomic_dec_return_release(...)					\
+	__atomic_op_release(atomic_dec_return, __VA_ARGS__)
+#endif
+
+#ifndef atomic_dec_return
+#define  atomic_dec_return(...)						\
+	__atomic_op_fence(atomic_dec_return, __VA_ARGS__)
+#endif
+#endif /* atomic_dec_return_relaxed */
+
 /* atomic_xchg_relaxed */
 #ifndef atomic_xchg_relaxed
 #define  atomic_xchg_relaxed		atomic_xchg
@@ -185,6 +233,31 @@
 #endif
 #endif /* atomic64_add_return_relaxed */
 
+/* atomic64_inc_return_relaxed */
+#ifndef atomic64_inc_return_relaxed
+#define  atomic64_inc_return_relaxed	atomic64_inc_return
+#define  atomic64_inc_return_acquire	atomic64_inc_return
+#define  atomic64_inc_return_release	atomic64_inc_return
+
+#else /* atomic64_inc_return_relaxed */
+
+#ifndef atomic64_inc_return_acquire
+#define  atomic64_inc_return_acquire(...)				\
+	__atomic_op_acquire(atomic64_inc_return, __VA_ARGS__)
+#endif
+
+#ifndef atomic64_inc_return_release
+#define  atomic64_inc_return_release(...)				\
+	__atomic_op_release(atomic64_inc_return, __VA_ARGS__)
+#endif
+
+#ifndef atomic64_inc_return
+#define  atomic64_inc_return(...)					\
+	__atomic_op_fence(atomic64_inc_return, __VA_ARGS__)
+#endif
+#endif /* atomic64_inc_return_relaxed */
+
+
 /* atomic64_sub_return_relaxed */
 #ifndef atomic64_sub_return_relaxed
 #define  atomic64_sub_return_relaxed	atomic64_sub_return
@@ -209,6 +282,30 @@
 #endif
 #endif /* atomic64_sub_return_relaxed */
 
+/* atomic64_dec_return_relaxed */
+#ifndef atomic64_dec_return_relaxed
+#define  atomic64_dec_return_relaxed	atomic64_dec_return
+#define  atomic64_dec_return_acquire	atomic64_dec_return
+#define  atomic64_dec_return_release	atomic64_dec_return
+
+#else /* atomic64_dec_return_relaxed */
+
+#ifndef atomic64_dec_return_acquire
+#define  atomic64_dec_return_acquire(...)				\
+	__atomic_op_acquire(atomic64_dec_return, __VA_ARGS__)
+#endif
+
+#ifndef atomic64_dec_return_release
+#define  atomic64_dec_return_release(...)				\
+	__atomic_op_release(atomic64_dec_return, __VA_ARGS__)
+#endif
+
+#ifndef atomic64_dec_return
+#define  atomic64_dec_return(...)					\
+	__atomic_op_fence(atomic64_dec_return, __VA_ARGS__)
+#endif
+#endif /* atomic64_dec_return_relaxed */
+
 /* atomic64_xchg_relaxed */
 #ifndef atomic64_xchg_relaxed
 #define  atomic64_xchg_relaxed		atomic64_xchg
-- 
2.1.4

[PATCH 2/5] locking/mutex: Relax to atomic/release semantics

[Davidlohr Bueso]

... 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

[PATCH 3/5] locking/rtmutex: Relax to acquire/release semantics

[Davidlohr Bueso]

... by using acquire semantics where appropiate. As such, weakly
ordered archs can benefit from more relaxed use of barriers when
issuing atomics.

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

[PATCH 4/5] locking/rwsem: Relax to acquire/release semantics

[Davidlohr Bueso]

... by using acquire semantics where appropiate. 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/rwsem.h | 14 +++++++-------
 kernel/locking/rwsem-xadd.c |  5 +++--
 2 files changed, 10 insertions(+), 9 deletions(-)

diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h
index d48bf5a..1a6bb87 100644
--- a/include/asm-generic/rwsem.h
+++ b/include/asm-generic/rwsem.h
@@ -33,7 +33,7 @@
  */
 static inline void __down_read(struct rw_semaphore *sem)
 {
-	if (unlikely(atomic_long_inc_return((atomic_long_t *)&sem->count) <= 0))
+	if (unlikely(atomic_long_inc_return_acquire((atomic_long_t *)&sem->count) <= 0))
 		rwsem_down_read_failed(sem);
 }
 
@@ -42,7 +42,7 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
 	long tmp;
 
 	while ((tmp = sem->count) >= 0) {
-		if (tmp == cmpxchg(&sem->count, tmp,
+		if (tmp == cmpxchg_acquire(&sem->count, tmp,
 				   tmp + RWSEM_ACTIVE_READ_BIAS)) {
 			return 1;
 		}
@@ -57,7 +57,7 @@ static inline void __down_write_nested(struct rw_semaphore *sem, int subclass)
 {
 	long tmp;
 
-	tmp = atomic_long_add_return(RWSEM_ACTIVE_WRITE_BIAS,
+	tmp = atomic_long_add_return_acquire(RWSEM_ACTIVE_WRITE_BIAS,
 				     (atomic_long_t *)&sem->count);
 	if (unlikely(tmp != RWSEM_ACTIVE_WRITE_BIAS))
 		rwsem_down_write_failed(sem);
@@ -72,7 +72,7 @@ static inline int __down_write_trylock(struct rw_semaphore *sem)
 {
 	long tmp;
 
-	tmp = cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE,
+	tmp = cmpxchg_acquire(&sem->count, RWSEM_UNLOCKED_VALUE,
 		      RWSEM_ACTIVE_WRITE_BIAS);
 	return tmp == RWSEM_UNLOCKED_VALUE;
 }
@@ -84,7 +84,7 @@ static inline void __up_read(struct rw_semaphore *sem)
 {
 	long tmp;
 
-	tmp = atomic_long_dec_return((atomic_long_t *)&sem->count);
+	tmp = atomic_long_dec_return_release((atomic_long_t *)&sem->count);
 	if (unlikely(tmp < -1 && (tmp & RWSEM_ACTIVE_MASK) == 0))
 		rwsem_wake(sem);
 }
@@ -94,7 +94,7 @@ static inline void __up_read(struct rw_semaphore *sem)
  */
 static inline void __up_write(struct rw_semaphore *sem)
 {
-	if (unlikely(atomic_long_sub_return(RWSEM_ACTIVE_WRITE_BIAS,
+	if (unlikely(atomic_long_sub_return_release(RWSEM_ACTIVE_WRITE_BIAS,
 				 (atomic_long_t *)&sem->count) < 0))
 		rwsem_wake(sem);
 }
@@ -114,7 +114,7 @@ static inline void __downgrade_write(struct rw_semaphore *sem)
 {
 	long tmp;
 
-	tmp = atomic_long_add_return(-RWSEM_WAITING_BIAS,
+	tmp = atomic_long_add_return_acquire(-RWSEM_WAITING_BIAS,
 				     (atomic_long_t *)&sem->count);
 	if (tmp < 0)
 		rwsem_downgrade_wake(sem);
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 0f18971..a4d4de0 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -262,7 +262,7 @@ static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem)
 	 * to reduce unnecessary expensive cmpxchg() operations.
 	 */
 	if (count == RWSEM_WAITING_BIAS &&
-	    cmpxchg(&sem->count, RWSEM_WAITING_BIAS,
+	    cmpxchg_acquire(&sem->count, RWSEM_WAITING_BIAS,
 		    RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_WAITING_BIAS) {
 		if (!list_is_singular(&sem->wait_list))
 			rwsem_atomic_update(RWSEM_WAITING_BIAS, sem);
@@ -285,7 +285,8 @@ static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
 		if (!(count == 0 || count == RWSEM_WAITING_BIAS))
 			return false;
 
-		old = cmpxchg(&sem->count, count, count + RWSEM_ACTIVE_WRITE_BIAS);
+		old = cmpxchg_acquire(&sem->count, count,
+				      count + RWSEM_ACTIVE_WRITE_BIAS);
 		if (old == count) {
 			rwsem_set_owner(sem);
 			return true;
-- 
2.1.4

[PATCH 5/5] locking/mcs: Relax to atomic/release semantics

[Davidlohr Bueso]

... 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>
---
 kernel/locking/mcs_spinlock.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/kernel/locking/mcs_spinlock.h b/kernel/locking/mcs_spinlock.h
index fd91aaa..5b9102a 100644
--- a/kernel/locking/mcs_spinlock.h
+++ b/kernel/locking/mcs_spinlock.h
@@ -67,7 +67,7 @@ void mcs_spin_lock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
 	node->locked = 0;
 	node->next   = NULL;
 
-	prev = xchg(lock, node);
+	prev = xchg_acquire(lock, node);
 	if (likely(prev == NULL)) {
 		/*
 		 * Lock acquired, don't need to set node->locked to 1. Threads
@@ -98,7 +98,7 @@ void mcs_spin_unlock(struct mcs_spinlock **lock, struct mcs_spinlock *node)
 		/*
 		 * Release the lock by setting it to NULL
 		 */
-		if (likely(cmpxchg(lock, node, NULL) == node))
+		if (likely(cmpxchg_release(lock, node, NULL) == node))
 			return;
 		/* Wait until the next pointer is set */
 		while (!(next = READ_ONCE(node->next)))
-- 
2.1.4

Re: [PATCH 2/5] locking/mutex: Relax to atomic/release semantics

[Linus Torvalds]

On Sun, Sep 20, 2015 at 3:11 AM, Davidlohr Bueso <dave@stgolabs.net> wrote:
> ... by using acquire semantics where appropriate As such, weakly
> ordered archs can benefit from more relaxed use of barriers when
> issuing atomics.

Subject line is wrong. I'm assuming it should be acquire/release
rather than atomic/release.

                    Linus

Re: [PATCH 5/5] locking/mcs: Relax to atomic/release semantics

[Linus Torvalds]

On Sun, Sep 20, 2015 at 3:11 AM, Davidlohr Bueso <dave@stgolabs.net> wrote:
> ... by using acquire semantics where appropriate As such, weakly
> ordered archs can benefit from more relaxed use of barriers when
> issuing atomics.

Again, subject is wrong.

Also, I actually think you should avoid using the word "relax", since
that actually has a separate meaning for the atomics, and it is *not*
using those relaxed semantics, and they wouldn't work anyway.

So you might as well just say something like

    "locking: use acquire/release semantics"

or similar. Ok?

                Linus

Re: [PATCH 2/5] locking/mutex: Relax to atomic/release semantics

[Davidlohr Bueso]

On Sun, 20 Sep 2015, Linus Torvalds wrote:

>On Sun, Sep 20, 2015 at 3:11 AM, Davidlohr Bueso <dave@stgolabs.net> wrote:
>> ... by using acquire semantics where appropriate As such, weakly
>> ordered archs can benefit from more relaxed use of barriers when
>> issuing atomics.
>
>Subject line is wrong. I'm assuming it should be acquire/release
>rather than atomic/release.

Dear me, yes.

[PATCH 1/5] asm-generic: Add _{relaxed|acquire|release}() variants for inc/dec atomics

[Davidlohr Bueso]

Similar to what we have for regular add/sub calls. For now, no actual arch
implements them, so everyone falls back to the default atomics... iow,
nothing changes. These will be used in future primitives.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
 include/asm-generic/atomic-long.h | 29 +++++++-----
 include/linux/atomic.h            | 97 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 114 insertions(+), 12 deletions(-)

diff --git a/include/asm-generic/atomic-long.h b/include/asm-generic/atomic-long.h
index a94cbeb..c3c7183 100644
--- a/include/asm-generic/atomic-long.h
+++ b/include/asm-generic/atomic-long.h
@@ -154,19 +154,24 @@ static inline int atomic_long_add_negative(long i, atomic_long_t *l)
 	return ATOMIC_LONG_PFX(_add_negative)(i, v);
 }
 
-static inline long atomic_long_inc_return(atomic_long_t *l)
-{
-	ATOMIC_LONG_PFX(_t) *v = (ATOMIC_LONG_PFX(_t) *)l;
-
-	return (long)ATOMIC_LONG_PFX(_inc_return)(v);
-}
-
-static inline long atomic_long_dec_return(atomic_long_t *l)
-{
-	ATOMIC_LONG_PFX(_t) *v = (ATOMIC_LONG_PFX(_t) *)l;
-
-	return (long)ATOMIC_LONG_PFX(_dec_return)(v);
+#define ATOMIC_LONG_INC_DEC_OP(op, mo)					\
+static inline long							\
+atomic_long_##op##_return##mo(atomic_long_t *l)				\
+{									\
+	ATOMIC_LONG_PFX(_t) *v = (ATOMIC_LONG_PFX(_t) *)l;		\
+									\
+	return (long)ATOMIC_LONG_PFX(_##op##_return##mo)(v);		\
 }
+ATOMIC_LONG_INC_DEC_OP(inc,)
+ATOMIC_LONG_INC_DEC_OP(inc, _relaxed)
+ATOMIC_LONG_INC_DEC_OP(inc, _acquire)
+ATOMIC_LONG_INC_DEC_OP(inc, _release)
+ATOMIC_LONG_INC_DEC_OP(dec,)
+ATOMIC_LONG_INC_DEC_OP(dec, _relaxed)
+ATOMIC_LONG_INC_DEC_OP(dec, _acquire)
+ATOMIC_LONG_INC_DEC_OP(dec, _release)
+
+#undef ATOMIC_LONG_INC_DEC_OP
 
 static inline long atomic_long_add_unless(atomic_long_t *l, long a, long u)
 {
diff --git a/include/linux/atomic.h b/include/linux/atomic.h
index 00a5763..fb44d3b 100644
--- a/include/linux/atomic.h
+++ b/include/linux/atomic.h
@@ -81,6 +81,30 @@
 #endif
 #endif /* atomic_add_return_relaxed */
 
+/* atomic_inc_return_relaxed */
+#ifndef atomic_inc_return_relaxed
+#define  atomic_inc_return_relaxed	atomic_inc_return
+#define  atomic_inc_return_acquire	atomic_inc_return
+#define  atomic_inc_return_release	atomic_inc_return
+
+#else /* atomic_inc_return_relaxed */
+
+#ifndef atomic_inc_return_acquire
+#define  atomic_inc_return_acquire(...)					\
+	__atomic_op_acquire(atomic_inc_return, __VA_ARGS__)
+#endif
+
+#ifndef atomic_inc_return_release
+#define  atomic_inc_return_release(...)					\
+	__atomic_op_release(atomic_inc_return, __VA_ARGS__)
+#endif
+
+#ifndef atomic_inc_return
+#define  atomic_inc_return(...)						\
+	__atomic_op_fence(atomic_inc_return, __VA_ARGS__)
+#endif
+#endif /* atomic_inc_return_relaxed */
+
 /* atomic_sub_return_relaxed */
 #ifndef atomic_sub_return_relaxed
 #define  atomic_sub_return_relaxed	atomic_sub_return
@@ -105,6 +129,30 @@
 #endif
 #endif /* atomic_sub_return_relaxed */
 
+/* atomic_dec_return_relaxed */
+#ifndef atomic_dec_return_relaxed
+#define  atomic_dec_return_relaxed	atomic_dec_return
+#define  atomic_dec_return_acquire	atomic_dec_return
+#define  atomic_dec_return_release	atomic_dec_return
+
+#else /* atomic_dec_return_relaxed */
+
+#ifndef atomic_dec_return_acquire
+#define  atomic_dec_return_acquire(...)					\
+	__atomic_op_acquire(atomic_dec_return, __VA_ARGS__)
+#endif
+
+#ifndef atomic_dec_return_release
+#define  atomic_dec_return_release(...)					\
+	__atomic_op_release(atomic_dec_return, __VA_ARGS__)
+#endif
+
+#ifndef atomic_dec_return
+#define  atomic_dec_return(...)						\
+	__atomic_op_fence(atomic_dec_return, __VA_ARGS__)
+#endif
+#endif /* atomic_dec_return_relaxed */
+
 /* atomic_xchg_relaxed */
 #ifndef atomic_xchg_relaxed
 #define  atomic_xchg_relaxed		atomic_xchg
@@ -185,6 +233,31 @@
 #endif
 #endif /* atomic64_add_return_relaxed */
 
+/* atomic64_inc_return_relaxed */
+#ifndef atomic64_inc_return_relaxed
+#define  atomic64_inc_return_relaxed	atomic64_inc_return
+#define  atomic64_inc_return_acquire	atomic64_inc_return
+#define  atomic64_inc_return_release	atomic64_inc_return
+
+#else /* atomic64_inc_return_relaxed */
+
+#ifndef atomic64_inc_return_acquire
+#define  atomic64_inc_return_acquire(...)				\
+	__atomic_op_acquire(atomic64_inc_return, __VA_ARGS__)
+#endif
+
+#ifndef atomic64_inc_return_release
+#define  atomic64_inc_return_release(...)				\
+	__atomic_op_release(atomic64_inc_return, __VA_ARGS__)
+#endif
+
+#ifndef atomic64_inc_return
+#define  atomic64_inc_return(...)					\
+	__atomic_op_fence(atomic64_inc_return, __VA_ARGS__)
+#endif
+#endif /* atomic64_inc_return_relaxed */
+
+
 /* atomic64_sub_return_relaxed */
 #ifndef atomic64_sub_return_relaxed
 #define  atomic64_sub_return_relaxed	atomic64_sub_return
@@ -209,6 +282,30 @@
 #endif
 #endif /* atomic64_sub_return_relaxed */
 
+/* atomic64_dec_return_relaxed */
+#ifndef atomic64_dec_return_relaxed
+#define  atomic64_dec_return_relaxed	atomic64_dec_return
+#define  atomic64_dec_return_acquire	atomic64_dec_return
+#define  atomic64_dec_return_release	atomic64_dec_return
+
+#else /* atomic64_dec_return_relaxed */
+
+#ifndef atomic64_dec_return_acquire
+#define  atomic64_dec_return_acquire(...)				\
+	__atomic_op_acquire(atomic64_dec_return, __VA_ARGS__)
+#endif
+
+#ifndef atomic64_dec_return_release
+#define  atomic64_dec_return_release(...)				\
+	__atomic_op_release(atomic64_dec_return, __VA_ARGS__)
+#endif
+
+#ifndef atomic64_dec_return
+#define  atomic64_dec_return(...)					\
+	__atomic_op_fence(atomic64_dec_return, __VA_ARGS__)
+#endif
+#endif /* atomic64_dec_return_relaxed */
+
 /* atomic64_xchg_relaxed */
 #ifndef atomic64_xchg_relaxed
 #define  atomic64_xchg_relaxed		atomic64_xchg
-- 
2.1.4

[PATCH 1/5] asm-generic: Add _{relaxed|acquire|release}() variants for inc/dec atomics

[Davidlohr Bueso]

Similar to what we have for regular add/sub calls. For now, no actual arch
implements them, so everyone falls back to the default atomics... iow,
nothing changes. These will be used in future primitives.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
 include/asm-generic/atomic-long.h | 29 +++++++-----
 include/linux/atomic.h            | 97 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 114 insertions(+), 12 deletions(-)

diff --git a/include/asm-generic/atomic-long.h b/include/asm-generic/atomic-long.h
index 8942cdc..f91093c 100644
--- a/include/asm-generic/atomic-long.h
+++ b/include/asm-generic/atomic-long.h
@@ -159,19 +159,24 @@ static inline int atomic_long_add_negative(long i, atomic_long_t *l)
 	return ATOMIC_LONG_PFX(_add_negative)(i, v);
 }
 
-static inline long atomic_long_inc_return(atomic_long_t *l)
-{
-	ATOMIC_LONG_PFX(_t) *v = (ATOMIC_LONG_PFX(_t) *)l;
-
-	return (long)ATOMIC_LONG_PFX(_inc_return)(v);
-}
-
-static inline long atomic_long_dec_return(atomic_long_t *l)
-{
-	ATOMIC_LONG_PFX(_t) *v = (ATOMIC_LONG_PFX(_t) *)l;
-
-	return (long)ATOMIC_LONG_PFX(_dec_return)(v);
+#define ATOMIC_LONG_INC_DEC_OP(op, mo)					\
+static inline long							\
+atomic_long_##op##_return##mo(atomic_long_t *l)				\
+{									\
+	ATOMIC_LONG_PFX(_t) *v = (ATOMIC_LONG_PFX(_t) *)l;		\
+									\
+	return (long)ATOMIC_LONG_PFX(_##op##_return##mo)(v);		\
 }
+ATOMIC_LONG_INC_DEC_OP(inc,)
+ATOMIC_LONG_INC_DEC_OP(inc, _relaxed)
+ATOMIC_LONG_INC_DEC_OP(inc, _acquire)
+ATOMIC_LONG_INC_DEC_OP(inc, _release)
+ATOMIC_LONG_INC_DEC_OP(dec,)
+ATOMIC_LONG_INC_DEC_OP(dec, _relaxed)
+ATOMIC_LONG_INC_DEC_OP(dec, _acquire)
+ATOMIC_LONG_INC_DEC_OP(dec, _release)
+
+#undef ATOMIC_LONG_INC_DEC_OP
 
 static inline long atomic_long_add_unless(atomic_long_t *l, long a, long u)
 {
diff --git a/include/linux/atomic.h b/include/linux/atomic.h
index e326469..27e580d 100644
--- a/include/linux/atomic.h
+++ b/include/linux/atomic.h
@@ -90,6 +90,30 @@ static inline int atomic_read_ctrl(const atomic_t *v)
 #endif
 #endif /* atomic_add_return_relaxed */
 
+/* atomic_inc_return_relaxed */
+#ifndef atomic_inc_return_relaxed
+#define  atomic_inc_return_relaxed	atomic_inc_return
+#define  atomic_inc_return_acquire	atomic_inc_return
+#define  atomic_inc_return_release	atomic_inc_return
+
+#else /* atomic_inc_return_relaxed */
+
+#ifndef atomic_inc_return_acquire
+#define  atomic_inc_return_acquire(...)					\
+	__atomic_op_acquire(atomic_inc_return, __VA_ARGS__)
+#endif
+
+#ifndef atomic_inc_return_release
+#define  atomic_inc_return_release(...)					\
+	__atomic_op_release(atomic_inc_return, __VA_ARGS__)
+#endif
+
+#ifndef atomic_inc_return
+#define  atomic_inc_return(...)						\
+	__atomic_op_fence(atomic_inc_return, __VA_ARGS__)
+#endif
+#endif /* atomic_inc_return_relaxed */
+
 /* atomic_sub_return_relaxed */
 #ifndef atomic_sub_return_relaxed
 #define  atomic_sub_return_relaxed	atomic_sub_return
@@ -114,6 +138,30 @@ static inline int atomic_read_ctrl(const atomic_t *v)
 #endif
 #endif /* atomic_sub_return_relaxed */
 
+/* atomic_dec_return_relaxed */
+#ifndef atomic_dec_return_relaxed
+#define  atomic_dec_return_relaxed	atomic_dec_return
+#define  atomic_dec_return_acquire	atomic_dec_return
+#define  atomic_dec_return_release	atomic_dec_return
+
+#else /* atomic_dec_return_relaxed */
+
+#ifndef atomic_dec_return_acquire
+#define  atomic_dec_return_acquire(...)					\
+	__atomic_op_acquire(atomic_dec_return, __VA_ARGS__)
+#endif
+
+#ifndef atomic_dec_return_release
+#define  atomic_dec_return_release(...)					\
+	__atomic_op_release(atomic_dec_return, __VA_ARGS__)
+#endif
+
+#ifndef atomic_dec_return
+#define  atomic_dec_return(...)						\
+	__atomic_op_fence(atomic_dec_return, __VA_ARGS__)
+#endif
+#endif /* atomic_dec_return_relaxed */
+
 /* atomic_xchg_relaxed */
 #ifndef atomic_xchg_relaxed
 #define  atomic_xchg_relaxed		atomic_xchg
@@ -194,6 +242,31 @@ static inline int atomic_read_ctrl(const atomic_t *v)
 #endif
 #endif /* atomic64_add_return_relaxed */
 
+/* atomic64_inc_return_relaxed */
+#ifndef atomic64_inc_return_relaxed
+#define  atomic64_inc_return_relaxed	atomic64_inc_return
+#define  atomic64_inc_return_acquire	atomic64_inc_return
+#define  atomic64_inc_return_release	atomic64_inc_return
+
+#else /* atomic64_inc_return_relaxed */
+
+#ifndef atomic64_inc_return_acquire
+#define  atomic64_inc_return_acquire(...)				\
+	__atomic_op_acquire(atomic64_inc_return, __VA_ARGS__)
+#endif
+
+#ifndef atomic64_inc_return_release
+#define  atomic64_inc_return_release(...)				\
+	__atomic_op_release(atomic64_inc_return, __VA_ARGS__)
+#endif
+
+#ifndef atomic64_inc_return
+#define  atomic64_inc_return(...)					\
+	__atomic_op_fence(atomic64_inc_return, __VA_ARGS__)
+#endif
+#endif /* atomic64_inc_return_relaxed */
+
+
 /* atomic64_sub_return_relaxed */
 #ifndef atomic64_sub_return_relaxed
 #define  atomic64_sub_return_relaxed	atomic64_sub_return
@@ -218,6 +291,30 @@ static inline int atomic_read_ctrl(const atomic_t *v)
 #endif
 #endif /* atomic64_sub_return_relaxed */
 
+/* atomic64_dec_return_relaxed */
+#ifndef atomic64_dec_return_relaxed
+#define  atomic64_dec_return_relaxed	atomic64_dec_return
+#define  atomic64_dec_return_acquire	atomic64_dec_return
+#define  atomic64_dec_return_release	atomic64_dec_return
+
+#else /* atomic64_dec_return_relaxed */
+
+#ifndef atomic64_dec_return_acquire
+#define  atomic64_dec_return_acquire(...)				\
+	__atomic_op_acquire(atomic64_dec_return, __VA_ARGS__)
+#endif
+
+#ifndef atomic64_dec_return_release
+#define  atomic64_dec_return_release(...)				\
+	__atomic_op_release(atomic64_dec_return, __VA_ARGS__)
+#endif
+
+#ifndef atomic64_dec_return
+#define  atomic64_dec_return(...)					\
+	__atomic_op_fence(atomic64_dec_return, __VA_ARGS__)
+#endif
+#endif /* atomic64_dec_return_relaxed */
+
 /* atomic64_xchg_relaxed */
 #ifndef atomic64_xchg_relaxed
 #define  atomic64_xchg_relaxed		atomic64_xchg
-- 
2.1.4

Re: [PATCH 1/5] asm-generic: Add _{relaxed|acquire|release}() variants for inc/dec atomics

[Peter Zijlstra]

On Wed, Sep 30, 2015 at 01:03:11PM -0700, Davidlohr Bueso wrote:
> Similar to what we have for regular add/sub calls. For now, no actual arch
> implements them, so everyone falls back to the default atomics... iow,
> nothing changes. These will be used in future primitives.
> 

Maybe also do an aargh64 patch for this?

Re: [PATCH 1/5] asm-generic: Add _{relaxed|acquire|release}() variants for inc/dec atomics

[Will Deacon]

On Thu, Oct 01, 2015 at 07:39:10AM +0100, Peter Zijlstra wrote:
> On Wed, Sep 30, 2015 at 01:03:11PM -0700, Davidlohr Bueso wrote:
> > Similar to what we have for regular add/sub calls. For now, no actual arch
> > implements them, so everyone falls back to the default atomics... iow,
> > nothing changes. These will be used in future primitives.
> > 
> 
> Maybe also do an aargh64 patch for this?

I can follow up with that, since I'm about 60% through adding the
relaxed/acquire/release versions of all the other primitives. It's a fair
amount of work thanks to the new atomic instructions making me do everything
twice... (that and I've been on holiday this past week!)

Will

Re: [PATCH 1/5] asm-generic: Add _{relaxed|acquire|release}() variants for inc/dec atomics

[Peter Zijlstra]

On Mon, Oct 05, 2015 at 06:32:43PM +0100, Will Deacon wrote:
> On Thu, Oct 01, 2015 at 07:39:10AM +0100, Peter Zijlstra wrote:
> > On Wed, Sep 30, 2015 at 01:03:11PM -0700, Davidlohr Bueso wrote:
> > > Similar to what we have for regular add/sub calls. For now, no actual arch
> > > implements them, so everyone falls back to the default atomics... iow,
> > > nothing changes. These will be used in future primitives.
> > > 
> > 
> > Maybe also do an aargh64 patch for this?
> 
> I can follow up with that, since I'm about 60% through adding the
> relaxed/acquire/release versions of all the other primitives. It's a fair
> amount of work thanks to the new atomic instructions making me do everything
> twice... (that and I've been on holiday this past week!)

Right, what I meant was arm (I thought argh64 was the one with _relaxed
bits in, but my memories tricked me, it was ARM).

If you add new _relaxed ops, we'd better convert all archs that support
_relaxed, which would be ARM (for now).

Re: [PATCH 1/5] asm-generic: Add _{relaxed|acquire|release}() variants for inc/dec atomics

[Will Deacon]

On Mon, Oct 05, 2015 at 09:48:24PM +0200, Peter Zijlstra wrote:
> On Mon, Oct 05, 2015 at 06:32:43PM +0100, Will Deacon wrote:
> > On Thu, Oct 01, 2015 at 07:39:10AM +0100, Peter Zijlstra wrote:
> > > On Wed, Sep 30, 2015 at 01:03:11PM -0700, Davidlohr Bueso wrote:
> > > > Similar to what we have for regular add/sub calls. For now, no actual arch
> > > > implements them, so everyone falls back to the default atomics... iow,
> > > > nothing changes. These will be used in future primitives.
> > > > 
> > > 
> > > Maybe also do an aargh64 patch for this?
> > 
> > I can follow up with that, since I'm about 60% through adding the
> > relaxed/acquire/release versions of all the other primitives. It's a fair
> > amount of work thanks to the new atomic instructions making me do everything
> > twice... (that and I've been on holiday this past week!)
> 
> Right, what I meant was arm (I thought argh64 was the one with _relaxed
> bits in, but my memories tricked me, it was ARM).
> 
> If you add new _relaxed ops, we'd better convert all archs that support
> _relaxed, which would be ARM (for now).

Right, yeah, arch/arm/ will need updating along with this series. Should
be straightforward to implement them in terms of atomic_{add,sub}_*.

Will