From fd41b33435ada87323cc86b50959fbffe35192c8 Mon Sep 17 00:00:00 2001 From: Michel Lespinasse Date: Mon, 9 Aug 2010 17:21:18 -0700 Subject: [PATCH] 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 Acked-by: David Howells Cc: Mike Waychison Cc: Suleiman Souhlal Cc: Ying Han Cc: Ingo Molnar Cc: Thomas Gleixner Cc: "H. Peter Anvin" Cc: Peter Zijlstra Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- lib/rwsem.c | 28 +++++++++++++++++----------- 1 file changed, 17 insertions(+), 11 deletions(-) diff --git a/lib/rwsem.c b/lib/rwsem.c index 94f2d7a9dc4f..a3e68bf5932e 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; } -- 2.34.1