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rwsem: let RWSEM_WAITING_BIAS represent any number of waiting threads

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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>
This commit is contained in:
Michel Lespinasse 2010-08-09 17:21:18 -07:00 committed by Linus Torvalds
parent 70bdc6e064
commit fd41b33435
1 changed files with 17 additions and 11 deletions
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28
lib/rwsem.c
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@ -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;
}