aboutsummaryrefslogtreecommitdiff
path: root/lib/rwsem.c
diff options
context:
space:
mode:
authorMichel Lespinasse <walken@google.com>2010-08-09 17:21:18 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2010-08-09 20:45:11 -0700
commitfd41b33435ada87323cc86b50959fbffe35192c8 (patch)
tree6d2a7e79557e5de21bc91fce5b939c1f7dc1355c /lib/rwsem.c
parent70bdc6e0644f3535e93bac5c364ca199397e507e (diff)
downloadlinux-linaro-precise-fd41b33435ada87323cc86b50959fbffe35192c8.tar.gz
rwsem: let RWSEM_WAITING_BIAS represent any number of waiting threads
Previously each waiting thread added a bias of RWSEM_WAITING_BIAS. With this change, the bias is added only once to indicate that 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> Acked-by: David Howells <dhowells@redhat.com> Cc: Mike Waychison <mikew@google.com> Cc: Suleiman Souhlal <suleiman@google.com> Cc: Ying Han <yinghan@google.com> Cc: Ingo Molnar <mingo@elte.hu> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: "H. Peter Anvin" <hpa@zytor.com> Cc: Peter Zijlstra <a.p.zijlstra@chello.nl> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'lib/rwsem.c')
-rw-r--r--lib/rwsem.c28
1 files changed, 17 insertions, 11 deletions
diff --git a/lib/rwsem.c b/lib/rwsem.c
index 94f2d7a9dc4..a3e68bf5932 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_write:
- 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_write;
@@ -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_write:
- 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_write;
}
@@ -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;
}