/*
 * Detect Hung Task
 *
 * kernel/hung_task.c - kernel thread for detecting tasks stuck in D state
 *
 */

#include <linux/mm.h>
#include <linux/cpu.h>
#include <linux/nmi.h>
#include <linux/init.h>
#include <linux/delay.h>
#include <linux/freezer.h>
#include <linux/kthread.h>
#include <linux/lockdep.h>
#include <linux/export.h>
#include <linux/sysctl.h>

/*
 * The number of tasks checked:
 */
unsigned long __read_mostly sysctl_hung_task_check_count = PID_MAX_LIMIT;

/*
 * Limit number of tasks checked in a batch.
 *
 * This value controls the preemptibility of khungtaskd since preemption
 * is disabled during the critical section. It also controls the size of
 * the RCU grace period. So it needs to be upper-bound.
 */
#define HUNG_TASK_BATCHING 1024

/*
 * Zero means infinite timeout - no checking done:
 */
unsigned long __read_mostly sysctl_hung_task_timeout_secs = CONFIG_DEFAULT_HUNG_TASK_TIMEOUT;

unsigned long __read_mostly sysctl_hung_task_warnings = 10;

static int __read_mostly did_panic;

static struct task_struct *watchdog_task;

/*
 * Should we panic (and reboot, if panic_timeout= is set) when a
 * hung task is detected:
 */
unsigned int __read_mostly sysctl_hung_task_panic =
                                CONFIG_BOOTPARAM_HUNG_TASK_PANIC_VALUE;

static int __init hung_task_panic_setup(char *str)
{
        sysctl_hung_task_panic = simple_strtoul(str, NULL, 0);

        return 1;
}
__setup("hung_task_panic=", hung_task_panic_setup);

static int
hung_task_panic(struct notifier_block *this, unsigned long event, void *ptr)
{
        did_panic = 1;

        return NOTIFY_DONE;
}

static struct notifier_block panic_block = {
        .notifier_call = hung_task_panic,
};

static void check_hung_task(struct task_struct *t, unsigned long timeout)
{
        unsigned long switch_count = t->nvcsw + t->nivcsw;

        /*
         * Ensure the task is not frozen.
         * Also, skip vfork and any other user process that freezer should skip.
         */
        if (unlikely(t->flags & (PF_FROZEN | PF_FREEZER_SKIP)))
            return;

        /*
         * When a freshly created task is scheduled once, changes its state to
         * TASK_UNINTERRUPTIBLE without having ever been switched out once, it
         * musn't be checked.
         */
        if (unlikely(!switch_count))
                return;

        if (switch_count != t->last_switch_count) {
                t->last_switch_count = switch_count;
                return;
        }
        if (!sysctl_hung_task_warnings)
                return;
        sysctl_hung_task_warnings--;

        /*
         * Ok, the task did not get scheduled for more than 2 minutes,
         * complain:
         */
        printk(KERN_ERR "INFO: task %s:%d blocked for more than "
                        "%ld seconds.\n", t->comm, t->pid, timeout);
        printk(KERN_ERR "\"echo 0 > /proc/sys/kernel/hung_task_timeout_secs\""
                        " disables this message.\n");
        sched_show_task(t);
        debug_show_held_locks(t);

        touch_nmi_watchdog();

        if (sysctl_hung_task_panic) {
                trigger_all_cpu_backtrace();
                panic("hung_task: blocked tasks");
        }
}

/*
 * To avoid extending the RCU grace period for an unbounded amount of time,
 * periodically exit the critical section and enter a new one.
 *
 * For preemptible RCU it is sufficient to call rcu_read_unlock in order
 * to exit the grace period. For classic RCU, a reschedule is required.
 */
static bool rcu_lock_break(struct task_struct *g, struct task_struct *t)
{
        bool can_cont;

        get_task_struct(g);
        get_task_struct(t);
        rcu_read_unlock();
        cond_resched();
        rcu_read_lock();
        can_cont = pid_alive(g) && pid_alive(t);
        put_task_struct(t);
        put_task_struct(g);

        return can_cont;
}

/*
 * Check whether a TASK_UNINTERRUPTIBLE does not get woken up for
 * a really long time (120 seconds). If that happens, print out
 * a warning.
 */
static void check_hung_uninterruptible_tasks(unsigned long timeout)
{
        int max_count = sysctl_hung_task_check_count;
        int batch_count = HUNG_TASK_BATCHING;
        struct task_struct *g, *t;

        /*
         * If the system crashed already then all bets are off,
         * do not report extra hung tasks:
         */
        if (test_taint(TAINT_DIE) || did_panic)
                return;

        rcu_read_lock();
        do_each_thread(g, t) {
                if (!max_count--)
                        goto unlock;
                if (!--batch_count) {
                        batch_count = HUNG_TASK_BATCHING;
                        if (!rcu_lock_break(g, t))
                                goto unlock;
                }
                /* use "==" to skip the TASK_KILLABLE tasks waiting on NFS */
                if (t->state == TASK_UNINTERRUPTIBLE)
                        check_hung_task(t, timeout);
        } while_each_thread(g, t);
 unlock:
        rcu_read_unlock();
}

static unsigned long timeout_jiffies(unsigned long timeout)
{
        /* timeout of 0 will disable the watchdog */
        return timeout ? timeout * HZ : MAX_SCHEDULE_TIMEOUT;
}

/*
 * Process updating of timeout sysctl
 */
int proc_dohung_task_timeout_secs(struct ctl_table *table, int write,
                                  void __user *buffer,
                                  size_t *lenp, loff_t *ppos)
{
        int ret;

        ret = proc_doulongvec_minmax(table, write, buffer, lenp, ppos);

        if (ret || !write)
                goto out;

        wake_up_process(watchdog_task);

 out:
        return ret;
}

/*
 * kthread which checks for tasks stuck in D state
 */
static int watchdog(void *dummy)
{
        set_user_nice(current, 0);

        for ( ; ; ) {
                unsigned long timeout = sysctl_hung_task_timeout_secs;

                while (schedule_timeout_interruptible(timeout_jiffies(timeout)))
                        timeout = sysctl_hung_task_timeout_secs;

                check_hung_uninterruptible_tasks(timeout);
        }

        return 0;
}

static int __init hung_task_init(void)
{
        atomic_notifier_chain_register(&panic_notifier_list, &panic_block);
        watchdog_task = kthread_run(watchdog, NULL, "khungtaskd");

        return 0;
}

module_init(hung_task_init);