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Subject[PATCH 04/10] rwsem: let RWSEM_WAITING_BIAS represent any number of waiting threads
Date
Previously each waiting thread added a bias of RWSEM_WAITING_BIAS. With this
change, the bias is added only once when the wait list is non-empty.

This has a few nice properties which will be used in following changes:
- when the spinlock is held and the waiter list is known to be non-empty,
count < RWSEM_WAITING_BIAS <=> there is an active writer on that sem
- count == RWSEM_WAITING_BIAS <=> there are waiting threads and no
active readers/writers on that sem

Signed-off-by: Michel Lespinasse <walken@google.com>
---
lib/rwsem.c | 28 +++++++++++++++++-----------
1 files changed, 17 insertions(+), 11 deletions(-)
diff --git a/lib/rwsem.c b/lib/rwsem.c
index ab0d306..b2dde5a 100644
--- a/lib/rwsem.c
+++ b/lib/rwsem.c
@@ -60,7 +60,7 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type)
struct rwsem_waiter *waiter;
struct task_struct *tsk;
struct list_head *next;
- signed long oldcount, woken, loop;
+ signed long oldcount, woken, loop, adjustment;

waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
if (!(waiter->flags & RWSEM_WAITING_FOR_WRITE))
@@ -73,9 +73,12 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type)
* write lock. However, we only wake this writer if we can transition
* the active part of the count from 0 -> 1
*/
+ adjustment = RWSEM_ACTIVE_WRITE_BIAS;
+ if (waiter->list.next == &sem->wait_list)
+ adjustment -= RWSEM_WAITING_BIAS;
+
try_again:
- oldcount = rwsem_atomic_update(RWSEM_ACTIVE_BIAS, sem)
- - RWSEM_ACTIVE_BIAS;
+ oldcount = rwsem_atomic_update(adjustment, sem) - adjustment;
if (oldcount & RWSEM_ACTIVE_MASK)
/* Someone grabbed the sem already */
goto undo;
@@ -128,13 +131,15 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type)

} while (waiter->flags & RWSEM_WAITING_FOR_READ);

- loop = woken;
- woken *= RWSEM_ACTIVE_BIAS - RWSEM_WAITING_BIAS;
+ adjustment = woken * RWSEM_ACTIVE_READ_BIAS;
+ if (waiter->flags & RWSEM_WAITING_FOR_READ)
+ /* hit end of list above */
+ adjustment -= RWSEM_WAITING_BIAS;

- rwsem_atomic_add(woken, sem);
+ rwsem_atomic_add(adjustment, sem);

next = sem->wait_list.next;
- for (; loop > 0; loop--) {
+ for (loop = woken; loop > 0; loop--) {
waiter = list_entry(next, struct rwsem_waiter, list);
next = waiter->list.next;
tsk = waiter->task;
@@ -153,7 +158,7 @@ __rwsem_do_wake(struct rw_semaphore *sem, int wake_type)
/* undo the change to the active count, but check for a transition
* 1->0 */
undo:
- if (rwsem_atomic_update(-RWSEM_ACTIVE_BIAS, sem) & RWSEM_ACTIVE_MASK)
+ if (rwsem_atomic_update(-adjustment, sem) & RWSEM_ACTIVE_MASK)
goto out;
goto try_again;
}
@@ -175,6 +180,8 @@ rwsem_down_failed_common(struct rw_semaphore *sem,
waiter->task = tsk;
get_task_struct(tsk);

+ if (list_empty(&sem->wait_list))
+ adjustment += RWSEM_WAITING_BIAS;
list_add_tail(&waiter->list, &sem->wait_list);

/* we're now waiting on the lock, but no longer actively locking */
@@ -208,8 +215,7 @@ rwsem_down_read_failed(struct rw_semaphore *sem)
struct rwsem_waiter waiter;

waiter.flags = RWSEM_WAITING_FOR_READ;
- rwsem_down_failed_common(sem, &waiter,
- RWSEM_WAITING_BIAS - RWSEM_ACTIVE_BIAS);
+ rwsem_down_failed_common(sem, &waiter, -RWSEM_ACTIVE_READ_BIAS);
return sem;
}

@@ -222,7 +228,7 @@ rwsem_down_write_failed(struct rw_semaphore *sem)
struct rwsem_waiter waiter;

waiter.flags = RWSEM_WAITING_FOR_WRITE;
- rwsem_down_failed_common(sem, &waiter, -RWSEM_ACTIVE_BIAS);
+ rwsem_down_failed_common(sem, &waiter, -RWSEM_ACTIVE_WRITE_BIAS);

return sem;
}
--
1.7.0.1


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