/*
   * 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.
  ", t->comm, t->pid, timeout);

  	printk(KERN_ERR "\"echo 0 > /proc/sys/kernel/hung_task_timeout_secs\""
  			" disables this message.
  ");
  	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);