From: Davidlohr Bueso <d...@stgolabs.net>
As such, weakly ordered archs can benefit from more relaxed use
of barriers when locking/unlocking.
Signed-off-by: Davidlohr Bueso <dbu...@suse.de>
---
Changes from v1:
Fix bogus acquire in generic lock downgrade (Linus)
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..d01e313 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, use 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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
