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Subject[PATCH] 2.5.3 remove global semaphore_lock spin lock.
Below is a patch for i386 that replaces the global spin lock semaphore_lock,
with the rwlock_t embedded in the wait_queue_head_t in the struct semaphore.

None of the data protected by semaphore_lock lock is global and there is
no need to restrict the system to only allow one semaphore to be dealt with
at a time.

This removes 2 lock round trips from __down() and __down_interruptible().
It also reduces the number the cache lines touched by 1 (i.e.: cache line with
semaphore_lock).

This work has only been done for i386 because that is the only arch I have
access to for testing. The same changes could be applied to arm, ia64, s390,
s390x and sparc if requested.

--
Bob Miller Email: rem@osdl.org
Open Software Development Lab Phone: 503.626.2455 Ext. 17


diff -ru linux.2.5.3-orig/arch/i386/kernel/semaphore.c linux.2.5.3/arch/i386/kernel/semaphore.c
--- linux.2.5.3-orig/arch/i386/kernel/semaphore.c Fri Nov 30 08:54:18 2001
+++ linux.2.5.3/arch/i386/kernel/semaphore.c Thu Jan 31 14:18:43 2002
@@ -28,7 +28,8 @@
* the increment operation.
*
* "sleeping" and the contention routine ordering is
- * protected by the semaphore spinlock.
+ * protected by the read/write lock in the semaphore's
+ * waitqueue head.
*
* Note that these functions are only called when there is
* contention on the lock, and as such all this is the
@@ -52,39 +53,41 @@
wake_up(&sem->wait);
}

-static spinlock_t semaphore_lock = SPIN_LOCK_UNLOCKED;
-
void __down(struct semaphore * sem)
{
struct task_struct *tsk = current;
DECLARE_WAITQUEUE(wait, tsk);
+ unsigned long flags;
+
tsk->state = TASK_UNINTERRUPTIBLE;
- add_wait_queue_exclusive(&sem->wait, &wait);
+ wq_write_lock_irqsave(&sem->wait.lock, flags);
+ add_wait_queue_exclusive_locked(&sem->wait, &wait);

- spin_lock_irq(&semaphore_lock);
sem->sleepers++;
for (;;) {
int sleepers = sem->sleepers;

/*
* Add "everybody else" into it. They aren't
- * playing, because we own the spinlock.
+ * playing, because we own the wait lock
+ * exclusivly.
*/
if (!atomic_add_negative(sleepers - 1, &sem->count)) {
sem->sleepers = 0;
break;
}
sem->sleepers = 1; /* us - see -1 above */
- spin_unlock_irq(&semaphore_lock);
+ wq_write_unlock_irqrestore(&sem->wait.lock, flags);

schedule();
+
+ wq_write_lock_irqsave(&sem->wait.lock, flags);
tsk->state = TASK_UNINTERRUPTIBLE;
- spin_lock_irq(&semaphore_lock);
}
- spin_unlock_irq(&semaphore_lock);
- remove_wait_queue(&sem->wait, &wait);
+ remove_wait_queue_locked(&sem->wait, &wait);
+ wake_up_locked(&sem->wait);
+ wq_write_unlock_irqrestore(&sem->wait.lock, flags);
tsk->state = TASK_RUNNING;
- wake_up(&sem->wait);
}

int __down_interruptible(struct semaphore * sem)
@@ -92,11 +95,13 @@
int retval = 0;
struct task_struct *tsk = current;
DECLARE_WAITQUEUE(wait, tsk);
+ unsigned long flags;
+
tsk->state = TASK_INTERRUPTIBLE;
- add_wait_queue_exclusive(&sem->wait, &wait);
+ wq_write_lock_irqsave(&sem->wait.lock, flags);
+ add_wait_queue_exclusive_locked(&sem->wait, &wait);

- spin_lock_irq(&semaphore_lock);
- sem->sleepers ++;
+ sem->sleepers++;
for (;;) {
int sleepers = sem->sleepers;

@@ -116,25 +121,27 @@

/*
* Add "everybody else" into it. They aren't
- * playing, because we own the spinlock. The
- * "-1" is because we're still hoping to get
- * the lock.
+ * playing, because we own the wait lock
+ * exclusivly. The "-1" is because we're still
+ * hoping to get the semaphore.
*/
if (!atomic_add_negative(sleepers - 1, &sem->count)) {
sem->sleepers = 0;
break;
}
sem->sleepers = 1; /* us - see -1 above */
- spin_unlock_irq(&semaphore_lock);
+ wq_write_unlock_irqrestore(&sem->wait.lock, flags);

schedule();
+
+ wq_write_lock_irqsave(&sem->wait.lock, flags);
tsk->state = TASK_INTERRUPTIBLE;
- spin_lock_irq(&semaphore_lock);
}
- spin_unlock_irq(&semaphore_lock);
+ remove_wait_queue_locked(&sem->wait, &wait);
+ wake_up_locked(&sem->wait);
+ wq_write_unlock_irqrestore(&sem->wait.lock, flags);
+
tsk->state = TASK_RUNNING;
- remove_wait_queue(&sem->wait, &wait);
- wake_up(&sem->wait);
return retval;
}

@@ -151,18 +158,19 @@
int sleepers;
unsigned long flags;

- spin_lock_irqsave(&semaphore_lock, flags);
+ wq_write_lock_irqsave(&sem->wait.lock, flags);
sleepers = sem->sleepers + 1;
sem->sleepers = 0;

/*
* Add "everybody else" and us into it. They aren't
- * playing, because we own the spinlock.
+ * playing, because we own the wait lock exclusivly.
*/
- if (!atomic_add_negative(sleepers, &sem->count))
- wake_up(&sem->wait);
+ if (!atomic_add_negative(sleepers, &sem->count)) {
+ wake_up_locked(&sem->wait);
+ }

- spin_unlock_irqrestore(&semaphore_lock, flags);
+ wq_write_unlock_irqrestore(&sem->wait.lock, flags);
return 1;
}

diff -ru linux.2.5.3-orig/include/linux/sched.h linux.2.5.3/include/linux/sched.h
--- linux.2.5.3-orig/include/linux/sched.h Tue Jan 29 21:41:12 2002
+++ linux.2.5.3/include/linux/sched.h Thu Jan 31 10:35:37 2002
@@ -537,6 +537,7 @@

extern void FASTCALL(__wake_up(wait_queue_head_t *q, unsigned int mode, int nr));
extern void FASTCALL(__wake_up_sync(wait_queue_head_t *q, unsigned int mode, int nr));
+extern void FASTCALL(__wake_up_locked(wait_queue_head_t *q, unsigned int mode, int nr));
extern void FASTCALL(sleep_on(wait_queue_head_t *q));
extern long FASTCALL(sleep_on_timeout(wait_queue_head_t *q,
signed long timeout));
@@ -556,6 +557,7 @@
#define wake_up_interruptible_all(x) __wake_up((x),TASK_INTERRUPTIBLE, 0)
#define wake_up_interruptible_sync(x) __wake_up_sync((x),TASK_INTERRUPTIBLE, 1)
#define wake_up_interruptible_sync_nr(x) __wake_up_sync((x),TASK_INTERRUPTIBLE, nr)
+#define wake_up_locked(x) __wake_up_locked((x),TASK_UNINTERRUPTIBLE | TASK_INTERRUPTIBLE, 1)
asmlinkage long sys_wait4(pid_t pid,unsigned int * stat_addr, int options, struct rusage * ru);

extern int in_group_p(gid_t);
@@ -757,7 +759,9 @@

extern void FASTCALL(add_wait_queue(wait_queue_head_t *q, wait_queue_t * wait));
extern void FASTCALL(add_wait_queue_exclusive(wait_queue_head_t *q, wait_queue_t * wait));
+extern void FASTCALL(add_wait_queue_exclusive_locked(wait_queue_head_t *q, wait_queue_t * wait));
extern void FASTCALL(remove_wait_queue(wait_queue_head_t *q, wait_queue_t * wait));
+extern void FASTCALL(remove_wait_queue_locked(wait_queue_head_t *q, wait_queue_t * wait));

extern void wait_task_inactive(task_t * p);
extern void kick_if_running(task_t * p);
diff -ru linux.2.5.3-orig/kernel/fork.c linux.2.5.3/kernel/fork.c
--- linux.2.5.3-orig/kernel/fork.c Mon Jan 28 15:11:45 2002
+++ linux.2.5.3/kernel/fork.c Thu Jan 31 09:55:20 2002
@@ -59,6 +59,15 @@
wq_write_unlock_irqrestore(&q->lock, flags);
}

+/*
+ * Must be called with wait queue lock held in write mode.
+ */
+void add_wait_queue_exclusive_locked(wait_queue_head_t *q, wait_queue_t * wait)
+{
+ wait->flags |= WQ_FLAG_EXCLUSIVE;
+ __add_wait_queue_tail(q, wait);
+}
+
void remove_wait_queue(wait_queue_head_t *q, wait_queue_t * wait)
{
unsigned long flags;
@@ -68,6 +77,14 @@
wq_write_unlock_irqrestore(&q->lock, flags);
}

+/*
+ * Must be called with wait queue lock held in write mode.
+ */
+void remove_wait_queue_locked(wait_queue_head_t *q, wait_queue_t * wait)
+{
+ __remove_wait_queue(q, wait);
+}
+
void __init fork_init(unsigned long mempages)
{
/*
diff -ru linux.2.5.3-orig/kernel/sched.c linux.2.5.3/kernel/sched.c
--- linux.2.5.3-orig/kernel/sched.c Mon Jan 28 15:12:47 2002
+++ linux.2.5.3/kernel/sched.c Thu Jan 31 09:55:20 2002
@@ -743,6 +743,17 @@
}
}

+/*
+ * Same as __wake_up but called with the wait_queue_head_t lock held
+ * in at least read mode.
+ */
+void __wake_up_locked(wait_queue_head_t *q, unsigned int mode, int nr)
+{
+ if (q) {
+ __wake_up_common(q, mode, nr, 0);
+ }
+}
+
void __wake_up_sync(wait_queue_head_t *q, unsigned int mode, int nr)
{
if (q) {
-
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