Commit 9b0fc9c09f1b checks for if there are known active lockers in
order to avoid write trylocking using expensive cmpxchg() when it
likely wouldn't get the lock.
However, a subsequent patch was added such that we directly check for
sem->count == RWSEM_WAITING_BIAS right before trying that cmpxchg().
Thus, commit 9b0fc9c09f1b now just adds extra overhead. This patch
deletes it.
Also, add a comment on why we do an "extra check" of sem->count before
the cmpxchg().
Signed-off-by: Jason Low <jason.l...@hp.com>
---
kernel/locking/rwsem-xadd.c | 24 +++++++++++++-----------
1 files changed, 13 insertions(+), 11 deletions(-)
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index d6203fa..63d3ef2 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -247,18 +247,20 @@ struct rw_semaphore __sched
*rwsem_down_read_failed(struct rw_semaphore *sem)
return sem;
}
-static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem)
+static inline bool rwsem_try_write_lock(struct rw_semaphore *sem)
{
- if (!(count & RWSEM_ACTIVE_MASK)) {
- /* try acquiring the write lock */
- if (sem->count == RWSEM_WAITING_BIAS &&
- cmpxchg(&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);
- return true;
- }
+ /*
+ * Try acquiring the write lock. Check sem->count first
+ * in order to reduce unnecessary expensive cmpxchg() operations.
+ */
+ if (sem->count == RWSEM_WAITING_BIAS &&
+ cmpxchg(&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);
+ return true;
}
+
return false;
}
@@ -446,7 +448,7 @@ struct rw_semaphore __sched *rwsem_down_write_failed(struct
rw_semaphore *sem)
/* wait until we successfully acquire the lock */
set_current_state(TASK_UNINTERRUPTIBLE);
while (true) {
- if (rwsem_try_write_lock(count, sem))
+ if (rwsem_try_write_lock(sem))
break;
raw_spin_unlock_irq(&sem->wait_lock);
--
1.7.1
--
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