Re: [patch] speed up / fix the new generic semaphore code (fix AIM7 40% regression with 2.6.26-rc1)

[Ingo Molnar <mingo@elte.hu>]

* Ingo Molnar <mingo@elte.hu> wrote:

> Peter pointed it out that because sem->count is u32, the <= 0 is in 
> fact a "== 0" condition - the patch below does that. As expected gcc 
> figured out the same thing too so the resulting code output did not 
> change. (so this is just a cleanup)

a second update patch, i've further simplified the semaphore wakeup 
logic: there's no need for the wakeup to remove the task from the wait 
list. This will make them a slighly bit more fair, but more importantly, 
this closes a race in my first patch for the unlikely case of a signal 
(or a timeout) and an unlock coming in at the same time and the task not 
getting removed from the wait-list.

( my performance testing with 2000 AIM7 tasks on a quad never hit that
  race but x86.git QA actually triggered it after about 30 random kernel
  bootups and it caused a nasty crash and lockup. )

	Ingo

---------------->
Subject: sem: simplify queue management
From: Ingo Molnar <mingo@elte.hu>
Date: Tue May 06 19:32:42 CEST 2008

kernel/semaphore.o:

   text	   data	    bss	    dec	    hex	filename
   1040	      0	      0	   1040	    410	semaphore.o.before
    975	      0	      0	    975	    3cf	semaphore.o.after

Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 kernel/semaphore.c |   32 ++++++++++++++++----------------
 1 file changed, 16 insertions(+), 16 deletions(-)

Index: linux/kernel/semaphore.c
===================================================================
--- linux.orig/kernel/semaphore.c
+++ linux/kernel/semaphore.c
@@ -202,33 +202,34 @@ static inline int __sched __down_common(
 {
 	struct task_struct *task = current;
 	struct semaphore_waiter waiter;
+	int ret = 0;
 
 	waiter.task = task;
+	list_add_tail(&waiter.list, &sem->wait_list);
 
 	for (;;) {
-		list_add_tail(&waiter.list, &sem->wait_list);
-
-		if (state == TASK_INTERRUPTIBLE && signal_pending(task))
-			goto interrupted;
-		if (state == TASK_KILLABLE && fatal_signal_pending(task))
-			goto interrupted;
-		if (timeout <= 0)
-			goto timed_out;
+		if (state == TASK_INTERRUPTIBLE && signal_pending(task)) {
+			ret = -EINTR;
+			break;
+		}
+		if (state == TASK_KILLABLE && fatal_signal_pending(task)) {
+			ret = -EINTR;
+			break;
+		}
+		if (timeout <= 0) {
+			ret = -ETIME;
+			break;
+		}
 		__set_task_state(task, state);
 		spin_unlock_irq(&sem->lock);
 		timeout = schedule_timeout(timeout);
 		spin_lock_irq(&sem->lock);
 		if (sem->count > 0)
-			return 0;
+			break;
 	}
 
- timed_out:
-	list_del(&waiter.list);
-	return -ETIME;
-
- interrupted:
 	list_del(&waiter.list);
-	return -EINTR;
+	return ret;
 }
 
 static noinline void __sched __down(struct semaphore *sem)
@@ -255,6 +256,5 @@ static noinline void __sched __up(struct
 {
 	struct semaphore_waiter *waiter = list_first_entry(&sem->wait_list,
 						struct semaphore_waiter, list);
-	list_del(&waiter->list);
 	wake_up_process(waiter->task);
 }