[PATCH] simple preemption debug check

[Robert Love <rml@tech9.net>]

Linus,

This simple check was first suggested by Andrew Morton.  Pretty basic -
whines if a task exits with a nonzero preempt_count value.

I put an identical check in the 2.4 preempt-kernel patch and - sure
enough - it was found that XFS essentially disables preemption as it
destroys data structures containing locks without first unlocking.  The
SGI folks are working on that.

Anyhow, its a quick and clean solution to debugging potential problems. 
Patch is against 2.5.7, please apply.

	Robert Love

diff -urN linux-2.5.7/include/linux/spinlock.h linux/include/linux/spinlock.h
--- linux-2.5.7/include/linux/spinlock.h	Mon Mar 18 15:37:15 2002
+++ linux/include/linux/spinlock.h	Wed Apr  3 18:46:00 2002
@@ -204,7 +204,7 @@
 
 #else
 
-#define preempt_get_count()	do { } while (0)
+#define preempt_get_count()	(0)
 #define preempt_disable()	do { } while (0)
 #define preempt_enable_no_resched()	do {} while(0)
 #define preempt_enable()	do { } while (0)
diff -urN linux-2.5.7/kernel/exit.c linux/kernel/exit.c
--- linux-2.5.7/kernel/exit.c	Mon Mar 18 15:37:10 2002
+++ linux/kernel/exit.c	Wed Apr  3 18:44:24 2002
@@ -492,6 +492,11 @@
 	tsk->flags |= PF_EXITING;
 	del_timer_sync(&tsk->real_timer);
 
+	if (unlikely(preempt_get_count()))
+		printk(KERN_ERR "error: %s[%d] exited with preempt_count %d\n",
+				current->comm, current->pid,
+				preempt_get_count());
+
 fake_volatile:
 #ifdef CONFIG_BSD_PROCESS_ACCT
 	acct_process(code);