From: Ingo Molnar <mingo@elte.hu>

Introduce trylock_kernel(), to be used by the early init code to acquire
the BKL in an atomic way.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
---

 include/linux/smp_lock.h |    1 +
 init/main.c              |   13 ++++++++-----
 lib/kernel_lock.c        |   34 ++++++++++++++++++++++++++++++++++
 3 files changed, 43 insertions(+), 5 deletions(-)

diff -puN include/linux/smp_lock.h~add-trylock_kernel include/linux/smp_lock.h
--- devel/include/linux/smp_lock.h~add-trylock_kernel	2006-01-18 18:51:58.000000000 -0800
+++ devel-akpm/include/linux/smp_lock.h	2006-01-18 18:51:58.000000000 -0800
@@ -39,6 +39,7 @@ static inline int reacquire_kernel_lock(
 }
 
 extern void __lockfunc lock_kernel(void)	__acquires(kernel_lock);
+extern int __lockfunc trylock_kernel(void);
 extern void __lockfunc unlock_kernel(void)	__releases(kernel_lock);
 
 #else
diff -puN init/main.c~add-trylock_kernel init/main.c
--- devel/init/main.c~add-trylock_kernel	2006-01-18 18:51:58.000000000 -0800
+++ devel-akpm/init/main.c	2006-01-18 18:51:58.000000000 -0800
@@ -442,11 +442,14 @@ asmlinkage void __init start_kernel(void
 {
 	char * command_line;
 	extern struct kernel_param __start___param[], __stop___param[];
-/*
- * Interrupts are still disabled. Do necessary setups, then
- * enable them
- */
-	lock_kernel();
+
+	/*
+	 * Interrupts are still disabled. Do necessary setups, then
+	 * enable them. This is the first time we take the BKL, so
+	 * it must succeed:
+	 */
+	if (!trylock_kernel())
+		WARN_ON(1);
 	page_address_init();
 	printk(KERN_NOTICE);
 	printk(linux_banner);
diff -puN lib/kernel_lock.c~add-trylock_kernel lib/kernel_lock.c
--- devel/lib/kernel_lock.c~add-trylock_kernel	2006-01-18 18:51:58.000000000 -0800
+++ devel-akpm/lib/kernel_lock.c	2006-01-18 18:51:58.000000000 -0800
@@ -76,6 +76,23 @@ void __lockfunc lock_kernel(void)
 	task->lock_depth = depth;
 }
 
+int __lockfunc trylock_kernel(void)
+{
+	struct task_struct *task = current;
+	int depth = task->lock_depth + 1;
+
+	if (likely(!depth)) {
+		if (unlikely(down_trylock(&kernel_sem)))
+			return 0;
+		else
+			__acquire(kernel_sem);
+	}
+
+	task->lock_depth = depth;
+	return 1;
+}
+
+
 void __lockfunc unlock_kernel(void)
 {
 	struct task_struct *task = current;
@@ -194,6 +211,22 @@ void __lockfunc lock_kernel(void)
 	current->lock_depth = depth;
 }
 
+int __lockfunc trylock_kernel(void)
+{
+	struct task_struct *task = current;
+	int depth = task->lock_depth + 1;
+
+	if (likely(!depth)) {
+		if (unlikely(!spin_trylock(&kernel_flag)))
+			return 0;
+		else
+			__acquire(kernel_sem);
+	}
+
+	task->lock_depth = depth;
+	return 1;
+}
+
 void __lockfunc unlock_kernel(void)
 {
 	BUG_ON(current->lock_depth < 0);
@@ -204,5 +237,6 @@ void __lockfunc unlock_kernel(void)
 #endif
 
 EXPORT_SYMBOL(lock_kernel);
+/* we do not export trylock_kernel(). BKL code should shrink :-) */
 EXPORT_SYMBOL(unlock_kernel);
 
_