[patch] xmon bitlock fix

[Nick Piggin <npiggin@suse.de>]

(sorry, untested for lack of hardware)
--
xmon uses a bit lock spinlock but doesn't close the critical section
when releasing it. It doesn't seem like a big deal because it will
eventually break out of the lock anyway, but presumably that's only
in exceptional cases where some error is tolerated, while the lack of a memory
barrier could allow incorrect results during normal functioning operation
as well.

Convert it to use a regular spinlock instead.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
---
Index: linux-2.6/arch/ppc/xmon/start.c
===================================================================
--- linux-2.6.orig/arch/ppc/xmon/start.c
+++ linux-2.6/arch/ppc/xmon/start.c
@@ -92,16 +92,14 @@ xmon_write(void *handle, void *ptr, int 
 {
 	char *p = ptr;
 	int i, c, ct;
-
-#ifdef CONFIG_SMP
-	static unsigned long xmon_write_lock;
+	static DEFINE_SPINLOCK(xmon_write_lock);
 	int lock_wait = 1000000;
 	int locked;
 
-	while ((locked = test_and_set_bit(0, &xmon_write_lock)) != 0)
+	while (!(locked = spin_trylock(&xmon_write_lock))) {
 		if (--lock_wait == 0)
 			break;
-#endif
+	}
 
 	if (!scc_initialized)
 		xmon_init_scc();
@@ -122,10 +120,9 @@ xmon_write(void *handle, void *ptr, int 
 		eieio();
 	}
 
-#ifdef CONFIG_SMP
-	if (!locked)
-		clear_bit(0, &xmon_write_lock);
-#endif
+	if (locked)
+		spin_unlock(&xmon_write_lock);
+
 	return nb;
 }