linux内核有main函数吗

/*
 *  linux/init/main.c
 *
 *  Copyright (C) 1991, 1992  Linus Torvalds
 *
 *  GK 2/5/95  -  Changed to support mounting root fs via NFS
 *  Added initrd & change_root: Werner Almesberger & Hans Lermen, Feb '96
 *  Moan early if gcc is old, avoiding bogus kernels - Paul Gortmaker, May '96
 *  Simplified starting of init:  Michael A. Griffith <grif@acm.org> 
 * start_kernel->rest_init->kernel_init创建用户init  pid=1
                          ->kthreadd管理内核线程     pid=x
                          ->pid=0，是idle线程
    在rest_init中，会创建kernel_init线程，它负责创建用户init进程,完成工作后，自己
    化身为idle线程
 */
 
#include <linux/types.h>
#include <linux/module.h>
#include <linux/proc_fs.h>
#include <linux/kernel.h>
#include <linux/syscalls.h>
#include <linux/stackprotector.h>
#include <linux/string.h>
#include <linux/ctype.h>
#include <linux/delay.h>
#include <linux/ioport.h>
#include <linux/init.h>
#include <linux/initrd.h>
#include <linux/bootmem.h>
#include <linux/acpi.h>
#include <linux/tty.h>
#include <linux/percpu.h>
#include <linux/kmod.h>
#include <linux/vmalloc.h>
#include <linux/kernel_stat.h>
#include <linux/start_kernel.h>
#include <linux/security.h>
#include <linux/smp.h>
#include <linux/profile.h>
#include <linux/rcupdate.h>
#include <linux/moduleparam.h>
#include <linux/kallsyms.h>
#include <linux/writeback.h>
#include <linux/cpu.h>
#include <linux/cpuset.h>
#include <linux/cgroup.h>
#include <linux/efi.h>
#include <linux/tick.h>
#include <linux/interrupt.h>
#include <linux/taskstats_kern.h>
#include <linux/delayacct.h>
#include <linux/unistd.h>
#include <linux/rmap.h>
#include <linux/mempolicy.h>
#include <linux/key.h>
#include <linux/buffer_head.h>
#include <linux/page_cgroup.h>
#include <linux/debug_locks.h>
#include <linux/debugobjects.h>
#include <linux/lockdep.h>
#include <linux/kmemleak.h>
#include <linux/pid_namespace.h>
#include <linux/device.h>
#include <linux/kthread.h>
#include <linux/sched.h>
#include <linux/signal.h>
#include <linux/idr.h>
#include <linux/kgdb.h>
#include <linux/ftrace.h>
#include <linux/async.h>
#include <linux/kmemcheck.h>
#include <linux/sfi.h>
#include <linux/shmem_fs.h>
#include <linux/slab.h>
#include <linux/perf_event.h>
 
#include <asm/io.h>
#include <asm/bugs.h>
#include <asm/setup.h>
#include <asm/sections.h>
#include <asm/cacheflush.h>
 
#ifdef CONFIG_X86_LOCAL_APIC
#include <asm/smp.h>
#endif
 
static int kernel_init(void *);
 
extern void init_IRQ(void);
extern void fork_init(unsigned long);
extern void mca_init(void);
extern void sbus_init(void);
extern void prio_tree_init(void);
extern void radix_tree_init(void);
#ifndef CONFIG_DEBUG_RODATA
static inline void mark_rodata_ro(void) { }
#endif
 
#ifdef CONFIG_TC
extern void tc_init(void);
#endif
 
/*
 * Debug helper: via this flag we know that we are in 'early bootup code'
 * where only the boot processor is running with IRQ disabled.  This means
 * two things - IRQ must not be enabled before the flag is cleared and some
 * operations which are not allowed with IRQ disabled are allowed while the
 * flag is set.
 */
bool early_boot_irqs_disabled __read_mostly;
 
enum system_states system_state __read_mostly;
EXPORT_SYMBOL(system_state);
 
/*
 * Boot command-line arguments
 */
#define MAX_INIT_ARGS CONFIG_INIT_ENV_ARG_LIMIT
#define MAX_INIT_ENVS CONFIG_INIT_ENV_ARG_LIMIT
 
extern void time_init(void);
/* Default late time init is NULL. archs can override this later. */
void (*__initdata late_time_init)(void);
extern void softirq_init(void);
 
/* Untouched command line saved by arch-specific code. */
char __initdata boot_command_line[COMMAND_LINE_SIZE];
/* Untouched saved command line (eg. for /proc) */
char *saved_command_line;
/* Command line for parameter parsing */
static char *static_command_line;
 
static char *execute_command;
static char *ramdisk_execute_command;
 
/*
 * If set, this is an indication to the drivers that reset the underlying
 * device before going ahead with the initialization otherwise driver might
 * rely on the BIOS and skip the reset operation.
 *
 * This is useful if kernel is booting in an unreliable environment.
 * For ex. kdump situaiton where previous kernel has crashed, BIOS has been
 * skipped and devices will be in unknown state.
 */
unsigned int reset_devices;
EXPORT_SYMBOL(reset_devices);
 
static int __init set_reset_devices(char *str)
{
    reset_devices = 1;
    return 1;
}
 
__setup("reset_devices", set_reset_devices);
 
static const char * argv_init[MAX_INIT_ARGS+2] = { "init", NULL, };
const char * envp_init[MAX_INIT_ENVS+2] = { "HOME=/", "TERM=linux", NULL, };
static const char *panic_later, *panic_param;
 
extern const struct obs_kernel_param __setup_start[], __setup_end[];
 
static int __init obsolete_checksetup(char *line)
{
    const struct obs_kernel_param *p;
    int had_early_param = 0;
 
    p = __setup_start;
    do {
        int n = strlen(p->str);
        if (parameqn(line, p->str, n)) {
            if (p->early) {
                /* Already done in parse_early_param?
                 * (Needs exact match on param part).
                 * Keep iterating, as we can have early
                 * params and __setups of same names 8( */
                if (line[n] == '\0' || line[n] == '=')
                    had_early_param = 1;
            } else if (!p->setup_func) {
                printk(KERN_WARNING "Parameter %s is obsolete,"
                       " ignored\n", p->str);
                return 1;
            } else if (p->setup_func(line + n))
                return 1;
        }
        p++;
    } while (p < __setup_end);
 
    return had_early_param;
}
 
/*
 * This should be approx 2 Bo*oMips to start (note initial shift), and will
 * still work even if initially too large, it will just take slightly longer
 */
unsigned long loops_per_jiffy = (1<<12);
EXPORT_SYMBOL(loops_per_jiffy);
static int __init debug_kernel(char *str)
{
    console_loglevel = 10;
    return 0;
}
static int __init quiet_kernel(char *str)
{
    console_loglevel = 4;
    return 0;
}
early_param("debug", debug_kernel);
early_param("quiet", quiet_kernel);
static int __init loglevel(char *str)
{
    int newlevel;
    /*
     * Only update loglevel value when a correct setting was passed,
     * to prevent blind crashes (when loglevel being set to 0) that
     * are quite hard to debug
     */
    if (get_option(&str, &newlevel)) {
        console_loglevel = newlevel;
        return 0;
    }
    return -EINVAL;
}
early_param("loglevel", loglevel);
/* Change NUL term back to "=", to make "param" the whole string. */
static int __init repair_env_string(char *param, char *val)
{
    if (val) {
        /* param=val or param="val"? */
        if (val == param+strlen(param)+1)
            val[-1] = '=';
        else if (val == param+strlen(param)+2) {
            val[-2] = '=';
            memmove(val-1, val, strlen(val)+1);
            val--;
        } else
            BUG();
    }
    return 0;
}
/*
 * Unknown boot options get handed to init, unless they look like
 * unused parameters (modprobe will find them in /proc/cmdline).
 */
static int __init unknown_bootoption(char *param, char *val)
{
    repair_env_string(param, val);
    /* Handle obsolete-style parameters */
    if (obsolete_checksetup(param))
        return 0;
    /* Unused module parameter. */
    if (strchr(param, '.') && (!val || strchr(param, '.') < val))
        return 0;
    if (panic_later)
        return 0;
    if (val) {
        /* Environment option */
        unsigned int i;
        for (i = 0; envp_init[i]; i++) {
            if (i == MAX_INIT_ENVS) {
                panic_later = "Too many boot env vars at `%s'";
                panic_param = param;
            }
            if (!strncmp(param, envp_init[i], val - param))
                break;
        }
        envp_init[i] = param;
    } else {
        /* Command line option */
        unsigned int i;
        for (i = 0; argv_init[i]; i++) {
            if (i == MAX_INIT_ARGS) {
                panic_later = "Too many boot init vars at `%s'";
                panic_param = param;
            }
        }
        argv_init[i] = param;
    }
    return 0;
}
static int __init init_setup(char *str)
{
    unsigned int i;
    execute_command = str;
    /*
     * In case LILO is going to boot us with default command line,
     * it prepends "auto" before the whole cmdline which makes
     * the shell think it should execute a script with such name.
     * So we ignore all arguments entered _before_ init=... [MJ]
     */
    for (i = 1; i < MAX_INIT_ARGS; i++)
        argv_init[i] = NULL;
    return 1;
}
__setup("init=", init_setup);
static int __init rdinit_setup(char *str)
{
    unsigned int i;
    ramdisk_execute_command = str;
    /* See "auto" comment in init_setup */
    for (i = 1; i < MAX_INIT_ARGS; i++)
        argv_init[i] = NULL;
    return 1;
}
__setup("rdinit=", rdinit_setup);
#ifndef CONFIG_SMP
static const unsigned int setup_max_cpus = NR_CPUS;
#ifdef CONFIG_X86_LOCAL_APIC
static void __init smp_init(void)
{
    APIC_init_uniprocessor();
}
#else
#define smp_init()  do { } while (0)
#endif
static inline void setup_nr_cpu_ids(void) { }
static inline void smp_prepare_cpus(unsigned int maxcpus) { }
#endif
/*
 * We need to store the untouched command line for future reference.
 * We also need to store the touched command line since the parameter
 * parsing is performed in place, and we should allow a component to
 * store reference of name/value for future reference.
 */
static void __init setup_command_line(char *command_line)
{
    saved_command_line = alloc_bootmem(strlen (boot_command_line)+1);
    static_command_line = alloc_bootmem(strlen (command_line)+1);
    strcpy (saved_command_line, boot_command_line);
    strcpy (static_command_line, command_line);
}
/*
 * We need to finalize in a non-__init function or else race conditions
 * between the root thread and the init thread may cause start_kernel to
 * be reaped by free_initmem before the root thread has proceeded to
 * cpu_idle.
 *
 * gcc-3.4 accidentally inlines this function, so use noinline.
 */
static __initdata DECLARE_COMPLETION(kthreadd_done);
static noinline void __init_refok rest_init(void)
{
    int pid;
    rcu_scheduler_starting();//READ-COPY UPDATE启动
    /*
     * We need to spawn init first so that it obtains pid 1, however
     * the init task will end up wanting to create kthreads, which, if
     * we schedule it before we create kthreadd, will OOPS.
     * 创建一个内核线程，它的线程函数是kernel_init,pid=1,内核进程
     */
    kernel_thread(kernel_init, NULL, CLONE_FS | CLONE_SIGHAND);
    //numa策略设置
    numa_default_policy();
    //全局链表kthread_create_list中的kthread内核线程都被运行
    //kthreadd线程管理和调度其它内核线程
    pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES);
    rcu_read_lock();
    //通过pid,ini_pid_ns取得kthreadd地址
    kthreadd_task = find_task_by_pid_ns(pid, &init_pid_ns);
    rcu_read_unlock();
    //通知在kthreadd_done条件的kernel_init线程
    complete(&kthreadd_done);
    /*
     * The boot idle thread must execute schedule()
     * at least once to get things moving:
     * idle 线程初始化
     */
    init_idle_bootup_task(current);
    //抢占禁用
    schedule_preempt_disabled();
    /* Call into cpu_idle with preempt disabled */
    cpu_idle();
}
/* Check for early params. */
static int __init do_early_param(char *param, char *val)
{
    const struct obs_kernel_param *p;
    for (p = __setup_start; p < __setup_end; p++) {
        if ((p->early && parameq(param, p->str)) ||
            (strcmp(param, "console") == 0 &&
             strcmp(p->str, "earlycon") == 0)
        ) {
            if (p->setup_func(val) != 0)
                printk(KERN_WARNING
                       "Malformed early option '%s'\n", param);
        }
    }
    /* We accept everything at this stage. */
    return 0;
}
void __init parse_early_options(char *cmdline)
{
    parse_args("early options", cmdline, NULL, 0, 0, 0, do_early_param);
}
/* Arch code calls this early on, or if not, just before other parsing. */
void __init parse_early_param(void)
{
    static __initdata int done = 0;
    static __initdata char tmp_cmdline[COMMAND_LINE_SIZE];
    if (done)
        return;
    /* All fall through to do_early_param. */
    strlcpy(tmp_cmdline, boot_command_line, COMMAND_LINE_SIZE);
    parse_early_options(tmp_cmdline);
    done = 1;
}
/*
 *  Activate the first processor.
 */
static void __init boot_cpu_init(void)
{
    int cpu = smp_processor_id();
    /* Mark the boot cpu "present", "online" etc for SMP and UP case */
    set_cpu_online(cpu, true);
    set_cpu_active(cpu, true);
    set_cpu_present(cpu, true);
    set_cpu_possible(cpu, true);
}
void __init __weak smp_setup_processor_id(void)
{
}
void __init __weak thread_info_cache_init(void)
{
}
/*
 * Set up kernel memory allocators
 */
static void __init mm_init(void)
{
    /*
     * page_cgroup requires contiguous pages,
     * bigger than MAX_ORDER unless SPARSEMEM.
     */
    page_cgroup_init_flatmem();
    mem_init();
    kmem_cache_init();
    percpu_init_late();
    pgtable_cache_init();
    vmalloc_init();
}
asmlinkage void __init start_kernel(void)
{
    char * command_line;
    extern const struct kernel_param __start___param[], __stop___param[];
    /*
     * Need to run as early as possible, to initialize the
     * lockdep hash:
     */
    //初始化2个hash表-Lock Dependency Validator(内核依赖的关系表)
    lockdep_init();
    smp_setup_processor_id(); //空函数
    debug_objects_early_init();//初始化内核调试相关
    /*
     * Set up the the initial canary ASAP:
     */
    boot_init_stack_canary();//栈溢出保护初始化
    //控制组初始化-cgroup-资源任务分组管理
    cgroup_init_early();
    local_irq_disable();//关中断
    early_boot_irqs_disabled = true;
/*
 * Interrupts are still disabled. Do necessary setups, then
 * enable them
 */
    tick_init();//时钟初始化
    boot_cpu_init();//启动cpu初始化
    page_address_init();//页面初始化
    printk(KERN_NOTICE "%s", linux_banner);
    setup_arch(&command_line);//架构相关初始化
    mm_init_owner(&init_mm, &init_task);//内存管理初始化
    mm_init_cpumask(&init_mm);//内存管理初始化
    setup_command_line(command_line);//处理命令行(保存2份)
    setup_nr_cpu_ids();//cpuid相关
    setup_per_cpu_areas();//每cpu变量申请空间(包括gdt)
    //smp中用来启动的cpu
    smp_prepare_boot_cpu(); /* arch-specific boot-cpu hooks */
     //建立系统内存页区链表
    build_all_zonelists(NULL);
    //内存页相关初始化
    page_alloc_init();
    printk(KERN_NOTICE "Kernel command line: %s\n", boot_command_line);
    //命令行boot_command_line
    parse_early_param();
    //解析参数
    parse_args("Booting kernel", static_command_line, __start___param,
           __stop___param - __start___param,
           -1, -1, &unknown_bootoption);
    //
    jump_label_init();
    /*
     * These use large bootmem allocations and must precede
     * kmem_cache_init()
     * 内存初始化相关
     */
    setup_log_buf(0);
    pidhash_init();
    vfs_caches_init_early();
    sort_main_extable();
    trap_init();
    mm_init();
    /*
     * Set up the scheduler prior starting any interrupts (such as the
     * timer interrupt). Full topology setup happens at smp_init()
     * time - but meanwhile we still have a functioning scheduler.
     * 调度初始化
     */
    sched_init();
    /*
     * Disable preemption - early bootup scheduling is extremely
     * fragile until we cpu_idle() for the first time.
     * 抢占禁用
     */
    preempt_disable();
    if (!irqs_disabled()) {
        printk(KERN_WARNING "start_kernel(): bug: interrupts were "
                "enabled *very* early, fixing it\n");
        local_irq_disable();
    }
    idr_init_cache();//idr
    perf_event_init();//performance event
    rcu_init();//read-copy-update 机制
    radix_tree_init();//radix树机制
    /* init some links before init_ISA_irqs() */
    early_irq_init();//中断请求
    init_IRQ();//中断请求
    prio_tree_init();//优先查找树
    init_timers();//时钟
    hrtimers_init();//High-resolution kernel timers高精度内核时钟
    softirq_init();//软中断
    timekeeping_init();//时间相关
    time_init();//时间
    profile_init();//分配内核性能统计保存的内存
    call_function_init();//smp中每cpu的call_single_queue初始化
    if (!irqs_disabled())
        printk(KERN_CRIT "start_kernel(): bug: interrupts were "
                 "enabled early\n");
    early_boot_irqs_disabled = false;//中断请求开
    local_irq_enable();//本地中断开
    kmem_cache_init_late();//kmem后期初始化
    /*
     * HACK ALERT! This is early. We're enabling the console before
     * we've done PCI setups etc, and console_init() must be aware of
     * this. But we do want output early, in case something goes wrong.
     */
    console_init();//初始化系统控制台结构
    if (panic_later)
        panic(panic_later, panic_param);
    //锁依赖信息
    lockdep_info();
    /*
     * Need to run this when irqs are enabled, because it wants
     * to self-test [hard/soft]-irqs on/off lock inversion bugs
     * too:
     */
    locking_selftest();
#ifdef CONFIG_BLK_DEV_INITRD
    if (initrd_start && !initrd_below_start_ok &&
        page_to_pfn(virt_to_page((void *)initrd_start)) < min_low_pfn) {
        printk(KERN_CRIT "initrd overwritten (0x%08lx < 0x%08lx) - "
            "disabling it.\n",
            page_to_pfn(virt_to_page((void *)initrd_start)),
            min_low_pfn);
        initrd_start = 0;
    }
#endif
    page_cgroup_init();//control groups初始化
    debug_objects_mem_init();//对象调试
    kmemleak_init();//检测内核内存泄漏的功能
    setup_per_cpu_pageset();//申请并初始化每cpu页set
    numa_policy_init();//numa相关
    if (late_time_init)
        late_time_init();
    //初始化每cpusched_clock_data=ktime_now
    sched_clock_init();
    calibrate_delay();//计算cpuMIPS百万条指令/s
    pidmap_init();//pid进程id表初始化
    anon_vma_init();//虚拟地址
#ifdef CONFIG_X86
    if (efi_enabled)//efi bois
        efi_enter_virtual_mode();
#endif
    thread_info_cache_init();//申请thread_info的内存
    cred_init();//credential健在分配 
    //根据物理内存大小，计算可创建进/线程数量
    fork_init(totalram_pages); 
    proc_caches_init();//进程内存初始化
    buffer_init();//页高速缓存
    key_init();//红黑树内存，存keys
    security_init();//安全相关
    dbg_late_init();//调试相关
    vfs_caches_init(totalram_pages);//虚拟文件系统初始化
    signals_init();//sigqueue申请内存，信号系统
    /* rootfs populating might need page-writeback */
    page_writeback_init();//页回写
#ifdef CONFIG_PROC_FS
    proc_root_init();//proc文件系统初始化
#endif
    cgroup_init();//cgroup相关
    cpuset_init();//cpuset相关
    taskstats_init_early();//进程计数器
    delayacct_init();//进程延时审计
    check_bugs();//系统bug相关测试
    //acpi总线
    acpi_early_init(); /* before LAPIC and SMP init */
    sfi_init_late();//Simple Firmware Interface
    //功能追踪初始化,一种调试工具
    ftrace_init();
    /* Do the rest non-__init'ed, we're now alive */
    rest_init();
}
/* Call all constructor functions linked into the kernel. */
static void __init do_ctors(void)
{
#ifdef CONFIG_CONSTRUCTORS
    ctor_fn_t *fn = (ctor_fn_t *) __ctors_start;
    for (; fn < (ctor_fn_t *) __ctors_end; fn++)
        (*fn)();
#endif
}
bool initcall_debug;
core_param(initcall_debug, initcall_debug, bool, 0644);
static char msgbuf[64];
static int __init_or_module do_one_initcall_debug(initcall_t fn)
{
    ktime_t calltime, delta, rettime;
    unsigned long long duration;
    int ret;
    printk(KERN_DEBUG "calling  %pF @ %i\n", fn, task_pid_nr(current));
    calltime = ktime_get();
    ret = fn();
    rettime = ktime_get();
    delta = ktime_sub(rettime, calltime);
    duration = (unsigned long long) ktime_to_ns(delta) >> 10;
    printk(KERN_DEBUG "initcall %pF returned %d after %lld usecs\n", fn,
        ret, duration);
    return ret;
}
int __init_or_module do_one_initcall(initcall_t fn)
{
    int count = preempt_count();
    int ret;
    if (initcall_debug)
        ret = do_one_initcall_debug(fn);
    else
        ret = fn();
    msgbuf[0] = 0;
    if (ret && ret != -ENODEV && initcall_debug)
        sprintf(msgbuf, "error code %d ", ret);
    if (preempt_count() != count) {
        strlcat(msgbuf, "preemption imbalance ", sizeof(msgbuf));
        preempt_count() = count;
    }
    if (irqs_disabled()) {
        strlcat(msgbuf, "disabled interrupts ", sizeof(msgbuf));
        local_irq_enable();
    }
    if (msgbuf[0]) {
        printk("initcall %pF returned with %s\n", fn, msgbuf);
    }
    return ret;
}
extern initcall_t __initcall_start[];
extern initcall_t __initcall0_start[];
extern initcall_t __initcall1_start[];
extern initcall_t __initcall2_start[];
extern initcall_t __initcall3_start[];
extern initcall_t __initcall4_start[];
extern initcall_t __initcall5_start[];
extern initcall_t __initcall6_start[];
extern initcall_t __initcall7_start[];
extern initcall_t __initcall_end[];
static initcall_t *initcall_levels[] __initdata = {
    __initcall0_start,
    __initcall1_start,
    __initcall2_start,
    __initcall3_start,
    __initcall4_start,
    __initcall5_start,
    __initcall6_start,
    __initcall7_start,
    __initcall_end,
};
static char *initcall_level_names[] __initdata = {
    "early parameters",
    "core parameters",
    "postcore parameters",
    "arch parameters",
    "subsys parameters",
    "fs parameters",
    "device parameters",
    "late parameters",
};
static void __init do_initcall_level(int level)
{
    extern const struct kernel_param __start___param[], __stop___param[];
    initcall_t *fn;
    strcpy(static_command_line, saved_command_line);
    parse_args(initcall_level_names[level],
           static_command_line, __start___param,
           __stop___param - __start___param,
           level, level,
           repair_env_string);
    for (fn = initcall_levels[level]; fn < initcall_levels[level+1]; fn++)
        do_one_initcall(*fn);
}
static void __init do_initcalls(void)
{
    int level;
    for (level = 0; level < ARRAY_SIZE(initcall_levels) - 1; level++)
        do_initcall_level(level);
}
/*
 * Ok, the machine is now initialized. None of the devices
 * have been touched yet, but the CPU subsystem is up and
 * running, and memory and process management works.
 *
 * Now we can finally start doing some real work..
 */
static void __init do_basic_setup(void)
{
    cpuset_init_smp();//smp cpuset相关
    usermodehelper_init();//khelper单线程工作队列
    shmem_init();//sheme机制
    driver_init();//驱动各子系统
    init_irq_proc();//proc中创建irq目录
    do_ctors();//内核中所有构造函数,介于.ctors段中的函数
    usermodehelper_enable();
    //所有编译进内核的驱动模块初始化函数
    do_initcalls();
}
static void __init do_pre_smp_initcalls(void)
{
    initcall_t *fn;
    for (fn = __initcall_start; fn < __initcall0_start; fn++)
        do_one_initcall(*fn);
}
static void run_init_process(const char *init_filename)
{
    argv_init[0] = init_filename;
    kernel_execve(init_filename, argv_init, envp_init);
}
/* This is a non __init function. Force it to be noinline otherwise gcc
 * makes it inline to init() and it becomes part of init.text section
 * 这是个非Init函数，防止gcc让它内联到init()，并成为Init.text段的一部分
 */
static noinline int init_post(void)
{
    /* need to finish all async __init code before freeing the memory 
     * 在释放init内存前，必须完成所有__init代码执行
     */
    async_synchronize_full();
    free_initmem();//释放init.*段中的内存
    //修改页表，保证只读数据段为只读属性read only
    mark_rodata_ro();
    //系统运行状态标志
    system_state = SYSTEM_RUNNING;
    //numa默认策略
    numa_default_policy();
    //当前进程不能被杀掉，只为它是init
    current->signal->flags |= SIGNAL_UNKILLABLE;
    //如果ramdisk_execute_command变量指定了init程序，执行它
    if (ramdisk_execute_command) {
        run_init_process(ramdisk_execute_command);
        printk(KERN_WARNING "Failed to execute %s\n",
                ramdisk_execute_command);
    }
    /*
     * We try each of these until one succeeds.
     *
     * The Bourne shell can be used instead of init if we are
     * trying to recover a really broken machine.
     * 又一个程序，看能不能执行，如果不能，则执行下面4个之一
     */
    if (execute_command) {
        run_init_process(execute_command);
        printk(KERN_WARNING "Failed to execute %s.  Attempting "
                    "defaults...\n", execute_command);
    }
    run_init_process("/sbin/init");
    run_init_process("/etc/init");
    run_init_process("/bin/init");
    run_init_process("/bin/sh");
    //两个变量和4个init都不能成功执行，报错
    panic("No init found.  Try passing init= option to kernel. "
          "See Linux Documentation/init.txt for guidance.");
}
static int __init kernel_init(void * unused)
{
    /*
     * Wait until kthreadd is all set-up.等待kthreadd的启动完成
     */
    wait_for_completion(&kthreadd_done);
    /* Now the scheduler is fully set up and can do blocking allocations 
     * 
     */
    gfp_allowed_mask = __GFP_BITS_MASK;
    /*
     * init can allocate pages on any node
     */
    set_mems_allowed(node_states[N_HIGH_MEMORY]);
    /*
     * init can run on any cpu.
     */
    set_cpus_allowed_ptr(current, cpu_all_mask);
    //cad_pid为接收Ctrl-alt-del操作的INT信号的进程ID,设置成了init的pid
    //说明init可接受这3个键
    cad_pid = task_pid(current);
    //smp系统准备、激活所有cpu
    smp_prepare_cpus(setup_max_cpus);
    do_pre_smp_initcalls();
    lockup_detector_init();
    smp_init();
    sched_init_smp();
    //初始化设备驱动、内核模块
    do_basic_setup();
    /* Open the /dev/console on the rootfs, this should never fail 
     * 打开/dev/console设备
     */
    if (sys_open((const char __user *) "/dev/console", O_RDWR, 0) < 0)
        printk(KERN_WARNING "Warning: unable to open an initial console.\n");
    /*
     * 复制两次标准输入0，一个是标准输入1，一个是标准错误2
     */
    (void) sys_dup(0);
    (void) sys_dup(0);
    /*
     * check if there is an early userspace init.  If yes, let it do all
     * the work
     * 是否有早期用户空间init进程，有的话，让其执行
     */
    if (!ramdisk_execute_command)
        ramdisk_execute_command = "/init";
    if (sys_access((const char __user *) ramdisk_execute_command, 0) != 0) {
        ramdisk_execute_command = NULL;
        prepare_namespace();
    }
    /*
     * Ok, we have completed the initial bootup, and
     * we're essentially up and running. Get rid of the
     * initmem segments and start the user-mode stuff..
     */
    //启动用户空间的init进程
    init_post();
    return 0;
}