[PATCH 02/11] locking, rwsem: drop explicit memory barriers

Michal Hocko
2016-02-29T12:58:16Z

From: Michal Hocko <mhocko@suse.com>

sh and xtensa seem to be the only architectures which use explicit
memory barriers for rw_semaphore operations even though they are not
really needed because there is the full memory barrier is always implied
by atomic_{inc,dec,add,sub}_return resp. cmpxchg. Remove them.

Signed-off-by: Michal Hocko <mhocko@suse.com>
---
arch/sh/include/asm/rwsem.h | 14 ++------------
arch/xtensa/include/asm/rwsem.h | 14 ++------------
2 files changed, 4 insertions(+), 24 deletions(-)

diff --git a/arch/sh/include/asm/rwsem.h b/arch/sh/include/asm/rwsem.h
index a5104bebd1eb..f6c951c7a875 100644
--- a/arch/sh/include/asm/rwsem.h
+++ b/arch/sh/include/asm/rwsem.h
@@ -24,9 +24,7 @@
*/
static inline void __down_read(struct rw_semaphore *sem)
{
- if (atomic_inc_return((atomic_t *)(&sem->count)) > 0)
- smp_wmb();
- else
+ if (atomic_inc_return((atomic_t *)(&sem->count)) <= 0)
rwsem_down_read_failed(sem);
}

@@ -37,7 +35,6 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
while ((tmp = sem->count) >= 0) {
if (tmp = cmpxchg(&sem->count, tmp,
tmp + RWSEM_ACTIVE_READ_BIAS)) {
- smp_wmb();
return 1;
}
}
@@ -53,9 +50,7 @@ static inline void __down_write(struct rw_semaphore *sem)

tmp = atomic_add_return(RWSEM_ACTIVE_WRITE_BIAS,
(atomic_t *)(&sem->count));
- if (tmp = RWSEM_ACTIVE_WRITE_BIAS)
- smp_wmb();
- else
+ if (tmp != RWSEM_ACTIVE_WRITE_BIAS)
rwsem_down_write_failed(sem);
}

@@ -65,7 +60,6 @@ static inline int __down_write_trylock(struct rw_semaphore *sem)

tmp = cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE,
RWSEM_ACTIVE_WRITE_BIAS);
- smp_wmb();
return tmp = RWSEM_UNLOCKED_VALUE;
}

@@ -76,7 +70,6 @@ static inline void __up_read(struct rw_semaphore *sem)
{
int tmp;

- smp_wmb();
tmp = atomic_dec_return((atomic_t *)(&sem->count));
if (tmp < -1 && (tmp & RWSEM_ACTIVE_MASK) = 0)
rwsem_wake(sem);
@@ -87,7 +80,6 @@ static inline void __up_read(struct rw_semaphore *sem)
*/
static inline void __up_write(struct rw_semaphore *sem)
{
- smp_wmb();
if (atomic_sub_return(RWSEM_ACTIVE_WRITE_BIAS,
(atomic_t *)(&sem->count)) < 0)
rwsem_wake(sem);
@@ -108,7 +100,6 @@ static inline void __downgrade_write(struct rw_semaphore *sem)
{
int tmp;

- smp_wmb();
tmp = atomic_add_return(-RWSEM_WAITING_BIAS, (atomic_t *)(&sem->count));
if (tmp < 0)
rwsem_downgrade_wake(sem);
@@ -119,7 +110,6 @@ static inline void __downgrade_write(struct rw_semaphore *sem)
*/
static inline int rwsem_atomic_update(int delta, struct rw_semaphore *sem)
{
- smp_mb();
return atomic_add_return(delta, (atomic_t *)(&sem->count));
}

diff --git a/arch/xtensa/include/asm/rwsem.h b/arch/xtensa/include/asm/rwsem.h
index 249619e7e7f2..593483f6e1ff 100644
--- a/arch/xtensa/include/asm/rwsem.h
+++ b/arch/xtensa/include/asm/rwsem.h
@@ -29,9 +29,7 @@
*/
static inline void __down_read(struct rw_semaphore *sem)
{
- if (atomic_add_return(1,(atomic_t *)(&sem->count)) > 0)
- smp_wmb();
- else
+ if (atomic_add_return(1,(atomic_t *)(&sem->count)) <= 0)
rwsem_down_read_failed(sem);
}

@@ -42,7 +40,6 @@ static inline int __down_read_trylock(struct rw_semaphore *sem)
while ((tmp = sem->count) >= 0) {
if (tmp = cmpxchg(&sem->count, tmp,
tmp + RWSEM_ACTIVE_READ_BIAS)) {
- smp_wmb();
return 1;
}
}
@@ -58,9 +55,7 @@ static inline void __down_write(struct rw_semaphore *sem)

tmp = atomic_add_return(RWSEM_ACTIVE_WRITE_BIAS,
(atomic_t *)(&sem->count));
- if (tmp = RWSEM_ACTIVE_WRITE_BIAS)
- smp_wmb();
- else
+ if (tmp != RWSEM_ACTIVE_WRITE_BIAS)
rwsem_down_write_failed(sem);
}

@@ -70,7 +65,6 @@ static inline int __down_write_trylock(struct rw_semaphore *sem)

tmp = cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE,
RWSEM_ACTIVE_WRITE_BIAS);
- smp_wmb();
return tmp = RWSEM_UNLOCKED_VALUE;
}

@@ -81,7 +75,6 @@ static inline void __up_read(struct rw_semaphore *sem)
{
int tmp;

- smp_wmb();
tmp = atomic_sub_return(1,(atomic_t *)(&sem->count));
if (tmp < -1 && (tmp & RWSEM_ACTIVE_MASK) = 0)
rwsem_wake(sem);
@@ -92,7 +85,6 @@ static inline void __up_read(struct rw_semaphore *sem)
*/
static inline void __up_write(struct rw_semaphore *sem)
{
- smp_wmb();
if (atomic_sub_return(RWSEM_ACTIVE_WRITE_BIAS,
(atomic_t *)(&sem->count)) < 0)
rwsem_wake(sem);
@@ -113,7 +105,6 @@ static inline void __downgrade_write(struct rw_semaphore *sem)
{
int tmp;

- smp_wmb();
tmp = atomic_add_return(-RWSEM_WAITING_BIAS, (atomic_t *)(&sem->count));
if (tmp < 0)
rwsem_downgrade_wake(sem);
@@ -124,7 +115,6 @@ static inline void __downgrade_write(struct rw_semaphore *sem)
*/
static inline int rwsem_atomic_update(int delta, struct rw_semaphore *sem)
{
- smp_mb();
return atomic_add_return(delta, (atomic_t *)(&sem->count));
}

--
2.7.0