1. 前言

　内核版本：linux 4.9.225。内核版本：linux 4.9.225。对于内核常用的might_sleep函数，如果没有调试的需要(没有定义CONFIG_DEBUG_ATOMIC_SLEEP)，这个宏/函数什么事情都不，might_sleep就是一个空函数，所以平常看code的时候可以忽略。内核只是用它来提醒开发人员，调用该函数的函数可能会sleep。

2. might_sleep的定义

# include/linux/kernel.
#ifdef CONFIG_PREEMPT_VOLUNTARY
extern int _cond_resched(void);
# define might_resched() _cond_resched()
#else
# define might_resched() do { } while (0)
#endif

#ifdef CONFIG_DEBUG_ATOMIC_SLEEP
  void ___might_sleep(const char *file, int line, int preempt_offset);
  void __might_sleep(const char *file, int line, int preempt_offset);
/**
 * might_sleep - annotation for functions that can sleep
 *
 * this macro will print a stack trace if it is executed in an atomic
 * context (spinlock, irq-handler, ...).
 *
 * This is a useful debugging help to be able to catch problems early and not
 * be bitten later when the calling function happens to sleep when it is not
 * supposed to.
 */
# define might_sleep() \
	do { __might_sleep(__FILE__, __LINE__, 0); might_resched(); } while (0)
#else
# define might_sleep() do { might_resched(); } while (0)
#endif

2.1 未开启CONFIG_DEBUG_ATOMIC_SLEEP选项

在没有调试需求，即在选项 CONFIG_DEBUG_ATOMIC_SLEEP和 CONFIG_PREEMPT_VOLUNTARY(自愿抢占，代码中增加抢占点，在中断退出后遇到抢占点时进行抢占切换)未打开的情况下：

# define might_resched() do { } while (0)
# define might_sleep() do { might_resched(); } while (0)

可以看到，这里什么事情都没做。其中内核源码对此也有明确的注释：might_sleep - annotation for functions that can sleep。所以对于release版的kernel 而言，might_sleep函数仅仅是一个annotation，用来提醒开发人员，一个使用might_sleep的函数在其后的代码执行中可能会sleep。

2.2 开启CONFIG_DEBUG_ATOMIC_SLEEP选项

如果有调试需求的话，就必须打开内核的 CONFIG_DEBUG_ATOMIC_SLEEP选项。 此时might_sleep定义如下：

# define might_resched() _cond_resched()
# define might_sleep() \
	do { __might_sleep(__FILE__, __LINE__, 0); might_resched(); } while (0)

CONFIG_DEBUG_ATOMIC_SLEEP选项主要用来排查是否在一个ATOMIC操作的上下文中有函数发生sleep行为，关于什么是 ATOMIC操作，内核源码在might_sleep函数前也有一段注释：this macro will print a stack trace if it is executed in an atomic context (spinlock, irq-handler, ...)

所以，一个进程获得了spinlock之后(进入所谓的atomic context)，或者是在一个irq-handle中(也就是一个中断上下文中)。这两种情况下理论上不应该让当前的execution path进入sleep状态(虽然不是强制规定，换句话说，一个拥有spinlock的进程进入sleep并不必然意味着系统就一定会deadlock 等，但是对内核编程而言，还是应该尽力避开这个雷区)。

在CONFIG_DEBUG_ATOMIC_SLEEP选项打开的情形下，might_sleep具体实现的功能分析如下：

# kernel/sched/core.c
void ___might_sleep(const char *file, int line, int preempt_offset)
{
	static unsigned long prev_jiffy;	/* ratelimiting */
	unsigned long preempt_disable_ip;

	rcu_sleep_check(); /* WARN_ON_ONCE() by default, no rate limit reqd. */
	if ((preempt_count_equals(preempt_offset) && !irqs_disabled() &&!is_idle_task(current)) ||
	    system_state != SYSTEM_RUNNING || oops_in_progress)  /* 核心判断 */
		return;
	if (time_before(jiffies, prev_jiffy + HZ) && prev_jiffy)
		return;
	prev_jiffy = jiffies;

	/* Save this before calling printk(), since that will clobber it */
	preempt_disable_ip = get_preempt_disable_ip(current);

	printk(KERN_ERR
		"BUG: sleeping function called from invalid context at %s:%d\n",
			file, line);
	printk(KERN_ERR
		"in_atomic(): %d, irqs_disabled(): %d, pid: %d, name: %s\n",
			in_atomic(), irqs_disabled(),
			current->pid, current->comm);

	if (task_stack_end_corrupted(current))
		printk(KERN_EMERG "Thread overran stack, or stack corrupted\n"