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

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