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cond_resched() 用于让出cpu给其他进程运行，相对于yield函数，这个函数使用更为广泛一点.其使用的例程如下：void *hpfs_map_sector(struct super_block *s, unsigned secno, struct buffer_head **bhp,		 int ahead){	struct buffer_head *bh;	hpfs_lock_assert(s);	hpfs_prefetch_sectors(s, secno, ahead);	cond_resched();#让出cpu后，下一次执行时从这里开始.	*bhp = bh = sb_bread(s, hpfs_search_hotfix_map(s, secno));	if (bh != NULL)		return bh->b_data;	else {		pr_err("%s(): read error\n", __func__);		return NULL;	}}其源码分析如下：#define cond_resched() ({			\	___might_sleep(__FILE__, __LINE__, 0);	\ #调用这个函数的thread 不能sleep，如果发生sleep的时候，___might_sleep 会打印处callstack	_cond_resched();			\ #调用此函数让出cpu})_cond_resched 成功让出cpu，返回1，失败的返回0，但是我们一般不关心这个返回值int __sched _cond_resched(void){#检测此thread 是否符合被抢占的条件，这个函数之前的博文分析过	if (should_resched(0)) {#让出cpu		preempt_schedule_common();		return 1;	}	return 0;}static void __sched notrace preempt_schedule_common(void){	do {		preempt_disable_notrace();		preempt_latency_start(1);#核心是通过时调用__schedule 来让出cpu		__schedule(true);		preempt_latency_stop(1);		preempt_enable_no_resched_notrace();		/*		 * Check again in case we missed a preemption opportunity		 * between schedule and now.		 */	} while (need_resched());}

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