Linux启动过程中init/main.c中的start_kernel()函数中的lock_kernel()函数

#ifndef __LINUX_SMPLOCK_H
#define __LINUX_SMPLOCK_H

#ifdef CONFIG_LOCK_KERNEL

//判断内核是否支持内核锁

//而s3c2410中arch/arm/configs/s3c2410的Code maturity level options下没有定义，所以lock_kernel()什么也不做

#include <linux/sched.h>
#include <linux/spinlock.h>

#define kernel_locked()		(current->lock_depth >= 0)

extern int __lockfunc __reacquire_kernel_lock(void);
extern void __lockfunc __release_kernel_lock(void);

/*
* Release/re-acquire global kernel lock for the scheduler
*/
#define release_kernel_lock(tsk) do { 		\
if (unlikely((tsk)->lock_depth >= 0))	\
__release_kernel_lock();	\
} while (0)

/*
* Non-SMP kernels will never block on the kernel lock,
* so we are better off returning a constant zero from
* reacquire_kernel_lock() so that the compiler can see
* it at compile-time.
*/
#if defined(CONFIG_SMP) && !defined(CONFIG_PREEMPT_BKL)
# define return_value_on_smp return
#else
# define return_value_on_smp
#endif

static inline int reacquire_kernel_lock(struct task_struct *task)
{
if (unlikely(task->lock_depth >= 0))
return_value_on_smp __reacquire_kernel_lock();
return 0;
}

extern void __lockfunc lock_kernel(void)	__acquires(kernel_lock);
extern void __lockfunc unlock_kernel(void)	__releases(kernel_lock);

#else

#define lock_kernel()				do { } while(0)
#define unlock_kernel()				do { } while(0)
#define release_kernel_lock(task)		do { } while(0)
#define reacquire_kernel_lock(task)		0
#define kernel_locked()				1

#endif /* CONFIG_LOCK_KERNEL */
#endif /* __LINUX_SMPLOCK_H */

//如果定义了CONFIG_LOCK_KERNEL，则转到下面代码

//判断是使用big kernel semaphore还是big kernel lock

//而s3c2410中arch/arm/configs/s3c2410中没有定义

#ifdef CONFIG_PREEMPT_BKL  //判断是使用big kernel semaphore还是big kernel lock
//而s3c2410中arch/arm/configs/s3c2410中没有定义
/*
* The 'big kernel semaphore'
*
* This mutex is taken and released recursively by lock_kernel()
* and unlock_kernel().  It is transparently dropped and reacquired
* over schedule().  It is used to protect legacy code that hasn't
* been migrated to a proper locking design yet.
*
* Note: code locked by this semaphore will only be serialized against
* other code using the same locking facility. The code guarantees that
* the task remains on the same CPU.
*
* Don't use in new code.
*/
static DECLARE_MUTEX(kernel_sem);

/*
* Re-acquire the kernel semaphore.
*
* This function is called with preemption off.
*
* We are executing in schedule() so the code must be extremely careful
* about recursion, both due to the down() and due to the enabling of
* preemption. schedule() will re-check the preemption flag after
* reacquiring the semaphore.
*/
int __lockfunc __reacquire_kernel_lock(void)
{
struct task_struct *task = current;
int saved_lock_depth = task->lock_depth;

BUG_ON(saved_lock_depth < 0);

task->lock_depth = -1;
preempt_enable_no_resched();

down(&kernel_sem);

preempt_disable();
task->lock_depth = saved_lock_depth;

return 0;
}

void __lockfunc __release_kernel_lock(void)
{
up(&kernel_sem);
}

/*
* Getting the big kernel semaphore.
*/
void __lockfunc lock_kernel(void)
{
struct task_struct *task = current;
int depth = task->lock_depth + 1;

if (likely(!depth))
/*
* No recursion worries - we set up lock_depth _after_
*/
down(&kernel_sem);

task->lock_depth = depth;
}

void __lockfunc unlock_kernel(void)
{
struct task_struct *task = current;

BUG_ON(task->lock_depth < 0);

if (likely(--task->lock_depth < 0))
up(&kernel_sem);
}

#else

/*
* The 'big kernel lock'
*
* This spinlock is taken and released recursively by lock_kernel()
* and unlock_kernel().  It is transparently dropped and reacquired
* over schedule().  It is used to protect legacy code that hasn't
* been migrated to a proper locking design yet.
*
* Don't use in new code.
*/
static  __cacheline_aligned_in_smp DEFINE_SPINLOCK(kernel_flag);

/*
* Acquire/release the underlying lock from the scheduler.
*
* This is called with preemption disabled, and should
* return an error value if it cannot get the lock and
* TIF_NEED_RESCHED gets set.
*
* If it successfully gets the lock, it should increment
* the preemption count like any spinlock does.
*
* (This works on UP too - _raw_spin_trylock will never
* return false in that case)
*/
int __lockfunc __reacquire_kernel_lock(void)
{
while (!_raw_spin_trylock(&kernel_flag)) {
if (test_thread_flag(TIF_NEED_RESCHED))
return -EAGAIN;
cpu_relax();
}
preempt_disable();
return 0;
}

void __lockfunc __release_kernel_lock(void)
{
_raw_spin_unlock(&kernel_flag);
preempt_enable_no_resched();
}

/*
* These are the BKL spinlocks - we try to be polite about preemption.
* If SMP is not on (ie UP preemption), this all goes away because the
* _raw_spin_trylock() will always succeed.
*/
#ifdef CONFIG_PREEMPT

//使用big kernel lock的情况下，判断内核是否支持抢占式调度,支持则执行下面的代码

//而我们使用的s3c2410是单处理器的，不存在多个CPU竞争资源的情况，所以不需要用大内核锁/信号量来解决资源竞争的问题

//CONFIG_PREEMPT在arch/arm/configs/s3c2410_deconfig中Kernel Features下，在s3c2410中定义为# CONFIG_PREEMPT is not set

static inline void __lock_kernel(void)
{
preempt_disable();
if (unlikely(!_raw_spin_trylock(&kernel_flag))) {
/*
* If preemption was disabled even before this
* was called, there's nothing we can be polite
* about - just spin.
*/
if (preempt_count() > 1) {
_raw_spin_lock(&kernel_flag);
return;
}

/*
* Otherwise, let's wait for the kernel lock
* with preemption enabled..
*/
do {
preempt_enable();
while (spin_is_locked(&kernel_flag))
cpu_relax();
preempt_disable();
} while (!_raw_spin_trylock(&kernel_flag));
}
}

#else

/*
* Non-preemption case - just get the spinlock
*/
static inline void __lock_kernel(void)
{
_raw_spin_lock(&kernel_flag);
}
#endif

static inline void __unlock_kernel(void)
{
/*
* the BKL is not covered by lockdep, so we open-code the
* unlocking sequence (and thus avoid the dep-chain ops):
*/
_raw_spin_unlock(&kernel_flag);
preempt_enable();
}

/*
* Getting the big kernel lock.
*
* This cannot happen asynchronously, so we only need to
* worry about other CPU's.
*/
void __lockfunc lock_kernel(void)
{
int depth = current->lock_depth+1;  //lock_depth的初始值为-1，所以depth=0
if (likely(!depth))  //通过
__lock_kernel();  //执行
current->lock_depth = depth;
}

void __lockfunc unlock_kernel(void)
{
BUG_ON(current->lock_depth < 0);
if (likely(--current->lock_depth < 0))
__unlock_kernel();
}

#endif

EXPORT_SYMBOL(lock_kernel);
EXPORT_SYMBOL(unlock_kernel);