单行注释应采用/*␣comment␣*/的形式，用空格把界定符和文字分开。多行注释最常见的是这种形式：

/*
␣*␣Multi-line
␣*␣comment
␣*/

也有更花哨的形式：

/*************\
* Multi-line  *
* comment     *
\*************/

使用注释的场合主要有以下几种。

1、整个源文件的顶部注释。说明此模块的相关信息，例如文件名、作者和版本历史等，顶头写不缩进。例如内核源代码目录下的kernel/sched.c文件的开头：

/*
 *  kernel/sched.c
 *
 *  Kernel scheduler and related syscalls
 *
 *  Copyright (C) 1991-2002  Linus Torvalds
 *
 *  1996-12-23  Modified by Dave Grothe to fix bugs in semaphores and
 *              make semaphores SMP safe
 *  1998-11-19  Implemented schedule_timeout() and related stuff
 *              by Andrea Arcangeli
 *  2002-01-04  New ultra-scalable O(1) scheduler by Ingo Molnar:
 *              hybrid priority-list and round-robin design with
 *              an array-switch method of distributing timeslices
 *              and per-CPU runqueues.  Cleanups and useful suggestions
 *              by Davide Libenzi, preemptible kernel bits by Robert Love.
 *  2003-09-03  Interactivity tuning by Con Kolivas.
 *  2004-04-02  Scheduler domains code by Nick Piggin
 */

2、函数注释。说明此函数的功能、参数、返回值、错误码等，写在函数定义上侧，和此函数定义之间不留空行，顶头写不缩进。

3、相对独立的语句组注释。对这一组语句做特别说明，写在语句组上侧，和此语句组之间不留空行，与当前语句组的缩进一致。

4、代码行右侧的简短注释。对当前代码行做特别说明，一般为单行注释，和代码之间至少用一个空格隔开，一个源文件中所有的右侧注释最好能上下对齐。尽管例 2.1 “带更多注释的Hello World”讲过注释可以穿插在一行代码中间，但不建议这么写。内核源代码目录下的lib/radix-tree.c文件中的一个函数包含了上述三种注释：

/**
 *      radix_tree_insert    -    insert into a radix tree
 *      @root:          radix tree root
 *      @index:         index key
 *      @item:          item to insert
 *
 *      Insert an item into the radix tree at position @index.
 */
int radix_tree_insert(struct radix_tree_root *root,
                        unsigned long index, void *item)
{
        struct radix_tree_node *node = NULL, *slot;
        unsigned int height, shift;
        int offset;
        int error;

        /* Make sure the tree is high enough.  */
        if ((!index && !root->rnode) ||
                        index > radix_tree_maxindex(root->height)) {
                error = radix_tree_extend(root, index);
                if (error)
                        return error;
        }

        slot = root->rnode;
        height = root->height;
        shift = (height-1) * RADIX_TREE_MAP_SHIFT;

        offset = 0;                     /* uninitialised var warning */
        do {
                if (slot == NULL) {
                        /* Have to add a child node.  */
                        if (!(slot = radix_tree_node_alloc(root)))
                                return -ENOMEM;
                        if (node) {
                                node->slots[offset] = slot;
                                node->count++;
                        } else
                                root->rnode = slot;
                }

                /* Go a level down */
                offset = (index >> shift) & RADIX_TREE_MAP_MASK;
                node = slot;
                slot = node->slots[offset];
                shift -= RADIX_TREE_MAP_SHIFT;
                height--;
        } while (height > 0);

        if (slot != NULL)
                return -EEXIST;

        BUG_ON(!node);
        node->count++;
        node->slots[offset] = item;
        BUG_ON(tag_get(node, 0, offset));
        BUG_ON(tag_get(node, 1, offset));

        return 0;
}

[CodingStyle]中特别指出，函数内的注释要尽可能少用。写注释主要是为了说明你的代码“能做什么”（比如函数接口定义），而不是为了说明“怎样做”，只要代码写得足够清晰，“怎样做”是一目了然的，如果你需要用注释才能解释清楚，那就表示你的代码可读性很差，除非是特别需要提醒注意的地方才使用函数内注释。

5、复杂的结构体定义比函数更需要注释。例如内核源代码目录下的kernel/sched.c文件中定义了这样一个结构体：

/*
 * This is the main, per-CPU runqueue data structure.
 *
 * Locking rule: those places that want to lock multiple runqueues
 * (such as the load balancing or the thread migration code), lock
 * acquire operations must be ordered by ascending &runqueue.
 */
struct runqueue {
        spinlock_t lock;

        /*
         * nr_running and cpu_load should be in the same cacheline because
         * remote CPUs use both these fields when doing load calculation.
         */
        unsigned long nr_running;
#ifdef CONFIG_SMP
        unsigned long cpu_load[3];
#endif
        unsigned long long nr_switches;

        /*
         * This is part of a global counter where only the total sum
         * over all CPUs matters. A task can increase this counter on
         * one CPU and if it got migrated afterwards it may decrease
         * it on another CPU. Always updated under the runqueue lock:
         */
        unsigned long nr_uninterruptible;

        unsigned long expired_timestamp;
        unsigned long long timestamp_last_tick;
        task_t *curr, *idle;
        struct mm_struct *prev_mm;
        prio_array_t *active, *expired, arrays[2];
        int best_expired_prio;
        atomic_t nr_iowait;

#ifdef CONFIG_SMP
        struct sched_domain *sd;

        /* For active balancing */
        int active_balance;
        int push_cpu;

        task_t *migration_thread;
        struct list_head migration_queue;
        int cpu;
#endif

#ifdef CONFIG_SCHEDSTATS
        /* latency stats */
        struct sched_info rq_sched_info;

        /* sys_sched_yield() stats */
        unsigned long yld_exp_empty;
        unsigned long yld_act_empty;
        unsigned long yld_both_empty;
        unsigned long yld_cnt;

        /* schedule() stats */
        unsigned long sched_switch;
        unsigned long sched_cnt;
        unsigned long sched_goidle;

        /* try_to_wake_up() stats */
        unsigned long ttwu_cnt;
        unsigned long ttwu_local;
#endif
};

6、复杂的宏定义和变量声明也需要注释。例如内核源代码目录下的include/linux/jiffies.h文件中的定义：

/* TICK_USEC_TO_NSEC is the time between ticks in nsec assuming real ACTHZ and  */
/* a value TUSEC for TICK_USEC (can be set bij adjtimex)                */
#define TICK_USEC_TO_NSEC(TUSEC) (SH_DIV (TUSEC * USER_HZ * 1000, ACTHZ, 8))

/* some arch's have a small-data section that can be accessed register-relative
 * but that can only take up to, say, 4-byte variables. jiffies being part of
 * an 8-byte variable may not be correctly accessed unless we force the issue
 */
#define __jiffy_data  __attribute__((section(".data")))

/*
 * The 64-bit value is not volatile - you MUST NOT read it
 * without sampling the sequence number in xtime_lock.
 * get_jiffies_64() will do this for you as appropriate.
 */
extern u64 __jiffy_data jiffies_64;
extern unsigned long volatile __jiffy_data jiffies;