debug_core.c

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/*
 * Kernel Debug Core
 *
 * Maintainer: Jason Wessel <[email protected]>
 *
 * Copyright (C) 2000-2001 VERITAS Software Corporation.
 * Copyright (C) 2002-2004 Timesys Corporation
 * Copyright (C) 2003-2004 Amit S. Kale <[email protected]>
 * Copyright (C) 2004 Pavel Machek <[email protected]>
 * Copyright (C) 2004-2006 Tom Rini <[email protected]>
 * Copyright (C) 2004-2006 LinSysSoft Technologies Pvt. Ltd.
 * Copyright (C) 2005-2009 Wind River Systems, Inc.
 * Copyright (C) 2007 MontaVista Software, Inc.
 * Copyright (C) 2008 Red Hat, Inc., Ingo Molnar <[email protected]>
 *
 * Contributors at various stages not listed above:
 *  Jason Wessel ( [email protected] )
 *  George Anzinger <[email protected]>
 *  Anurekh Saxena ([email protected])
 *  Lake Stevens Instrument Division (Glenn Engel)
 *  Jim Kingdon, Cygnus Support.
 *
 * Original KGDB stub: David Grothe <[email protected]>,
 * Tigran Aivazian <[email protected]>
 *
 * This file is licensed under the terms of the GNU General Public License
 * version 2. This program is licensed "as is" without any warranty of any
 * kind, whether express or implied.
 */
#include <linux/pid_namespace.h>
#include <linux/clocksource.h>
#include <linux/interrupt.h>
#include <linux/spinlock.h>
#include <linux/console.h>
#include <linux/threads.h>
#include <linux/uaccess.h>
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/ptrace.h>
#include <linux/string.h>
#include <linux/delay.h>
#include <linux/sched.h>
#include <linux/sysrq.h>
#include <linux/reboot.h>
#include <linux/init.h>
#include <linux/kgdb.h>
#include <linux/kdb.h>
#include <linux/pid.h>
#include <linux/smp.h>
#include <linux/mm.h>
#include <linux/rcupdate.h>

#include <asm/cacheflush.h>
#include <asm/byteorder.h>
#include <linux/atomic.h>

#include "debug_core.h"

static int kgdb_break_asap;

struct debuggerinfo_struct kgdb_info[NR_CPUS];

int             kgdb_connected;
EXPORT_SYMBOL_GPL(kgdb_connected);

/* All the KGDB handlers are installed */
int         kgdb_io_module_registered;

/* Guard for recursive entry */
static int          exception_level;

struct kgdb_io      *dbg_io_ops;
static DEFINE_SPINLOCK(kgdb_registration_lock);

/* Action for the reboot notifiter, a global allow kdb to change it */
static int kgdbreboot;
/* kgdb console driver is loaded */
static int kgdb_con_registered;
/* determine if kgdb console output should be used */
static int kgdb_use_con;
/* Flag for alternate operations for early debugging */
bool dbg_is_early = true;
/* Next cpu to become the master debug core */
int dbg_switch_cpu;

/* Use kdb or gdbserver mode */
int dbg_kdb_mode = 1;

static int __init opt_kgdb_con(char *str)
{
    kgdb_use_con = 1;
    return 0;
}

early_param("kgdbcon", opt_kgdb_con);

module_param(kgdb_use_con, int, 0644);
module_param(kgdbreboot, int, 0644);

/*
 * Holds information about breakpoints in a kernel. These breakpoints are
 * added and removed by gdb.
 */
static struct kgdb_bkpt     kgdb_break[KGDB_MAX_BREAKPOINTS] = {
    [0 ... KGDB_MAX_BREAKPOINTS-1] = { .state = BP_UNDEFINED }
};

/*
 * The CPU# of the active CPU, or -1 if none:
 */
atomic_t            kgdb_active = ATOMIC_INIT(-1);
EXPORT_SYMBOL_GPL(kgdb_active);
static DEFINE_RAW_SPINLOCK(dbg_master_lock);
static DEFINE_RAW_SPINLOCK(dbg_slave_lock);

/*
 * We use NR_CPUs not PERCPU, in case kgdb is used to debug early
 * bootup code (which might not have percpu set up yet):
 */
static atomic_t         masters_in_kgdb;
static atomic_t         slaves_in_kgdb;
static atomic_t         kgdb_break_tasklet_var;
atomic_t            kgdb_setting_breakpoint;

struct task_struct      *kgdb_usethread;
struct task_struct      *kgdb_contthread;

int             kgdb_single_step;
static pid_t            kgdb_sstep_pid;

/* to keep track of the CPU which is doing the single stepping*/
atomic_t            kgdb_cpu_doing_single_step = ATOMIC_INIT(-1);

/*
 * If you are debugging a problem where roundup (the collection of
 * all other CPUs) is a problem [this should be extremely rare],
 * then use the nokgdbroundup option to avoid roundup. In that case
 * the other CPUs might interfere with your debugging context, so
 * use this with care:
 */
static int kgdb_do_roundup = 1;

static int __init opt_nokgdbroundup(char *str)
{
    kgdb_do_roundup = 0;

    return 0;
}

early_param("nokgdbroundup", opt_nokgdbroundup);

/*
 * Finally, some KGDB code :-)
 */

/*
 * Weak aliases for breakpoint management,
 * can be overriden by architectures when needed:
 */
int __weak kgdb_arch_set_breakpoint(struct kgdb_bkpt *bpt)
{
    int err;

    err = probe_kernel_read(bpt->saved_instr, (char *)bpt->bpt_addr,
                BREAK_INSTR_SIZE);
    if (err)
        return err;
    err = probe_kernel_write((char *)bpt->bpt_addr,
                 arch_kgdb_ops.gdb_bpt_instr, BREAK_INSTR_SIZE);
    return err;
}

int __weak kgdb_arch_remove_breakpoint(struct kgdb_bkpt *bpt)
{
    return probe_kernel_write((char *)bpt->bpt_addr,
                  (char *)bpt->saved_instr, BREAK_INSTR_SIZE);
}

int __weak kgdb_validate_break_address(unsigned long addr)
{
    struct kgdb_bkpt tmp;
    int err;
    /* Validate setting the breakpoint and then removing it.  If the
     * remove fails, the kernel needs to emit a bad message because we
     * are deep trouble not being able to put things back the way we
     * found them.
     */
    tmp.bpt_addr = addr;
    err = kgdb_arch_set_breakpoint(&tmp);
    if (err)
        return err;
    err = kgdb_arch_remove_breakpoint(&tmp);
    if (err)
        printk(KERN_ERR "KGDB: Critical breakpoint error, kernel "
           "memory destroyed at: %lx", addr);
    return err;
}

unsigned long __weak kgdb_arch_pc(int exception, struct pt_regs *regs)
{
    return instruction_pointer(regs);
}

int __weak kgdb_arch_init(void)
{
    return 0;
}

int __weak kgdb_skipexception(int exception, struct pt_regs *regs)
{
    return 0;
}

/*
 * Some architectures need cache flushes when we set/clear a
 * breakpoint:
 */
static void kgdb_flush_swbreak_addr(unsigned long addr)
{
    if (!CACHE_FLUSH_IS_SAFE)
        return;

    if (current->mm && current->mm->mmap_cache) {
        flush_cache_range(current->mm->mmap_cache,
                  addr, addr + BREAK_INSTR_SIZE);
    }
    /* Force flush instruction cache if it was outside the mm */
    flush_icache_range(addr, addr + BREAK_INSTR_SIZE);
}

/*
 * SW breakpoint management:
 */
int dbg_activate_sw_breakpoints(void)
{
    int error;
    int ret = 0;
    int i;

    for (i = 0; i < KGDB_MAX_BREAKPOINTS; i++) {
        if (kgdb_break[i].state != BP_SET)
            continue;

        error = kgdb_arch_set_breakpoint(&kgdb_break[i]);
        if (error) {
            ret = error;
            printk(KERN_INFO "KGDB: BP install failed: %lx",
                   kgdb_break[i].bpt_addr);
            continue;
        }

        kgdb_flush_swbreak_addr(kgdb_break[i].bpt_addr);
        kgdb_break[i].state = BP_ACTIVE;
    }
    return ret;
}

int dbg_set_sw_break(unsigned long addr)
{
    int err = kgdb_validate_break_address(addr);
    int breakno = -1;
    int i;

    if (err)
        return err;

    for (i = 0; i < KGDB_MAX_BREAKPOINTS; i++) {
        if ((kgdb_break[i].state == BP_SET) &&
                    (kgdb_break[i].bpt_addr == addr))
            return -EEXIST;
    }
    for (i = 0; i < KGDB_MAX_BREAKPOINTS; i++) {
        if (kgdb_break[i].state == BP_REMOVED &&
                    kgdb_break[i].bpt_addr == addr) {
            breakno = i;
            break;
        }
    }

    if (breakno == -1) {
        for (i = 0; i < KGDB_MAX_BREAKPOINTS; i++) {
            if (kgdb_break[i].state == BP_UNDEFINED) {
                breakno = i;
                break;
            }
        }
    }

    if (breakno == -1)
        return -E2BIG;

    kgdb_break[breakno].state = BP_SET;
    kgdb_break[breakno].type = BP_BREAKPOINT;
    kgdb_break[breakno].bpt_addr = addr;

    return 0;
}

int dbg_deactivate_sw_breakpoints(void)
{
    int error;
    int ret = 0;
    int i;

    for (i = 0; i < KGDB_MAX_BREAKPOINTS; i++) {
        if (kgdb_break[i].state != BP_ACTIVE)
            continue;
        error = kgdb_arch_remove_breakpoint(&kgdb_break[i]);
        if (error) {
            printk(KERN_INFO "KGDB: BP remove failed: %lx\n",
                   kgdb_break[i].bpt_addr);
            ret = error;
        }

        kgdb_flush_swbreak_addr(kgdb_break[i].bpt_addr);
        kgdb_break[i].state = BP_SET;
    }
    return ret;
}

int dbg_remove_sw_break(unsigned long addr)
{
    int i;

    for (i = 0; i < KGDB_MAX_BREAKPOINTS; i++) {
        if ((kgdb_break[i].state == BP_SET) &&
                (kgdb_break[i].bpt_addr == addr)) {
            kgdb_break[i].state = BP_REMOVED;
            return 0;
        }
    }
    return -ENOENT;
}

int kgdb_isremovedbreak(unsigned long addr)
{
    int i;

    for (i = 0; i < KGDB_MAX_BREAKPOINTS; i++) {
        if ((kgdb_break[i].state == BP_REMOVED) &&
                    (kgdb_break[i].bpt_addr == addr))
            return 1;
    }
    return 0;
}

int dbg_remove_all_break(void)
{
    int error;
    int i;

    /* Clear memory breakpoints. */
    for (i = 0; i < KGDB_MAX_BREAKPOINTS; i++) {
        if (kgdb_break[i].state != BP_ACTIVE)
            goto setundefined;
        error = kgdb_arch_remove_breakpoint(&kgdb_break[i]);
        if (error)
            printk(KERN_ERR "KGDB: breakpoint remove failed: %lx\n",
                   kgdb_break[i].bpt_addr);
setundefined:
        kgdb_break[i].state = BP_UNDEFINED;
    }

    /* Clear hardware breakpoints. */
    if (arch_kgdb_ops.remove_all_hw_break)
        arch_kgdb_ops.remove_all_hw_break();

    return 0;
}

/*
 * Return true if there is a valid kgdb I/O module.  Also if no
 * debugger is attached a message can be printed to the console about
 * waiting for the debugger to attach.
 *
 * The print_wait argument is only to be true when called from inside
 * the core kgdb_handle_exception, because it will wait for the
 * debugger to attach.
 */
static int kgdb_io_ready(int print_wait)
{
    if (!dbg_io_ops)
        return 0;
    if (kgdb_connected)
        return 1;
    if (atomic_read(&kgdb_setting_breakpoint))
        return 1;
    if (print_wait) {
#ifdef CONFIG_KGDB_KDB
        if (!dbg_kdb_mode)
            printk(KERN_CRIT "KGDB: waiting... or $3#33 for KDB\n");
#else
        printk(KERN_CRIT "KGDB: Waiting for remote debugger\n");
#endif
    }
    return 1;
}

static int kgdb_reenter_check(struct kgdb_state *ks)
{
    unsigned long addr;

    if (atomic_read(&kgdb_active) != raw_smp_processor_id())
        return 0;

    /* Panic on recursive debugger calls: */
    exception_level++;
    addr = kgdb_arch_pc(ks->ex_vector, ks->linux_regs);
    dbg_deactivate_sw_breakpoints();

    /*
     * If the break point removed ok at the place exception
     * occurred, try to recover and print a warning to the end
     * user because the user planted a breakpoint in a place that
     * KGDB needs in order to function.
     */
    if (dbg_remove_sw_break(addr) == 0) {
        exception_level = 0;
        kgdb_skipexception(ks->ex_vector, ks->linux_regs);
        dbg_activate_sw_breakpoints();
        printk(KERN_CRIT "KGDB: re-enter error: breakpoint removed %lx\n",
            addr);
        WARN_ON_ONCE(1);

        return 1;
    }
    dbg_remove_all_break();
    kgdb_skipexception(ks->ex_vector, ks->linux_regs);

    if (exception_level > 1) {
        dump_stack();
        panic("Recursive entry to debugger");
    }

    printk(KERN_CRIT "KGDB: re-enter exception: ALL breakpoints killed\n");
#ifdef CONFIG_KGDB_KDB
    /* Allow kdb to debug itself one level */
    return 0;
#endif
    dump_stack();
    panic("Recursive entry to debugger");

    return 1;
}

static void dbg_touch_watchdogs(void)
{
    touch_softlockup_watchdog_sync();
    clocksource_touch_watchdog();
    rcu_cpu_stall_reset();
}

static int kgdb_cpu_enter(struct kgdb_state *ks, struct pt_regs *regs,
        int exception_state)
{
    unsigned long flags;
    int sstep_tries = 100;
    int error;
    int cpu;
    int trace_on = 0;
    int online_cpus = num_online_cpus();

    kgdb_info[ks->cpu].enter_kgdb++;
    kgdb_info[ks->cpu].exception_state |= exception_state;

    if (exception_state == DCPU_WANT_MASTER)
        atomic_inc(&masters_in_kgdb);
    else
        atomic_inc(&slaves_in_kgdb);

    if (arch_kgdb_ops.disable_hw_break)
        arch_kgdb_ops.disable_hw_break(regs);

acquirelock:
    /*
     * Interrupts will be restored by the 'trap return' code, except when
     * single stepping.
     */
    local_irq_save(flags);

    cpu = ks->cpu;
    kgdb_info[cpu].debuggerinfo = regs;
    kgdb_info[cpu].task = current;
    kgdb_info[cpu].ret_state = 0;
    kgdb_info[cpu].irq_depth = hardirq_count() >> HARDIRQ_SHIFT;

    /* Make sure the above info reaches the primary CPU */
    smp_mb();

    if (exception_level == 1) {
        if (raw_spin_trylock(&dbg_master_lock))
            atomic_xchg(&kgdb_active, cpu);
        goto cpu_master_loop;
    }

    /*
     * CPU will loop if it is a slave or request to become a kgdb
     * master cpu and acquire the kgdb_active lock:
     */
    while (1) {
cpu_loop:
        if (kgdb_info[cpu].exception_state & DCPU_NEXT_MASTER) {
            kgdb_info[cpu].exception_state &= ~DCPU_NEXT_MASTER;
            goto cpu_master_loop;
        } else if (kgdb_info[cpu].exception_state & DCPU_WANT_MASTER) {
            if (raw_spin_trylock(&dbg_master_lock)) {
                atomic_xchg(&kgdb_active, cpu);
                break;
            }
        } else if (kgdb_info[cpu].exception_state & DCPU_IS_SLAVE) {
            if (!raw_spin_is_locked(&dbg_slave_lock))
                goto return_normal;
        } else {
return_normal:
            /* Return to normal operation by executing any
             * hw breakpoint fixup.
             */
            if (arch_kgdb_ops.correct_hw_break)
                arch_kgdb_ops.correct_hw_break();
            if (trace_on)
                tracing_on();
            kgdb_info[cpu].exception_state &=
                ~(DCPU_WANT_MASTER | DCPU_IS_SLAVE);
            kgdb_info[cpu].enter_kgdb--;
            smp_mb__before_atomic_dec();
            atomic_dec(&slaves_in_kgdb);
            dbg_touch_watchdogs();
            local_irq_restore(flags);
            return 0;
        }
        cpu_relax();
    }

    /*
     * For single stepping, try to only enter on the processor
     * that was single stepping.  To guard against a deadlock, the
     * kernel will only try for the value of sstep_tries before
     * giving up and continuing on.
     */
    if (atomic_read(&kgdb_cpu_doing_single_step) != -1 &&
        (kgdb_info[cpu].task &&
         kgdb_info[cpu].task->pid != kgdb_sstep_pid) && --sstep_tries) {
        atomic_set(&kgdb_active, -1);
        raw_spin_unlock(&dbg_master_lock);
        dbg_touch_watchdogs();
        local_irq_restore(flags);

        goto acquirelock;
    }

    if (!kgdb_io_ready(1)) {
        kgdb_info[cpu].ret_state = 1;
        goto kgdb_restore; /* No I/O connection, resume the system */
    }

    /*
     * Don't enter if we have hit a removed breakpoint.
     */
    if (kgdb_skipexception(ks->ex_vector, ks->linux_regs))
        goto kgdb_restore;

    /* Call the I/O driver's pre_exception routine */
    if (dbg_io_ops->pre_exception)
        dbg_io_ops->pre_exception();

    /*
     * Get the passive CPU lock which will hold all the non-primary
     * CPU in a spin state while the debugger is active
     */
    if (!kgdb_single_step)
        raw_spin_lock(&dbg_slave_lock);

#ifdef CONFIG_SMP
    /* Signal the other CPUs to enter kgdb_wait() */
    if ((!kgdb_single_step) && kgdb_do_roundup)
        kgdb_roundup_cpus(flags);
#endif

    /*
     * Wait for the other CPUs to be notified and be waiting for us:
     */
    while (kgdb_do_roundup && (atomic_read(&masters_in_kgdb) +
                atomic_read(&slaves_in_kgdb)) != online_cpus)
        cpu_relax();

    /*
     * At this point the primary processor is completely
     * in the debugger and all secondary CPUs are quiescent
     */
    dbg_deactivate_sw_breakpoints();
    kgdb_single_step = 0;
    kgdb_contthread = current;
    exception_level = 0;
    trace_on = tracing_is_on();
    if (trace_on)
        tracing_off();

    while (1) {
cpu_master_loop:
        if (dbg_kdb_mode) {
            kgdb_connected = 1;
            error = kdb_stub(ks);
            if (error == -1)
                continue;
            kgdb_connected = 0;
        } else {
            error = gdb_serial_stub(ks);
        }

        if (error == DBG_PASS_EVENT) {
            dbg_kdb_mode = !dbg_kdb_mode;
        } else if (error == DBG_SWITCH_CPU_EVENT) {
            kgdb_info[dbg_switch_cpu].exception_state |=
                DCPU_NEXT_MASTER;
            goto cpu_loop;
        } else {
            kgdb_info[cpu].ret_state = error;
            break;
        }
    }

    /* Call the I/O driver's post_exception routine */
    if (dbg_io_ops->post_exception)
        dbg_io_ops->post_exception();

    if (!kgdb_single_step) {
        raw_spin_unlock(&dbg_slave_lock);
        /* Wait till all the CPUs have quit from the debugger. */
        while (kgdb_do_roundup && atomic_read(&slaves_in_kgdb))
            cpu_relax();
    }

kgdb_restore:
    if (atomic_read(&kgdb_cpu_doing_single_step) != -1) {
        int sstep_cpu = atomic_read(&kgdb_cpu_doing_single_step);
        if (kgdb_info[sstep_cpu].task)
            kgdb_sstep_pid = kgdb_info[sstep_cpu].task->pid;
        else
            kgdb_sstep_pid = 0;
    }
    if (arch_kgdb_ops.correct_hw_break)
        arch_kgdb_ops.correct_hw_break();
    if (trace_on)
        tracing_on();

    kgdb_info[cpu].exception_state &=
        ~(DCPU_WANT_MASTER | DCPU_IS_SLAVE);
    kgdb_info[cpu].enter_kgdb--;
    smp_mb__before_atomic_dec();
    atomic_dec(&masters_in_kgdb);
    /* Free kgdb_active */
    atomic_set(&kgdb_active, -1);
    raw_spin_unlock(&dbg_master_lock);
    dbg_touch_watchdogs();
    local_irq_restore(flags);

    return kgdb_info[cpu].ret_state;
}

/*
 * kgdb_handle_exception() - main entry point from a kernel exception
 *
 * Locking hierarchy:
 *  interface locks, if any (begin_session)
 *  kgdb lock (kgdb_active)
 */
int
kgdb_handle_exception(int evector, int signo, int ecode, struct pt_regs *regs)
{
    struct kgdb_state kgdb_var;
    struct kgdb_state *ks = &kgdb_var;
    int ret = 0;

    if (arch_kgdb_ops.enable_nmi)
        arch_kgdb_ops.enable_nmi(0);

    ks->cpu         = raw_smp_processor_id();
    ks->ex_vector       = evector;
    ks->signo       = signo;
    ks->err_code        = ecode;
    ks->kgdb_usethreadid    = 0;
    ks->linux_regs      = regs;

    if (kgdb_reenter_check(ks))
        goto out; /* Ouch, double exception ! */
    if (kgdb_info[ks->cpu].enter_kgdb != 0)
        goto out;

    ret = kgdb_cpu_enter(ks, regs, DCPU_WANT_MASTER);
out:
    if (arch_kgdb_ops.enable_nmi)
        arch_kgdb_ops.enable_nmi(1);
    return ret;
}

/*
 * GDB places a breakpoint at this function to know dynamically
 * loaded objects. It's not defined static so that only one instance with this
 * name exists in the kernel.
 */

static int module_event(struct notifier_block *self, unsigned long val,
    void *data)
{
    return 0;
}

static struct notifier_block dbg_module_load_nb = {
    .notifier_call  = module_event,
};

int kgdb_nmicallback(int cpu, void *regs)
{
#ifdef CONFIG_SMP
    struct kgdb_state kgdb_var;
    struct kgdb_state *ks = &kgdb_var;

    memset(ks, 0, sizeof(struct kgdb_state));
    ks->cpu         = cpu;
    ks->linux_regs      = regs;

    if (kgdb_info[ks->cpu].enter_kgdb == 0 &&
            raw_spin_is_locked(&dbg_master_lock)) {
        kgdb_cpu_enter(ks, regs, DCPU_IS_SLAVE);
        return 0;
    }
#endif
    return 1;
}

static void kgdb_console_write(struct console *co, const char *s,
   unsigned count)
{
    unsigned long flags;

    /* If we're debugging, or KGDB has not connected, don't try
     * and print. */
    if (!kgdb_connected || atomic_read(&kgdb_active) != -1 || dbg_kdb_mode)
        return;

    local_irq_save(flags);
    gdbstub_msg_write(s, count);
    local_irq_restore(flags);
}

static struct console kgdbcons = {
    .name       = "kgdb",
    .write      = kgdb_console_write,
    .flags      = CON_PRINTBUFFER | CON_ENABLED,
    .index      = -1,
};

#ifdef CONFIG_MAGIC_SYSRQ
static void sysrq_handle_dbg(int key)
{
    if (!dbg_io_ops) {
        printk(KERN_CRIT "ERROR: No KGDB I/O module available\n");
        return;
    }
    if (!kgdb_connected) {
#ifdef CONFIG_KGDB_KDB
        if (!dbg_kdb_mode)
            printk(KERN_CRIT "KGDB or $3#33 for KDB\n");
#else
        printk(KERN_CRIT "Entering KGDB\n");
#endif
    }

    kgdb_breakpoint();
}

static struct sysrq_key_op sysrq_dbg_op = {
    .handler    = sysrq_handle_dbg,
    .help_msg   = "debug(G)",
    .action_msg = "DEBUG",
};
#endif

static int kgdb_panic_event(struct notifier_block *self,
                unsigned long val,
                void *data)
{
    if (dbg_kdb_mode)
        kdb_printf("PANIC: %s\n", (char *)data);
    kgdb_breakpoint();
    return NOTIFY_DONE;
}

static struct notifier_block kgdb_panic_event_nb = {
       .notifier_call   = kgdb_panic_event,
       .priority    = INT_MAX,
};

void __weak kgdb_arch_late(void)
{
}

void __init dbg_late_init(void)
{
    dbg_is_early = false;
    if (kgdb_io_module_registered)
        kgdb_arch_late();
    kdb_init(KDB_INIT_FULL);
}

static int
dbg_notify_reboot(struct notifier_block *this, unsigned long code, void *x)
{
    /*
     * Take the following action on reboot notify depending on value:
     *    1 == Enter debugger
     *    0 == [the default] detatch debug client
     *   -1 == Do nothing... and use this until the board resets
     */
    switch (kgdbreboot) {
    case 1:
        kgdb_breakpoint();
    case -1:
        goto done;
    }
    if (!dbg_kdb_mode)
        gdbstub_exit(code);
done:
    return NOTIFY_DONE;
}

static struct notifier_block dbg_reboot_notifier = {
    .notifier_call      = dbg_notify_reboot,
    .next           = NULL,
    .priority       = INT_MAX,
};

static void kgdb_register_callbacks(void)
{
    if (!kgdb_io_module_registered) {
        kgdb_io_module_registered = 1;
        kgdb_arch_init();
        if (!dbg_is_early)
            kgdb_arch_late();
        register_module_notifier(&dbg_module_load_nb);
        register_reboot_notifier(&dbg_reboot_notifier);
        atomic_notifier_chain_register(&panic_notifier_list,
                           &kgdb_panic_event_nb);
#ifdef CONFIG_MAGIC_SYSRQ
        register_sysrq_key('g', &sysrq_dbg_op);
#endif
        if (kgdb_use_con && !kgdb_con_registered) {
            register_console(&kgdbcons);
            kgdb_con_registered = 1;
        }
    }
}

static void kgdb_unregister_callbacks(void)
{
    /*
     * When this routine is called KGDB should unregister from the
     * panic handler and clean up, making sure it is not handling any
     * break exceptions at the time.
     */
    if (kgdb_io_module_registered) {
        kgdb_io_module_registered = 0;
        unregister_reboot_notifier(&dbg_reboot_notifier);
        unregister_module_notifier(&dbg_module_load_nb);
        atomic_notifier_chain_unregister(&panic_notifier_list,
                           &kgdb_panic_event_nb);
        kgdb_arch_exit();
#ifdef CONFIG_MAGIC_SYSRQ
        unregister_sysrq_key('g', &sysrq_dbg_op);
#endif
        if (kgdb_con_registered) {
            unregister_console(&kgdbcons);
            kgdb_con_registered = 0;
        }
    }
}

/*
 * There are times a tasklet needs to be used vs a compiled in
 * break point so as to cause an exception outside a kgdb I/O module,
 * such as is the case with kgdboe, where calling a breakpoint in the
 * I/O driver itself would be fatal.
 */
static void kgdb_tasklet_bpt(unsigned long ing)
{
    kgdb_breakpoint();
    atomic_set(&kgdb_break_tasklet_var, 0);
}

static DECLARE_TASKLET(kgdb_tasklet_breakpoint, kgdb_tasklet_bpt, 0);

void kgdb_schedule_breakpoint(void)
{
    if (atomic_read(&kgdb_break_tasklet_var) ||
        atomic_read(&kgdb_active) != -1 ||
        atomic_read(&kgdb_setting_breakpoint))
        return;
    atomic_inc(&kgdb_break_tasklet_var);
    tasklet_schedule(&kgdb_tasklet_breakpoint);
}
EXPORT_SYMBOL_GPL(kgdb_schedule_breakpoint);

static void kgdb_initial_breakpoint(void)
{
    kgdb_break_asap = 0;

    printk(KERN_CRIT "kgdb: Waiting for connection from remote gdb...\n");
    kgdb_breakpoint();
}

int kgdb_register_io_module(struct kgdb_io *new_dbg_io_ops)
{
    int err;

    spin_lock(&kgdb_registration_lock);

    if (dbg_io_ops) {
        spin_unlock(&kgdb_registration_lock);

        printk(KERN_ERR "kgdb: Another I/O driver is already "
                "registered with KGDB.\n");
        return -EBUSY;
    }

    if (new_dbg_io_ops->init) {
        err = new_dbg_io_ops->init();
        if (err) {
            spin_unlock(&kgdb_registration_lock);
            return err;
        }
    }

    dbg_io_ops = new_dbg_io_ops;

    spin_unlock(&kgdb_registration_lock);

    printk(KERN_INFO "kgdb: Registered I/O driver %s.\n",
           new_dbg_io_ops->name);

    /* Arm KGDB now. */
    kgdb_register_callbacks();

    if (kgdb_break_asap)
        kgdb_initial_breakpoint();

    return 0;
}
EXPORT_SYMBOL_GPL(kgdb_register_io_module);

void kgdb_unregister_io_module(struct kgdb_io *old_dbg_io_ops)
{
    BUG_ON(kgdb_connected);

    /*
     * KGDB is no longer able to communicate out, so
     * unregister our callbacks and reset state.
     */
    kgdb_unregister_callbacks();

    spin_lock(&kgdb_registration_lock);

    WARN_ON_ONCE(dbg_io_ops != old_dbg_io_ops);
    dbg_io_ops = NULL;

    spin_unlock(&kgdb_registration_lock);

    printk(KERN_INFO
        "kgdb: Unregistered I/O driver %s, debugger disabled.\n",
        old_dbg_io_ops->name);
}
EXPORT_SYMBOL_GPL(kgdb_unregister_io_module);

int dbg_io_get_char(void)
{
    int ret = dbg_io_ops->read_char();
    if (ret == NO_POLL_CHAR)
        return -1;
    if (!dbg_kdb_mode)
        return ret;
    if (ret == 127)
        return 8;
    return ret;
}

void kgdb_breakpoint(void)
{
    atomic_inc(&kgdb_setting_breakpoint);
    wmb(); /* Sync point before breakpoint */
    arch_kgdb_breakpoint();
    wmb(); /* Sync point after breakpoint */
    atomic_dec(&kgdb_setting_breakpoint);
}
EXPORT_SYMBOL_GPL(kgdb_breakpoint);

static int __init opt_kgdb_wait(char *str)
{
    kgdb_break_asap = 1;

    kdb_init(KDB_INIT_EARLY);
    if (kgdb_io_module_registered)
        kgdb_initial_breakpoint();

    return 0;
}

early_param("kgdbwait", opt_kgdb_wait);