[patch] add trylock_kernel()

[Ingo Molnar]

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

> the way i fixed it in my tree was to add a trylock_kernel(), and to 
> check for success in init/main.c. See the patch below.

i had a silly bug in the spinlock variant, and some extra unneeded 
change from another debug patch - fixed patch is below. Tested on x86, 
with and without CONFIG_PREEMPT_BKL.

	Ingo

--
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 at elte.hu>

----

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

Index: linux/include/linux/smp_lock.h
===================================================================
--- linux.orig/include/linux/smp_lock.h
+++ linux/include/linux/smp_lock.h
@@ -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
Index: linux/init/main.c
===================================================================
--- linux.orig/init/main.c
+++ linux/init/main.c
@@ -443,11 +443,14 @@ 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);
Index: linux/lib/kernel_lock.c
===================================================================
--- linux.orig/lib/kernel_lock.c
+++ linux/lib/kernel_lock.c
@@ -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);