一、自旋锁
自旋锁是专为防止多处理器并发而引入的一种锁，它在内核中大量应用于中断处理等部分(对于单处理器来说，防止中断处理中的并发可简单采用关闭中断的方式，即在标志寄存器中关闭/打开中断标志位，不需要自旋锁)。
自旋不能抢占，但能中断。
首先定义
Spinlock_t lock;
/*
* ARMv6 Spin-locking.
*
* We (exclusively) read the old value, and decrement it. If it
* hits zero, we may have won the lock, so we try (exclusively)
* storing it.
*
* Unlocked value: 0
* Locked value: 1
*/
typedef struct {
volatile unsigned int lock;
#ifdef CONFIG_PREEMPT
unsigned int break_lock;
#endif
} spinlock_t;
补上x86平台
#define SPINLOCK_MAGIC0x1D244B3C
typedef struct {
unsigned long magic;
volatile unsigned long lock;
volatile unsigned int babble;
const char *module; // 所属模块
char *owner;
int oline;
} spinlock_t;
Lock为0时可以用，1是等待。0像锁孔，当没有钥匙插进去时，它才可以插进去
怎么初始化呢？
#define spin_lock_init(x)
do {
(x)->magic = SPINLOCK_MAGIC;
(x)->lock = 0; ；0初始化，表示可用
(x)->babble = 5;
(x)->module = __FILE__;
(x)->owner = NULL;
(x)->oline = 0;
} while (0)
定义一个自旋锁的方法很有意思，
Spinlock_t lock=？？？？？
可以通过spin_lock
Spin_lock_irqsave 来调用自旋锁，后者不允许中断。前者有可能在上锁中发生中断。
还有spin_trylock 这是一个绝不妥协的函数，它不等待。
恢复为spin_unlock
Spin_unlock_irqrestore
考查下面代码
#define spin_lock_irqsave(lock, flags)_spin_lock_irqsave(lock, flags)
#define _spin_lock_irqsave(lock, flags)
do {
local_irq_save(flags); 保存中断请求标志
preempt_disable(); 不允许抢占
_raw_spin_lock(lock);
__acquire(lock);
} while (0)
二、自旋锁综合使用
下面是一个使用的例子，你可以使用source insight查到它
/* never called when PTRS_PER_PMD > 1 */
void pgd_dtor(void *pgd, kmem_cache_t *cache, unsigned long unused)
{
unsigned long flags; /* can be called from interrupt context */
spin_lock_irqsave(&pgd_lock, flags); 枷锁
pgd_list_del(pgd);
spin_unlock_irqrestore(&pgd_lock, flags); 释放
}
中断枷锁
#define spin_lock_irqsave(lock, flags)_spin_lock_irqsave(lock, flags)
分析
unsigned long __lockfunc _spin_lock_irqsave(spinlock_t *lock)
{
unsigned long flags;
local_irq_save(flags); 将寄存器存入flags，并关中断
preempt_disable(); 抢占锁
_raw_spin_lock_flags(lock, flags); 枷锁
return flags;
}
EXPORT_SYMBOL(_spin_lock_irqsave);
继续
/* For spinlocks etc */
#define local_irq_save(x)__asm__ __volatile__("pushfl ; popl %0 ; cli":"=g" (x): /* no input */ :"memory")
将标志寄存器的内容放在内存x中。请查看gcc汇编
继续
static inline void _raw_spin_lock_flags (spinlock_t *lock, unsigned long flags)
{
#ifdef CONFIG_DEBUG_SPINLOCK
if (unlikely(lock->magic != SPINLOCK_MAGIC)) {
printk("eip: %p ", __builtin_return_address(0));
BUG();
}
#endif
__asm__ __volatile__(
spin_lock_string_flags
:"=m" (lock->slock) : "r" (flags) : "memory");
}
继续
#define spin_lock_string_flags
" 1: "
"lock ; decb %0 " ；lock总线锁住，原子操作
"jns 4f "
"2: "
"testl $0x200, %1 "
"jz 3f "
"sti "
"3: "
"rep;nop "
"cmpb $0, %0 "
"jle 3b "
"cli "
"jmp 1b "
"4: "
理解一下大概意思，就可以了。当lock-1后大于等于0就可以关中断继续执行了，否则nop空操作。Nop期间，cpu可以执行其他任务的代码。
解锁
#define spin_unlock_irqrestore(lock, flags)_spin_unlock_irqrestore(lock, flags)
void __lockfunc _spin_unlock_irqrestore(spinlock_t *lock, unsigned long flags)
{
_raw_spin_unlock(lock);
local_irq_restore(flags);
preempt_enable();
}
static inline void _raw_spin_unlock(spinlock_t *lock)
{
#ifdef CONFIG_DEBUG_SPINLOCK
BUG_ON(lock->magic != SPINLOCK_MAGIC);
BUG_ON(!spin_is_locked(lock));
#endif
__asm__ __volatile__(
spin_unlock_string
);
}
Raw赤裸的解锁，表示最低沉的解锁原理。
#define spin_unlock_string
"xchgb %b0, %1"
:"=q" (oldval), "=m" (lock->slock)
:"0" (oldval) : "memory"
加1.解锁