單行註釋應採用/*␣comment␣*/的形式，用空格把界定符和文字分開。多行註釋最常見的是這種形式：
/* ␣*␣Multi-line ␣*␣comment ␣*/
也有更花哨的形式：
/*************\ * Multi-line * * comment * \*************/
使用註釋的場合主要有以下幾種。
1、整個源檔案的頂部註釋。說明此模組的相關信息，例如檔案名、作者和版本歷史等，頂頭寫不縮進。例如內核原始碼目錄下的kernel/sched.c檔案的開頭：
/* * 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、函數註釋。說明此函數的功能、參數、返回值、錯誤碼等，寫在函數定義上側，和此函數定義之間不留空行，頂頭寫不縮進。
3、相對獨立的語句組註釋。對這一組語句做特別說明，寫在語句組上側，和此語句組之間不留空行，與當前語句組的縮進一致。
4、代碼行右側的簡短註釋。對當前代碼行做特別說明，一般為單行註釋，和代碼之間至少用一個空格隔開，一個源檔案中所有的右側註釋最好能上下對齊。儘管例 2.1 “帶更多註釋的Hello World”講過註釋可以穿插在一行代碼中間，但不建議這麼寫。內核原始碼目錄下的lib/radix-tree.c檔案中的一個函數包含了上述三種註釋：
/**
* 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]中特別指出，函數內的註釋要儘可能少用。寫註釋主要是為了說明你的代碼“能做什麼”（比如函數介面定義），而不是為了說明“怎樣做”，只要代碼寫得足夠清晰，“怎樣做”是一目瞭然的，如果你需要用註釋才能解釋清楚，那就表示你的代碼可讀性很差，除非是特別需要提醒注意的地方纔使用函數內註釋。
5、複雜的結構體定義比函數更需要註釋。例如內核原始碼目錄下的kernel/sched.c檔案中定義了這樣一個結構體：
/*
* 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、複雜的宏定義和變數聲明也需要註釋。例如內核原始碼目錄下的include/linux/jiffies.h檔案中的定義：
/* 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;