Doing cmpxchg will cause cache bouncing when checking
sem->count. This could cause scalability issue
in a large machine (e.g. a 80 cores box).
A pre-read of sem->count can mitigate this.
Signed-off-by: Alex Shi <alex....@intel.com>
Signed-off-by: Tim Chen <tim.c.c...@linux.intel.com>
---
include/asm-generic/rwsem.h | 8 ++++----
lib/rwsem.c | 21 +++++++++++++--------
2 files changed, 17 insertions(+), 12 deletions(-)
diff --git a/include/asm-generic/rwsem.h b/include/asm-generic/rwsem.h
index bb1e2cd..052d973 100644
--- a/include/asm-generic/rwsem.h
+++ b/include/asm-generic/rwsem.h
@@ -70,11 +70,11 @@ static inline void __down_write(struct rw_semaphore *sem)
static inline int __down_write_trylock(struct rw_semaphore *sem)
{
- long tmp;
+ if (unlikely(&sem->count != RWSEM_UNLOCKED_VALUE))
+ return 0;
- tmp = cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE,
- RWSEM_ACTIVE_WRITE_BIAS);
- return tmp == RWSEM_UNLOCKED_VALUE;
+ return cmpxchg(&sem->count, RWSEM_UNLOCKED_VALUE,
+ RWSEM_ACTIVE_WRITE_BIAS) == RWSEM_UNLOCKED_VALUE;
}
/*
diff --git a/lib/rwsem.c b/lib/rwsem.c
index 19c5fa9..2072af5 100644
--- a/lib/rwsem.c
+++ b/lib/rwsem.c
@@ -75,7 +75,7 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum
rwsem_wake_type wake_type)
* will block as they will notice the queued writer.
*/
wake_up_process(waiter->task);
- goto out;
+ return sem;
}
/* Writers might steal the lock before we grant it to the next reader.
@@ -85,15 +85,21 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum
rwsem_wake_type wake_type)
adjustment = 0;
if (wake_type != RWSEM_WAKE_READ_OWNED) {
adjustment = RWSEM_ACTIVE_READ_BIAS;
- try_reader_grant:
- oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
- if (unlikely(oldcount < RWSEM_WAITING_BIAS)) {
- /* A writer stole the lock. Undo our reader grant. */
+ while (1) {
+ /* A writer stole the lock. */
+ if (sem->count < RWSEM_WAITING_BIAS)
+ return sem;
+
+ oldcount = rwsem_atomic_update(adjustment, sem)
+ - adjustment;
+ if (likely(oldcount >= RWSEM_WAITING_BIAS))
+ break;
+
+ /* A writer stole the lock. Undo our reader grant. */
if (rwsem_atomic_update(-adjustment, sem) &
RWSEM_ACTIVE_MASK)
- goto out;
+ return sem;
/* Last active locker left. Retry waking readers. */
- goto try_reader_grant;
}
}
@@ -136,7 +142,6 @@ __rwsem_do_wake(struct rw_semaphore *sem, enum
rwsem_wake_type wake_type)
sem->wait_list.next = next;
next->prev = &sem->wait_list;
- out:
return sem;
}
--
1.7.4.4
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