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Groups > linux.kernel > #1236724

[PATCH 5/5] locking/rwsem: Use acquire/release semantics

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As of 654672d4ba1 (locking/atomics: Add _{acquire|release|relaxed}()
variants of some atomic operations) and 6d79ef2d30e (locking, asm-generic:
Add _{relaxed|acquire|release}() variants for 'atomic_long_t'), weakly
ordered archs can benefit from more relaxed use of barriers when locking
and unlocking, instead of regular full barrier semantics. While currently
only arm64 supports such optimizations, updating corresponding locking
primitives serves for other archs to immediately benefit as well, once the
necessary machinery is implemented of course.

Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
---
 include/asm-generic/rwsem.h | 21 ++++++++++++++-------
 kernel/locking/rwsem-xadd.c |  5 +++--
 2 files changed, 17 insertions(+), 9 deletions(-)

diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h
index d48bf5a..d6d5dc9 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 +114,14 @@ static inline void __downgrade_write(struct rw_semaphore *sem)
 {
 	long tmp;
 
-	tmp = atomic_long_add_return(-RWSEM_WAITING_BIAS,
+	/*
+	 * When downgrading from exclusive to shared ownership,
+	 * anything inside the write-locked region cannot leak
+	 * into the read side. In contrast, anything in the
+	 * read-locked region is ok to be re-ordered into the
+	 * write side. As such, rely on RELEASE semantics.
+	 */
+	tmp = atomic_long_add_return_release(-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

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[PATCH -tip v3 0/5] locking: Use acquire/release semantics Davidlohr Bueso <dave@stgolabs.net> - 2015-09-30 22:10 +0200
  [PATCH 5/5] locking/rwsem: Use acquire/release semantics Davidlohr Bueso <dave@stgolabs.net> - 2015-09-30 22:10 +0200
  [PATCH 1/5] asm-generic: Add _{relaxed|acquire|release}() variants for inc/dec atomics Davidlohr Bueso <dave@stgolabs.net> - 2015-09-30 22:10 +0200
    Re: [PATCH 1/5] asm-generic: Add _{relaxed|acquire|release}()  variants for inc/dec atomics Peter Zijlstra <peterz@infradead.org> - 2015-10-01 08:40 +0200
      Re: [PATCH 1/5] asm-generic: Add _{relaxed|acquire|release}()  variants for inc/dec atomics Will Deacon <will.deacon@arm.com> - 2015-10-05 19:40 +0200
        Re: [PATCH 1/5] asm-generic: Add _{relaxed|acquire|release}()  variants for inc/dec atomics Peter Zijlstra <peterz@infradead.org> - 2015-10-05 21:50 +0200
  [PATCH 3/5] locking/rtmutex: Use acquire/release semantics Davidlohr Bueso <dave@stgolabs.net> - 2015-09-30 22:10 +0200
  Re: [PATCH -tip v3 0/5] locking: Use acquire/release semantics Thomas Gleixner <tglx@linutronix.de> - 2015-09-30 22:40 +0200

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