ext4: Fix up comments in inode.c

[linux-2.6/linux-acpi-2.6/ibm-acpi-2.6.git] / kernel / printk.c

/*
 *  linux/kernel/printk.c
 *
 *  Copyright (C) 1991, 1992  Linus Torvalds
 *
 * Modified to make sys_syslog() more flexible: added commands to
 * return the last 4k of kernel messages, regardless of whether
 * they've been read or not.  Added option to suppress kernel printk's
 * to the console.  Added hook for sending the console messages
 * elsewhere, in preparation for a serial line console (someday).
 * Ted Ts'o, 2/11/93.
 * Modified for sysctl support, 1/8/97, Chris Horn.
 * Fixed SMP synchronization, 08/08/99, Manfred Spraul
 *     manfred@colorfullife.com
 * Rewrote bits to get rid of console_lock
 *      01Mar01 Andrew Morton
 */

#include <linux/kernel.h>
#include <linux/mm.h>
#include <linux/tty.h>
#include <linux/tty_driver.h>
#include <linux/console.h>
#include <linux/init.h>
#include <linux/jiffies.h>
#include <linux/nmi.h>
#include <linux/module.h>
#include <linux/moduleparam.h>
#include <linux/interrupt.h>                    /* For in_interrupt() */
#include <linux/delay.h>
#include <linux/smp.h>
#include <linux/security.h>
#include <linux/bootmem.h>
#include <linux/syscalls.h>
#include <linux/kexec.h>
#include <linux/kdb.h>
#include <linux/ratelimit.h>
#include <linux/kmsg_dump.h>
#include <linux/syslog.h>
#include <linux/cpu.h>
#include <linux/notifier.h>

#include <asm/uaccess.h>

/*
 * for_each_console() allows you to iterate on each console
 */
#define for_each_console(con) \
        for (con = console_drivers; con != NULL; con = con->next)

/*
 * Architectures can override it:
 */
void asmlinkage __attribute__((weak)) early_printk(const char *fmt, ...)
{
}

#define __LOG_BUF_LEN   (1 << CONFIG_LOG_BUF_SHIFT)

/* printk's without a loglevel use this.. */
#define DEFAULT_MESSAGE_LOGLEVEL 4 /* KERN_WARNING */

/* We show everything that is MORE important than this.. */
#define MINIMUM_CONSOLE_LOGLEVEL 1 /* Minimum loglevel we let people use */
#define DEFAULT_CONSOLE_LOGLEVEL 7 /* anything MORE serious than KERN_DEBUG */

DECLARE_WAIT_QUEUE_HEAD(log_wait);

int console_printk[4] = {
        DEFAULT_CONSOLE_LOGLEVEL,       /* console_loglevel */
        DEFAULT_MESSAGE_LOGLEVEL,       /* default_message_loglevel */
        MINIMUM_CONSOLE_LOGLEVEL,       /* minimum_console_loglevel */
        DEFAULT_CONSOLE_LOGLEVEL,       /* default_console_loglevel */
};

/*
 * Low level drivers may need that to know if they can schedule in
 * their unblank() callback or not. So let's export it.
 */
int oops_in_progress;
EXPORT_SYMBOL(oops_in_progress);

/*
 * console_sem protects the console_drivers list, and also
 * provides serialisation for access to the entire console
 * driver system.
 */
static DEFINE_SEMAPHORE(console_sem);
struct console *console_drivers;
EXPORT_SYMBOL_GPL(console_drivers);

/*
 * This is used for debugging the mess that is the VT code by
 * keeping track if we have the console semaphore held. It's
 * definitely not the perfect debug tool (we don't know if _WE_
 * hold it are racing, but it helps tracking those weird code
 * path in the console code where we end up in places I want
 * locked without the console sempahore held
 */
static int console_locked, console_suspended;

/*
 * logbuf_lock protects log_buf, log_start, log_end, con_start and logged_chars
 * It is also used in interesting ways to provide interlocking in
 * release_console_sem().
 */
static DEFINE_SPINLOCK(logbuf_lock);

#define LOG_BUF_MASK (log_buf_len-1)
#define LOG_BUF(idx) (log_buf[(idx) & LOG_BUF_MASK])

/*
 * The indices into log_buf are not constrained to log_buf_len - they
 * must be masked before subscripting
 */
static unsigned log_start;      /* Index into log_buf: next char to be read by syslog() */
static unsigned con_start;      /* Index into log_buf: next char to be sent to consoles */
static unsigned log_end;        /* Index into log_buf: most-recently-written-char + 1 */

/*
 *      Array of consoles built from command line options (console=)
 */
struct console_cmdline
{
        char    name[8];                        /* Name of the driver       */
        int     index;                          /* Minor dev. to use        */
        char    *options;                       /* Options for the driver   */
#ifdef CONFIG_A11Y_BRAILLE_CONSOLE
        char    *brl_options;                   /* Options for braille driver */
#endif
};

#define MAX_CMDLINECONSOLES 8

static struct console_cmdline console_cmdline[MAX_CMDLINECONSOLES];
static int selected_console = -1;
static int preferred_console = -1;
int console_set_on_cmdline;
EXPORT_SYMBOL(console_set_on_cmdline);

/* Flag: console code may call schedule() */
static int console_may_schedule;

#ifdef CONFIG_PRINTK

static char __log_buf[__LOG_BUF_LEN];
static char *log_buf = __log_buf;
static int log_buf_len = __LOG_BUF_LEN;
static unsigned logged_chars; /* Number of chars produced since last read+clear operation */
static int saved_console_loglevel = -1;

#ifdef CONFIG_KEXEC
/*
 * This appends the listed symbols to /proc/vmcoreinfo
 *
 * /proc/vmcoreinfo is used by various utiilties, like crash and makedumpfile to
 * obtain access to symbols that are otherwise very difficult to locate.  These
 * symbols are specifically used so that utilities can access and extract the
 * dmesg log from a vmcore file after a crash.
 */
void log_buf_kexec_setup(void)
{
        VMCOREINFO_SYMBOL(log_buf);
        VMCOREINFO_SYMBOL(log_end);
        VMCOREINFO_SYMBOL(log_buf_len);
        VMCOREINFO_SYMBOL(logged_chars);
}
#endif

static int __init log_buf_len_setup(char *str)
{
        unsigned size = memparse(str, &str);
        unsigned long flags;

        if (size)
                size = roundup_pow_of_two(size);
        if (size > log_buf_len) {
                unsigned start, dest_idx, offset;
                char *new_log_buf;

                new_log_buf = alloc_bootmem(size);
                if (!new_log_buf) {
                        printk(KERN_WARNING "log_buf_len: allocation failed\n");
                        goto out;
                }

                spin_lock_irqsave(&logbuf_lock, flags);
                log_buf_len = size;
                log_buf = new_log_buf;

                offset = start = min(con_start, log_start);
                dest_idx = 0;
                while (start != log_end) {
                        log_buf[dest_idx] = __log_buf[start & (__LOG_BUF_LEN - 1)];
                        start++;
                        dest_idx++;
                }
                log_start -= offset;
                con_start -= offset;
                log_end -= offset;
                spin_unlock_irqrestore(&logbuf_lock, flags);

                printk(KERN_NOTICE "log_buf_len: %d\n", log_buf_len);
        }
out:
        return 1;
}

__setup("log_buf_len=", log_buf_len_setup);

#ifdef CONFIG_BOOT_PRINTK_DELAY

static int boot_delay; /* msecs delay after each printk during bootup */
static unsigned long long loops_per_msec;       /* based on boot_delay */

static int __init boot_delay_setup(char *str)
{
        unsigned long lpj;

        lpj = preset_lpj ? preset_lpj : 1000000;        /* some guess */
        loops_per_msec = (unsigned long long)lpj / 1000 * HZ;

        get_option(&str, &boot_delay);
        if (boot_delay > 10 * 1000)
                boot_delay = 0;

        pr_debug("boot_delay: %u, preset_lpj: %ld, lpj: %lu, "
                "HZ: %d, loops_per_msec: %llu\n",
                boot_delay, preset_lpj, lpj, HZ, loops_per_msec);
        return 1;
}
__setup("boot_delay=", boot_delay_setup);

static void boot_delay_msec(void)
{
        unsigned long long k;
        unsigned long timeout;

        if (boot_delay == 0 || system_state != SYSTEM_BOOTING)
                return;

        k = (unsigned long long)loops_per_msec * boot_delay;

        timeout = jiffies + msecs_to_jiffies(boot_delay);
        while (k) {
                k--;
                cpu_relax();
                /*
                 * use (volatile) jiffies to prevent
                 * compiler reduction; loop termination via jiffies
                 * is secondary and may or may not happen.
                 */
                if (time_after(jiffies, timeout))
                        break;
                touch_nmi_watchdog();
        }
}
#else
static inline void boot_delay_msec(void)
{
}
#endif

#ifdef CONFIG_SECURITY_DMESG_RESTRICT
int dmesg_restrict = 1;
#else
int dmesg_restrict;
#endif

int do_syslog(int type, char __user *buf, int len, bool from_file)
{
        unsigned i, j, limit, count;
        int do_clear = 0;
        char c;
        int error = 0;

        /*
         * If this is from /proc/kmsg we only do the capabilities checks
         * at open time.
         */
        if (type == SYSLOG_ACTION_OPEN || !from_file) {
                if (dmesg_restrict && !capable(CAP_SYS_ADMIN))
                        return -EPERM;
                if ((type != SYSLOG_ACTION_READ_ALL &&
                     type != SYSLOG_ACTION_SIZE_BUFFER) &&
                    !capable(CAP_SYS_ADMIN))
                        return -EPERM;
        }

        error = security_syslog(type);
        if (error)
                return error;

        switch (type) {
        case SYSLOG_ACTION_CLOSE:       /* Close log */
                break;
        case SYSLOG_ACTION_OPEN:        /* Open log */
                break;
        case SYSLOG_ACTION_READ:        /* Read from log */
                error = -EINVAL;
                if (!buf || len < 0)
                        goto out;
                error = 0;
                if (!len)
                        goto out;
                if (!access_ok(VERIFY_WRITE, buf, len)) {
                        error = -EFAULT;
                        goto out;
                }
                error = wait_event_interruptible(log_wait,
                                                        (log_start - log_end));
                if (error)
                        goto out;
                i = 0;
                spin_lock_irq(&logbuf_lock);
                while (!error && (log_start != log_end) && i < len) {
                        c = LOG_BUF(log_start);
                        log_start++;
                        spin_unlock_irq(&logbuf_lock);
                        error = __put_user(c,buf);
                        buf++;
                        i++;
                        cond_resched();
                        spin_lock_irq(&logbuf_lock);
                }
                spin_unlock_irq(&logbuf_lock);
                if (!error)
                        error = i;
                break;
        /* Read/clear last kernel messages */
        case SYSLOG_ACTION_READ_CLEAR:
                do_clear = 1;
                /* FALL THRU */
        /* Read last kernel messages */
        case SYSLOG_ACTION_READ_ALL:
                error = -EINVAL;
                if (!buf || len < 0)
                        goto out;
                error = 0;
                if (!len)
                        goto out;
                if (!access_ok(VERIFY_WRITE, buf, len)) {
                        error = -EFAULT;
                        goto out;
                }
                count = len;
                if (count > log_buf_len)
                        count = log_buf_len;
                spin_lock_irq(&logbuf_lock);
                if (count > logged_chars)
                        count = logged_chars;
                if (do_clear)
                        logged_chars = 0;
                limit = log_end;
                /*
                 * __put_user() could sleep, and while we sleep
                 * printk() could overwrite the messages
                 * we try to copy to user space. Therefore
                 * the messages are copied in reverse. <manfreds>
                 */
                for (i = 0; i < count && !error; i++) {
                        j = limit-1-i;
                        if (j + log_buf_len < log_end)
                                break;
                        c = LOG_BUF(j);
                        spin_unlock_irq(&logbuf_lock);
                        error = __put_user(c,&buf[count-1-i]);
                        cond_resched();
                        spin_lock_irq(&logbuf_lock);
                }
                spin_unlock_irq(&logbuf_lock);
                if (error)
                        break;
                error = i;
                if (i != count) {
                        int offset = count-error;
                        /* buffer overflow during copy, correct user buffer. */
                        for (i = 0; i < error; i++) {
                                if (__get_user(c,&buf[i+offset]) ||
                                    __put_user(c,&buf[i])) {
                                        error = -EFAULT;
                                        break;
                                }
                                cond_resched();
                        }
                }
                break;
        /* Clear ring buffer */
        case SYSLOG_ACTION_CLEAR:
                logged_chars = 0;
                break;
        /* Disable logging to console */
        case SYSLOG_ACTION_CONSOLE_OFF:
                if (saved_console_loglevel == -1)
                        saved_console_loglevel = console_loglevel;
                console_loglevel = minimum_console_loglevel;
                break;
        /* Enable logging to console */
        case SYSLOG_ACTION_CONSOLE_ON:
                if (saved_console_loglevel != -1) {
                        console_loglevel = saved_console_loglevel;
                        saved_console_loglevel = -1;
                }
                break;
        /* Set level of messages printed to console */
        case SYSLOG_ACTION_CONSOLE_LEVEL:
                error = -EINVAL;
                if (len < 1 || len > 8)
                        goto out;
                if (len < minimum_console_loglevel)
                        len = minimum_console_loglevel;
                console_loglevel = len;
                /* Implicitly re-enable logging to console */
                saved_console_loglevel = -1;
                error = 0;
                break;
        /* Number of chars in the log buffer */
        case SYSLOG_ACTION_SIZE_UNREAD:
                error = log_end - log_start;
                break;
        /* Size of the log buffer */
        case SYSLOG_ACTION_SIZE_BUFFER:
                error = log_buf_len;
                break;
        default:
                error = -EINVAL;
                break;
        }
out:
        return error;
}

SYSCALL_DEFINE3(syslog, int, type, char __user *, buf, int, len)
{
        return do_syslog(type, buf, len, SYSLOG_FROM_CALL);
}

#ifdef  CONFIG_KGDB_KDB
/* kdb dmesg command needs access to the syslog buffer.  do_syslog()
 * uses locks so it cannot be used during debugging.  Just tell kdb
 * where the start and end of the physical and logical logs are.  This
 * is equivalent to do_syslog(3).
 */
void kdb_syslog_data(char *syslog_data[4])
{
        syslog_data[0] = log_buf;
        syslog_data[1] = log_buf + log_buf_len;
        syslog_data[2] = log_buf + log_end -
                (logged_chars < log_buf_len ? logged_chars : log_buf_len);
        syslog_data[3] = log_buf + log_end;
}
#endif  /* CONFIG_KGDB_KDB */

/*
 * Call the console drivers on a range of log_buf
 */
static void __call_console_drivers(unsigned start, unsigned end)
{
        struct console *con;

        for_each_console(con) {
                if ((con->flags & CON_ENABLED) && con->write &&
                                (cpu_online(smp_processor_id()) ||
                                (con->flags & CON_ANYTIME)))
                        con->write(con, &LOG_BUF(start), end - start);
        }
}

static int __read_mostly ignore_loglevel;

static int __init ignore_loglevel_setup(char *str)
{
        ignore_loglevel = 1;
        printk(KERN_INFO "debug: ignoring loglevel setting.\n");

        return 0;
}

early_param("ignore_loglevel", ignore_loglevel_setup);

/*
 * Write out chars from start to end - 1 inclusive
 */
static void _call_console_drivers(unsigned start,
                                unsigned end, int msg_log_level)
{
        if ((msg_log_level < console_loglevel || ignore_loglevel) &&
                        console_drivers && start != end) {
                if ((start & LOG_BUF_MASK) > (end & LOG_BUF_MASK)) {
                        /* wrapped write */
                        __call_console_drivers(start & LOG_BUF_MASK,
                                                log_buf_len);
                        __call_console_drivers(0, end & LOG_BUF_MASK);
                } else {
                        __call_console_drivers(start, end);
                }
        }
}

/*
 * Call the console drivers, asking them to write out
 * log_buf[start] to log_buf[end - 1].
 * The console_sem must be held.
 */
static void call_console_drivers(unsigned start, unsigned end)
{
        unsigned cur_index, start_print;
        static int msg_level = -1;

        BUG_ON(((int)(start - end)) > 0);

        cur_index = start;
        start_print = start;
        while (cur_index != end) {
                if (msg_level < 0 && ((end - cur_index) > 2) &&
                                LOG_BUF(cur_index + 0) == '<' &&
                                LOG_BUF(cur_index + 1) >= '0' &&
                                LOG_BUF(cur_index + 1) <= '7' &&
                                LOG_BUF(cur_index + 2) == '>') {
                        msg_level = LOG_BUF(cur_index + 1) - '0';
                        cur_index += 3;
                        start_print = cur_index;
                }
                while (cur_index != end) {
                        char c = LOG_BUF(cur_index);

                        cur_index++;
                        if (c == '\n') {
                                if (msg_level < 0) {
                                        /*
                                         * printk() has already given us loglevel tags in
                                         * the buffer.  This code is here in case the
                                         * log buffer has wrapped right round and scribbled
                                         * on those tags
                                         */
                                        msg_level = default_message_loglevel;
                                }
                                _call_console_drivers(start_print, cur_index, msg_level);
                                msg_level = -1;
                                start_print = cur_index;
                                break;
                        }
                }
        }
        _call_console_drivers(start_print, end, msg_level);
}

static void emit_log_char(char c)
{
        LOG_BUF(log_end) = c;
        log_end++;
        if (log_end - log_start > log_buf_len)
                log_start = log_end - log_buf_len;
        if (log_end - con_start > log_buf_len)
                con_start = log_end - log_buf_len;
        if (logged_chars < log_buf_len)
                logged_chars++;
}

/*
 * Zap console related locks when oopsing. Only zap at most once
 * every 10 seconds, to leave time for slow consoles to print a
 * full oops.
 */
static void zap_locks(void)
{
        static unsigned long oops_timestamp;

        if (time_after_eq(jiffies, oops_timestamp) &&
                        !time_after(jiffies, oops_timestamp + 30 * HZ))
                return;

        oops_timestamp = jiffies;

        /* If a crash is occurring, make sure we can't deadlock */
        spin_lock_init(&logbuf_lock);
        /* And make sure that we print immediately */
        sema_init(&console_sem, 1);
}

#if defined(CONFIG_PRINTK_TIME)
static int printk_time = 1;
#else
static int printk_time = 0;
#endif
module_param_named(time, printk_time, bool, S_IRUGO | S_IWUSR);

/* Check if we have any console registered that can be called early in boot. */
static int have_callable_console(void)
{
        struct console *con;

        for_each_console(con)
                if (con->flags & CON_ANYTIME)
                        return 1;

        return 0;
}

/**
 * printk - print a kernel message
 * @fmt: format string
 *
 * This is printk().  It can be called from any context.  We want it to work.
 *
 * We try to grab the console_sem.  If we succeed, it's easy - we log the output and
 * call the console drivers.  If we fail to get the semaphore we place the output
 * into the log buffer and return.  The current holder of the console_sem will
 * notice the new output in release_console_sem() and will send it to the
 * consoles before releasing the semaphore.
 *
 * One effect of this deferred printing is that code which calls printk() and
 * then changes console_loglevel may break. This is because console_loglevel
 * is inspected when the actual printing occurs.
 *
 * See also:
 * printf(3)
 *
 * See the vsnprintf() documentation for format string extensions over C99.
 */

asmlinkage int printk(const char *fmt, ...)
{
        va_list args;
        int r;

#ifdef CONFIG_KGDB_KDB
        if (unlikely(kdb_trap_printk)) {
                va_start(args, fmt);
                r = vkdb_printf(fmt, args);
                va_end(args);
                return r;
        }
#endif
        va_start(args, fmt);
        r = vprintk(fmt, args);
        va_end(args);

        return r;
}

/* cpu currently holding logbuf_lock */
static volatile unsigned int printk_cpu = UINT_MAX;

/*
 * Can we actually use the console at this time on this cpu?
 *
 * Console drivers may assume that per-cpu resources have
 * been allocated. So unless they're explicitly marked as
 * being able to cope (CON_ANYTIME) don't call them until
 * this CPU is officially up.
 */
static inline int can_use_console(unsigned int cpu)
{
        return cpu_online(cpu) || have_callable_console();
}

/*
 * Try to get console ownership to actually show the kernel
 * messages from a 'printk'. Return true (and with the
 * console_semaphore held, and 'console_locked' set) if it
 * is successful, false otherwise.
 *
 * This gets called with the 'logbuf_lock' spinlock held and
 * interrupts disabled. It should return with 'lockbuf_lock'
 * released but interrupts still disabled.
 */
static int acquire_console_semaphore_for_printk(unsigned int cpu)
        __releases(&logbuf_lock)
{
        int retval = 0;

        if (!try_acquire_console_sem()) {
                retval = 1;

                /*
                 * If we can't use the console, we need to release
                 * the console semaphore by hand to avoid flushing
                 * the buffer. We need to hold the console semaphore
                 * in order to do this test safely.
                 */
                if (!can_use_console(cpu)) {
                        console_locked = 0;
                        up(&console_sem);
                        retval = 0;
                }
        }
        printk_cpu = UINT_MAX;
        spin_unlock(&logbuf_lock);
        return retval;
}
static const char recursion_bug_msg [] =
                KERN_CRIT "BUG: recent printk recursion!\n";
static int recursion_bug;
static int new_text_line = 1;
static char printk_buf[1024];

int printk_delay_msec __read_mostly;

static inline void printk_delay(void)
{
        if (unlikely(printk_delay_msec)) {
                int m = printk_delay_msec;

                while (m--) {
                        mdelay(1);
                        touch_nmi_watchdog();
                }
        }
}

asmlinkage int vprintk(const char *fmt, va_list args)
{
        int printed_len = 0;
        int current_log_level = default_message_loglevel;
        unsigned long flags;
        int this_cpu;
        char *p;

        boot_delay_msec();
        printk_delay();

        preempt_disable();
        /* This stops the holder of console_sem just where we want him */
        raw_local_irq_save(flags);
        this_cpu = smp_processor_id();

        /*
         * Ouch, printk recursed into itself!
         */
        if (unlikely(printk_cpu == this_cpu)) {
                /*
                 * If a crash is occurring during printk() on this CPU,
                 * then try to get the crash message out but make sure
                 * we can't deadlock. Otherwise just return to avoid the
                 * recursion and return - but flag the recursion so that
                 * it can be printed at the next appropriate moment:
                 */
                if (!oops_in_progress) {
                        recursion_bug = 1;
                        goto out_restore_irqs;
                }
                zap_locks();
        }

        lockdep_off();
        spin_lock(&logbuf_lock);
        printk_cpu = this_cpu;

        if (recursion_bug) {
                recursion_bug = 0;
                strcpy(printk_buf, recursion_bug_msg);
                printed_len = strlen(recursion_bug_msg);
        }
        /* Emit the output into the temporary buffer */
        printed_len += vscnprintf(printk_buf + printed_len,
                                  sizeof(printk_buf) - printed_len, fmt, args);


        p = printk_buf;

        /* Do we have a loglevel in the string? */
        if (p[0] == '<') {
                unsigned char c = p[1];
                if (c && p[2] == '>') {
                        switch (c) {
                        case '0' ... '7': /* loglevel */
                                current_log_level = c - '0';
                        /* Fallthrough - make sure we're on a new line */
                        case 'd': /* KERN_DEFAULT */
                                if (!new_text_line) {
                                        emit_log_char('\n');
                                        new_text_line = 1;
                                }
                        /* Fallthrough - skip the loglevel */
                        case 'c': /* KERN_CONT */
                                p += 3;
                                break;
                        }
                }
        }

        /*
         * Copy the output into log_buf.  If the caller didn't provide
         * appropriate log level tags, we insert them here
         */
        for ( ; *p; p++) {
                if (new_text_line) {
                        /* Always output the token */
                        emit_log_char('<');
                        emit_log_char(current_log_level + '0');
                        emit_log_char('>');
                        printed_len += 3;
                        new_text_line = 0;

                        if (printk_time) {
                                /* Follow the token with the time */
                                char tbuf[50], *tp;
                                unsigned tlen;
                                unsigned long long t;
                                unsigned long nanosec_rem;

                                t = cpu_clock(printk_cpu);
                                nanosec_rem = do_div(t, 1000000000);
                                tlen = sprintf(tbuf, "[%5lu.%06lu] ",
                                                (unsigned long) t,
                                                nanosec_rem / 1000);

                                for (tp = tbuf; tp < tbuf + tlen; tp++)
                                        emit_log_char(*tp);
                                printed_len += tlen;
                        }

                        if (!*p)
                                break;
                }

                emit_log_char(*p);
                if (*p == '\n')
                        new_text_line = 1;
        }

        /*
         * Try to acquire and then immediately release the
         * console semaphore. The release will do all the
         * actual magic (print out buffers, wake up klogd,
         * etc). 
         *
         * The acquire_console_semaphore_for_printk() function
         * will release 'logbuf_lock' regardless of whether it
         * actually gets the semaphore or not.
         */
        if (acquire_console_semaphore_for_printk(this_cpu))
                release_console_sem();

        lockdep_on();
out_restore_irqs:
        raw_local_irq_restore(flags);

        preempt_enable();
        return printed_len;
}
EXPORT_SYMBOL(printk);
EXPORT_SYMBOL(vprintk);

#else

static void call_console_drivers(unsigned start, unsigned end)
{
}

#endif

static int __add_preferred_console(char *name, int idx, char *options,
                                   char *brl_options)
{
        struct console_cmdline *c;
        int i;

        /*
         *      See if this tty is not yet registered, and
         *      if we have a slot free.
         */
        for (i = 0; i < MAX_CMDLINECONSOLES && console_cmdline[i].name[0]; i++)
                if (strcmp(console_cmdline[i].name, name) == 0 &&
                          console_cmdline[i].index == idx) {
                                if (!brl_options)
                                        selected_console = i;
                                return 0;
                }
        if (i == MAX_CMDLINECONSOLES)
                return -E2BIG;
        if (!brl_options)
                selected_console = i;
        c = &console_cmdline[i];
        strlcpy(c->name, name, sizeof(c->name));
        c->options = options;
#ifdef CONFIG_A11Y_BRAILLE_CONSOLE
        c->brl_options = brl_options;
#endif
        c->index = idx;
        return 0;
}
/*
 * Set up a list of consoles.  Called from init/main.c
 */
static int __init console_setup(char *str)
{
        char buf[sizeof(console_cmdline[0].name) + 4]; /* 4 for index */
        char *s, *options, *brl_options = NULL;
        int idx;

#ifdef CONFIG_A11Y_BRAILLE_CONSOLE
        if (!memcmp(str, "brl,", 4)) {
                brl_options = "";
                str += 4;
        } else if (!memcmp(str, "brl=", 4)) {
                brl_options = str + 4;
                str = strchr(brl_options, ',');
                if (!str) {
                        printk(KERN_ERR "need port name after brl=\n");
                        return 1;
                }
                *(str++) = 0;
        }
#endif

        /*
         * Decode str into name, index, options.
         */
        if (str[0] >= '0' && str[0] <= '9') {
                strcpy(buf, "ttyS");
                strncpy(buf + 4, str, sizeof(buf) - 5);
        } else {
                strncpy(buf, str, sizeof(buf) - 1);
        }
        buf[sizeof(buf) - 1] = 0;
        if ((options = strchr(str, ',')) != NULL)
                *(options++) = 0;
#ifdef __sparc__
        if (!strcmp(str, "ttya"))
                strcpy(buf, "ttyS0");
        if (!strcmp(str, "ttyb"))
                strcpy(buf, "ttyS1");
#endif
        for (s = buf; *s; s++)
                if ((*s >= '0' && *s <= '9') || *s == ',')
                        break;
        idx = simple_strtoul(s, NULL, 10);
        *s = 0;

        __add_preferred_console(buf, idx, options, brl_options);
        console_set_on_cmdline = 1;
        return 1;
}
__setup("console=", console_setup);

/**
 * add_preferred_console - add a device to the list of preferred consoles.
 * @name: device name
 * @idx: device index
 * @options: options for this console
 *
 * The last preferred console added will be used for kernel messages
 * and stdin/out/err for init.  Normally this is used by console_setup
 * above to handle user-supplied console arguments; however it can also
 * be used by arch-specific code either to override the user or more
 * commonly to provide a default console (ie from PROM variables) when
 * the user has not supplied one.
 */
int add_preferred_console(char *name, int idx, char *options)
{
        return __add_preferred_console(name, idx, options, NULL);
}

int update_console_cmdline(char *name, int idx, char *name_new, int idx_new, char *options)
{
        struct console_cmdline *c;
        int i;

        for (i = 0; i < MAX_CMDLINECONSOLES && console_cmdline[i].name[0]; i++)
                if (strcmp(console_cmdline[i].name, name) == 0 &&
                          console_cmdline[i].index == idx) {
                                c = &console_cmdline[i];
                                strlcpy(c->name, name_new, sizeof(c->name));
                                c->name[sizeof(c->name) - 1] = 0;
                                c->options = options;
                                c->index = idx_new;
                                return i;
                }
        /* not found */
        return -1;
}

int console_suspend_enabled = 1;
EXPORT_SYMBOL(console_suspend_enabled);

static int __init console_suspend_disable(char *str)
{
        console_suspend_enabled = 0;
        return 1;
}
__setup("no_console_suspend", console_suspend_disable);

/**
 * suspend_console - suspend the console subsystem
 *
 * This disables printk() while we go into suspend states
 */
void suspend_console(void)
{
        if (!console_suspend_enabled)
                return;
        printk("Suspending console(s) (use no_console_suspend to debug)\n");
        acquire_console_sem();
        console_suspended = 1;
        up(&console_sem);
}

void resume_console(void)
{
        if (!console_suspend_enabled)
                return;
        down(&console_sem);
        console_suspended = 0;
        release_console_sem();
}

/**
 * console_cpu_notify - print deferred console messages after CPU hotplug
 * @self: notifier struct
 * @action: CPU hotplug event
 * @hcpu: unused
 *
 * If printk() is called from a CPU that is not online yet, the messages
 * will be spooled but will not show up on the console.  This function is
 * called when a new CPU comes online (or fails to come up), and ensures
 * that any such output gets printed.
 */
static int __cpuinit console_cpu_notify(struct notifier_block *self,
        unsigned long action, void *hcpu)
{
        switch (action) {
        case CPU_ONLINE:
        case CPU_DEAD:
        case CPU_DYING:
        case CPU_DOWN_FAILED:
        case CPU_UP_CANCELED:
                acquire_console_sem();
                release_console_sem();
        }
        return NOTIFY_OK;
}

/**
 * acquire_console_sem - lock the console system for exclusive use.
 *
 * Acquires a semaphore which guarantees that the caller has
 * exclusive access to the console system and the console_drivers list.
 *
 * Can sleep, returns nothing.
 */
void acquire_console_sem(void)
{
        BUG_ON(in_interrupt());
        down(&console_sem);
        if (console_suspended)
                return;
        console_locked = 1;
        console_may_schedule = 1;
}
EXPORT_SYMBOL(acquire_console_sem);

int try_acquire_console_sem(void)
{
        if (down_trylock(&console_sem))
                return -1;
        if (console_suspended) {
                up(&console_sem);
                return -1;
        }
        console_locked = 1;
        console_may_schedule = 0;
        return 0;
}
EXPORT_SYMBOL(try_acquire_console_sem);

int is_console_locked(void)
{
        return console_locked;
}

static DEFINE_PER_CPU(int, printk_pending);

void printk_tick(void)
{
        if (__get_cpu_var(printk_pending)) {
                __get_cpu_var(printk_pending) = 0;
                wake_up_interruptible(&log_wait);
        }
}

int printk_needs_cpu(int cpu)
{
        if (unlikely(cpu_is_offline(cpu)))
                printk_tick();
        return per_cpu(printk_pending, cpu);
}

void wake_up_klogd(void)
{
        if (waitqueue_active(&log_wait))
                this_cpu_write(printk_pending, 1);
}

/**
 * release_console_sem - unlock the console system
 *
 * Releases the semaphore which the caller holds on the console system
 * and the console driver list.
 *
 * While the semaphore was held, console output may have been buffered
 * by printk().  If this is the case, release_console_sem() emits
 * the output prior to releasing the semaphore.
 *
 * If there is output waiting for klogd, we wake it up.
 *
 * release_console_sem() may be called from any context.
 */
void release_console_sem(void)
{
        unsigned long flags;
        unsigned _con_start, _log_end;
        unsigned wake_klogd = 0;

        if (console_suspended) {
                up(&console_sem);
                return;
        }

        console_may_schedule = 0;

        for ( ; ; ) {
                spin_lock_irqsave(&logbuf_lock, flags);
                wake_klogd |= log_start - log_end;
                if (con_start == log_end)
                        break;                  /* Nothing to print */
                _con_start = con_start;
                _log_end = log_end;
                con_start = log_end;            /* Flush */
                spin_unlock(&logbuf_lock);
                stop_critical_timings();        /* don't trace print latency */
                call_console_drivers(_con_start, _log_end);
                start_critical_timings();
                local_irq_restore(flags);
        }
        console_locked = 0;
        up(&console_sem);
        spin_unlock_irqrestore(&logbuf_lock, flags);
        if (wake_klogd)
                wake_up_klogd();
}
EXPORT_SYMBOL(release_console_sem);

/**
 * console_conditional_schedule - yield the CPU if required
 *
 * If the console code is currently allowed to sleep, and
 * if this CPU should yield the CPU to another task, do
 * so here.
 *
 * Must be called within acquire_console_sem().
 */
void __sched console_conditional_schedule(void)
{
        if (console_may_schedule)
                cond_resched();
}
EXPORT_SYMBOL(console_conditional_schedule);

void console_unblank(void)
{
        struct console *c;

        /*
         * console_unblank can no longer be called in interrupt context unless
         * oops_in_progress is set to 1..
         */
        if (oops_in_progress) {
                if (down_trylock(&console_sem) != 0)
                        return;
        } else
                acquire_console_sem();

        console_locked = 1;
        console_may_schedule = 0;
        for_each_console(c)
                if ((c->flags & CON_ENABLED) && c->unblank)
                        c->unblank();
        release_console_sem();
}

/*
 * Return the console tty driver structure and its associated index
 */
struct tty_driver *console_device(int *index)
{
        struct console *c;
        struct tty_driver *driver = NULL;

        acquire_console_sem();
        for_each_console(c) {
                if (!c->device)
                        continue;
                driver = c->device(c, index);
                if (driver)
                        break;
        }
        release_console_sem();
        return driver;
}

/*
 * Prevent further output on the passed console device so that (for example)
 * serial drivers can disable console output before suspending a port, and can
 * re-enable output afterwards.
 */
void console_stop(struct console *console)
{
        acquire_console_sem();
        console->flags &= ~CON_ENABLED;
        release_console_sem();
}
EXPORT_SYMBOL(console_stop);

void console_start(struct console *console)
{
        acquire_console_sem();
        console->flags |= CON_ENABLED;
        release_console_sem();
}
EXPORT_SYMBOL(console_start);

/*
 * The console driver calls this routine during kernel initialization
 * to register the console printing procedure with printk() and to
 * print any messages that were printed by the kernel before the
 * console driver was initialized.
 *
 * This can happen pretty early during the boot process (because of
 * early_printk) - sometimes before setup_arch() completes - be careful
 * of what kernel features are used - they may not be initialised yet.
 *
 * There are two types of consoles - bootconsoles (early_printk) and
 * "real" consoles (everything which is not a bootconsole) which are
 * handled differently.
 *  - Any number of bootconsoles can be registered at any time.
 *  - As soon as a "real" console is registered, all bootconsoles
 *    will be unregistered automatically.
 *  - Once a "real" console is registered, any attempt to register a
 *    bootconsoles will be rejected
 */
void register_console(struct console *newcon)
{
        int i;
        unsigned long flags;
        struct console *bcon = NULL;

        /*
         * before we register a new CON_BOOT console, make sure we don't
         * already have a valid console
         */
        if (console_drivers && newcon->flags & CON_BOOT) {
                /* find the last or real console */
                for_each_console(bcon) {
                        if (!(bcon->flags & CON_BOOT)) {
                                printk(KERN_INFO "Too late to register bootconsole %s%d\n",
                                        newcon->name, newcon->index);
                                return;
                        }
                }
        }

        if (console_drivers && console_drivers->flags & CON_BOOT)
                bcon = console_drivers;

        if (preferred_console < 0 || bcon || !console_drivers)
                preferred_console = selected_console;

        if (newcon->early_setup)
                newcon->early_setup();

        /*
         *      See if we want to use this console driver. If we
         *      didn't select a console we take the first one
         *      that registers here.
         */
        if (preferred_console < 0) {
                if (newcon->index < 0)
                        newcon->index = 0;
                if (newcon->setup == NULL ||
                    newcon->setup(newcon, NULL) == 0) {
                        newcon->flags |= CON_ENABLED;
                        if (newcon->device) {
                                newcon->flags |= CON_CONSDEV;
                                preferred_console = 0;
                        }
                }
        }

        /*
         *      See if this console matches one we selected on
         *      the command line.
         */
        for (i = 0; i < MAX_CMDLINECONSOLES && console_cmdline[i].name[0];
                        i++) {
                if (strcmp(console_cmdline[i].name, newcon->name) != 0)
                        continue;
                if (newcon->index >= 0 &&
                    newcon->index != console_cmdline[i].index)
                        continue;
                if (newcon->index < 0)
                        newcon->index = console_cmdline[i].index;
#ifdef CONFIG_A11Y_BRAILLE_CONSOLE
                if (console_cmdline[i].brl_options) {
                        newcon->flags |= CON_BRL;
                        braille_register_console(newcon,
                                        console_cmdline[i].index,
                                        console_cmdline[i].options,
                                        console_cmdline[i].brl_options);
                        return;
                }
#endif
                if (newcon->setup &&
                    newcon->setup(newcon, console_cmdline[i].options) != 0)
                        break;
                newcon->flags |= CON_ENABLED;
                newcon->index = console_cmdline[i].index;
                if (i == selected_console) {
                        newcon->flags |= CON_CONSDEV;
                        preferred_console = selected_console;
                }
                break;
        }

        if (!(newcon->flags & CON_ENABLED))
                return;

        /*
         * If we have a bootconsole, and are switching to a real console,
         * don't print everything out again, since when the boot console, and
         * the real console are the same physical device, it's annoying to
         * see the beginning boot messages twice
         */
        if (bcon && ((newcon->flags & (CON_CONSDEV | CON_BOOT)) == CON_CONSDEV))
                newcon->flags &= ~CON_PRINTBUFFER;

        /*
         *      Put this console in the list - keep the
         *      preferred driver at the head of the list.
         */
        acquire_console_sem();
        if ((newcon->flags & CON_CONSDEV) || console_drivers == NULL) {
                newcon->next = console_drivers;
                console_drivers = newcon;
                if (newcon->next)
                        newcon->next->flags &= ~CON_CONSDEV;
        } else {
                newcon->next = console_drivers->next;
                console_drivers->next = newcon;
        }
        if (newcon->flags & CON_PRINTBUFFER) {
                /*
                 * release_console_sem() will print out the buffered messages
                 * for us.
                 */
                spin_lock_irqsave(&logbuf_lock, flags);
                con_start = log_start;
                spin_unlock_irqrestore(&logbuf_lock, flags);
        }
        release_console_sem();

        /*
         * By unregistering the bootconsoles after we enable the real console
         * we get the "console xxx enabled" message on all the consoles -
         * boot consoles, real consoles, etc - this is to ensure that end
         * users know there might be something in the kernel's log buffer that
         * went to the bootconsole (that they do not see on the real console)
         */
        if (bcon && ((newcon->flags & (CON_CONSDEV | CON_BOOT)) == CON_CONSDEV)) {
                /* we need to iterate through twice, to make sure we print
                 * everything out, before we unregister the console(s)
                 */
                printk(KERN_INFO "console [%s%d] enabled, bootconsole disabled\n",
                        newcon->name, newcon->index);
                for_each_console(bcon)
                        if (bcon->flags & CON_BOOT)
                                unregister_console(bcon);
        } else {
                printk(KERN_INFO "%sconsole [%s%d] enabled\n",
                        (newcon->flags & CON_BOOT) ? "boot" : "" ,
                        newcon->name, newcon->index);
        }
}
EXPORT_SYMBOL(register_console);

int unregister_console(struct console *console)
{
        struct console *a, *b;
        int res = 1;

#ifdef CONFIG_A11Y_BRAILLE_CONSOLE
        if (console->flags & CON_BRL)
                return braille_unregister_console(console);
#endif

        acquire_console_sem();
        if (console_drivers == console) {
                console_drivers=console->next;
                res = 0;
        } else if (console_drivers) {
                for (a=console_drivers->next, b=console_drivers ;
                     a; b=a, a=b->next) {
                        if (a == console) {
                                b->next = a->next;
                                res = 0;
                                break;
                        }
                }
        }

        /*
         * If this isn't the last console and it has CON_CONSDEV set, we
         * need to set it on the next preferred console.
         */
        if (console_drivers != NULL && console->flags & CON_CONSDEV)
                console_drivers->flags |= CON_CONSDEV;

        release_console_sem();
        return res;
}
EXPORT_SYMBOL(unregister_console);

static int __init printk_late_init(void)
{
        struct console *con;

        for_each_console(con) {
                if (con->flags & CON_BOOT) {
                        printk(KERN_INFO "turn off boot console %s%d\n",
                                con->name, con->index);
                        unregister_console(con);
                }
        }
        hotcpu_notifier(console_cpu_notify, 0);
        return 0;
}
late_initcall(printk_late_init);

#if defined CONFIG_PRINTK

/*
 * printk rate limiting, lifted from the networking subsystem.
 *
 * This enforces a rate limit: not more than 10 kernel messages
 * every 5s to make a denial-of-service attack impossible.
 */
DEFINE_RATELIMIT_STATE(printk_ratelimit_state, 5 * HZ, 10);

int __printk_ratelimit(const char *func)
{
        return ___ratelimit(&printk_ratelimit_state, func);
}
EXPORT_SYMBOL(__printk_ratelimit);

/**
 * printk_timed_ratelimit - caller-controlled printk ratelimiting
 * @caller_jiffies: pointer to caller's state
 * @interval_msecs: minimum interval between prints
 *
 * printk_timed_ratelimit() returns true if more than @interval_msecs
 * milliseconds have elapsed since the last time printk_timed_ratelimit()
 * returned true.
 */
bool printk_timed_ratelimit(unsigned long *caller_jiffies,
                        unsigned int interval_msecs)
{
        if (*caller_jiffies == 0
                        || !time_in_range(jiffies, *caller_jiffies,
                                        *caller_jiffies
                                        + msecs_to_jiffies(interval_msecs))) {
                *caller_jiffies = jiffies;
                return true;
        }
        return false;
}
EXPORT_SYMBOL(printk_timed_ratelimit);

static DEFINE_SPINLOCK(dump_list_lock);
static LIST_HEAD(dump_list);

/**
 * kmsg_dump_register - register a kernel log dumper.
 * @dumper: pointer to the kmsg_dumper structure
 *
 * Adds a kernel log dumper to the system. The dump callback in the
 * structure will be called when the kernel oopses or panics and must be
 * set. Returns zero on success and %-EINVAL or %-EBUSY otherwise.
 */
int kmsg_dump_register(struct kmsg_dumper *dumper)
{
        unsigned long flags;
        int err = -EBUSY;

        /* The dump callback needs to be set */
        if (!dumper->dump)
                return -EINVAL;

        spin_lock_irqsave(&dump_list_lock, flags);
        /* Don't allow registering multiple times */
        if (!dumper->registered) {
                dumper->registered = 1;
                list_add_tail(&dumper->list, &dump_list);
                err = 0;
        }
        spin_unlock_irqrestore(&dump_list_lock, flags);

        return err;
}
EXPORT_SYMBOL_GPL(kmsg_dump_register);

/**
 * kmsg_dump_unregister - unregister a kmsg dumper.
 * @dumper: pointer to the kmsg_dumper structure
 *
 * Removes a dump device from the system. Returns zero on success and
 * %-EINVAL otherwise.
 */
int kmsg_dump_unregister(struct kmsg_dumper *dumper)
{
        unsigned long flags;
        int err = -EINVAL;

        spin_lock_irqsave(&dump_list_lock, flags);
        if (dumper->registered) {
                dumper->registered = 0;
                list_del(&dumper->list);
                err = 0;
        }
        spin_unlock_irqrestore(&dump_list_lock, flags);

        return err;
}
EXPORT_SYMBOL_GPL(kmsg_dump_unregister);

static const char * const kmsg_reasons[] = {
        [KMSG_DUMP_OOPS]        = "oops",
        [KMSG_DUMP_PANIC]       = "panic",
        [KMSG_DUMP_KEXEC]       = "kexec",
};

static const char *kmsg_to_str(enum kmsg_dump_reason reason)
{
        if (reason >= ARRAY_SIZE(kmsg_reasons) || reason < 0)
                return "unknown";

        return kmsg_reasons[reason];
}

/**
 * kmsg_dump - dump kernel log to kernel message dumpers.
 * @reason: the reason (oops, panic etc) for dumping
 *
 * Iterate through each of the dump devices and call the oops/panic
 * callbacks with the log buffer.
 */
void kmsg_dump(enum kmsg_dump_reason reason)
{
        unsigned long end;
        unsigned chars;
        struct kmsg_dumper *dumper;
        const char *s1, *s2;
        unsigned long l1, l2;
        unsigned long flags;

        /* Theoretically, the log could move on after we do this, but
           there's not a lot we can do about that. The new messages
           will overwrite the start of what we dump. */
        spin_lock_irqsave(&logbuf_lock, flags);
        end = log_end & LOG_BUF_MASK;
        chars = logged_chars;
        spin_unlock_irqrestore(&logbuf_lock, flags);

        if (chars > end) {
                s1 = log_buf + log_buf_len - chars + end;
                l1 = chars - end;

                s2 = log_buf;
                l2 = end;
        } else {
                s1 = "";
                l1 = 0;

                s2 = log_buf + end - chars;
                l2 = chars;
        }

        if (!spin_trylock_irqsave(&dump_list_lock, flags)) {
                printk(KERN_ERR "dump_kmsg: dump list lock is held during %s, skipping dump\n",
                                kmsg_to_str(reason));
                return;
        }
        list_for_each_entry(dumper, &dump_list, list)
                dumper->dump(dumper, reason, s1, l1, s2, l2);
        spin_unlock_irqrestore(&dump_list_lock, flags);
}
#endif