When tasks with short lock hold time acquire a rwsem consecutively,
it is possible that they all try to wake up the same waiting task at
unlock time. Beside the first wakeup, the rests are a waste of precious
CPU cycles. This patch limits the actual wakeup call to only one.
To simplify sychronization of the waking flag, the inner schedule
loop of __rwsem_down_write_failed_common() is removed and
raw_spin_lock_irq() is always called after wakeup.
Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
kernel/locking/rwsem-xadd.c | 39 +++++++++++++++++++++------------------
1 file changed, 21 insertions(+), 18 deletions(-)
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index ba00795..3bdbf39 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -70,6 +70,7 @@ struct rwsem_waiter {
struct list_head list;
struct task_struct *task;
enum rwsem_waiter_type type;
+ bool waking; /* For writer, protected by wait_lock */
unsigned long timeout;
};
@@ -129,6 +130,12 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
if (wake_type == RWSEM_WAKE_ANY) {
/*
+ * No redundant wakeup if the waiter is waking up.
+ */
+ if (waiter->waking)
+ return;
+
+ /*
* Mark writer at the front of the queue for wakeup.
* Until the task is actually later awoken later by
* the caller, other writers are able to steal it.
@@ -136,6 +143,7 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
* will notice the queued writer.
*/
wake_q_add(wake_q, waiter->task);
+ waiter->waking = true;
}
return;
@@ -600,6 +608,7 @@ static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
*/
waiter.task = current;
waiter.type = RWSEM_WAITING_FOR_WRITE;
+ waiter.waking = false;
waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
raw_spin_lock_irq(&sem->wait_lock);
@@ -646,29 +655,24 @@ static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
if (rwsem_try_write_lock(count, sem, first))
break;
- raw_spin_unlock_irq(&sem->wait_lock);
+ if (signal_pending_state(state, current))
+ goto out_nolock;
- /* Block until there are no active lockers. */
- for (;;) {
- if (signal_pending_state(state, current))
- goto out_nolock;
+ if (!first)
+ first = rwsem_waiter_is_first(sem, &waiter);
- schedule();
- set_current_state(state);
- count = atomic_read(&sem->count);
+ waiter.waking = false;
+ raw_spin_unlock_irq(&sem->wait_lock);
- if (!first)
- first = rwsem_waiter_is_first(sem, &waiter);
+ if (first && !RWSEM_COUNT_HANDOFF(count) &&
+ time_after(jiffies, waiter.timeout))
+ atomic_or(RWSEM_FLAG_WHANDOFF, &sem->count);
- if (!RWSEM_COUNT_LOCKED(count))
- break;
-
- if (first && !RWSEM_COUNT_HANDOFF(count) &&
- time_after(jiffies, waiter.timeout))
- atomic_or(RWSEM_FLAG_WHANDOFF, &sem->count);
- }
+ schedule();
+ set_current_state(state);
raw_spin_lock_irq(&sem->wait_lock);
+ count = atomic_read(&sem->count);
}
__set_current_state(TASK_RUNNING);
list_del(&waiter.list);
@@ -678,7 +682,6 @@ static inline bool rwsem_has_spinner(struct rw_semaphore *sem)
out_nolock:
__set_current_state(TASK_RUNNING);
- raw_spin_lock_irq(&sem->wait_lock);
list_del(&waiter.list);
adjustment = 0;
if (list_empty(&sem->wait_list))
--
1.8.3.1
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