> Of course, someone who wrote it could do better ;-)
Here is Willy's code in patch format (against linux-next
tree tag next-20080326 which includes his re-write of the
semaphore code).
This looks a lot cleaner than my ia64 specific code that
used cmpxchg() for the down() operation and fetchadd for
the up() ... using a brand new semaphore_spin data type.
It appears to work ... I tried to do some timing comparisons
of this generic version against my arch specific one, but the
hackbench test case has a run to run variation of a factor of
three (from 1min9sec to 3min44sec) so it is hopeless to try
and see some small percentage difference.
commit 0359fbb64297d44328f26ec5fda3a3c26f1c5ba7
Author: Matthew Wilcox <matthew@wil.cx>
Date: Wed Mar 26 11:08:18 2008 -0700
Add "down_spin()" API for semaphores
For those places that need semaphore semantics but cannot sleep
Signed-off-by: Tony Luck <tony.luck@intel.com>
diff --git a/include/linux/semaphore.h b/include/linux/semaphore.h
index a7125da..3404ce5 100644
--- a/include/linux/semaphore.h
+++ b/include/linux/semaphore.h
@@ -53,6 +53,12 @@ static inline void sema_init(struct semaphore *sem, int val)
extern void down(struct semaphore *sem);
/*
+ * Attempt to acquire the semaphore. If another task is already holding the
+ * semaphore, spin until the semaphore is released.
+ */
+extern void down_spin(struct semaphore *sem);
+
+/*
* As down(), except the sleep may be interrupted by a signal. If it is,
* this function will return -EINTR.
*/
diff --git a/kernel/semaphore.c b/kernel/semaphore.c
index bef977b..d3eb559 100644
--- a/kernel/semaphore.c
+++ b/kernel/semaphore.c
@@ -23,6 +23,7 @@
*/
static noinline void __down(struct semaphore *sem);
+static noinline void __down_spin(struct semaphore *sem);
static noinline int __down_interruptible(struct semaphore *sem);
static noinline int __down_killable(struct semaphore *sem);
static noinline int __down_timeout(struct semaphore *sem, long jiffies);
@@ -41,6 +42,21 @@ void down(struct semaphore *sem)
}
EXPORT_SYMBOL(down);
+void down_spin(struct semaphore *sem)
+{
+ unsigned long flags;
+ int count;
+
+ spin_lock_irqsave(&sem->lock, flags);
+ count = sem->count - 1;
+ if (likely(count >= 0))
+ sem->count = count;
+ else
+ __down_spin(sem);
+ spin_unlock_irqrestore(&sem->lock, flags);
+}
+EXPORT_SYMBOL(down_spin);
+
int down_interruptible(struct semaphore *sem)
{
unsigned long flags;
@@ -197,6 +213,20 @@ static noinline int __sched __down_timeout(struct semaphore *sem, long jiffies)
return __down_common(sem, TASK_UNINTERRUPTIBLE, jiffies);
}
+static noinline void __sched __down_spin(struct semaphore *sem)
+{
+ struct semaphore_waiter waiter;
+
+ list_add_tail(&waiter.list, &sem->wait_list);
+ waiter.task = current;
+ waiter.up = 0;
+
+ spin_unlock_irq(&sem->lock);
+ while (!waiter.up)
+ cpu_relax();
+ spin_lock_irq(&sem->lock);
+}
+
static noinline void __sched __up(struct semaphore *sem)
{
struct semaphore_waiter *waiter = list_first_entry(&sem->wait_list,
--
