/*
   * Context tracking: Probe on high level context boundaries such as kernel
   * and userspace. This includes syscalls and exceptions entry/exit.
   *
   * This is used by RCU to remove its dependency on the timer tick while a CPU
   * runs in userspace.
   *
   *  Started by Frederic Weisbecker:
   *
   * Copyright (C) 2012 Red Hat, Inc., Frederic Weisbecker <fweisbec@redhat.com>
   *
   * Many thanks to Gilad Ben-Yossef, Paul McKenney, Ingo Molnar, Andrew Morton,
   * Steven Rostedt, Peter Zijlstra for suggestions and improvements.
   *
   */

  #include <linux/context_tracking.h>
  #include <linux/rcupdate.h>
  #include <linux/sched.h>

  #include <linux/hardirq.h>

  #include <linux/export.h>

  #include <linux/kprobes.h>

  #define CREATE_TRACE_POINTS
  #include <trace/events/context_tracking.h>

  struct static_key context_tracking_enabled = STATIC_KEY_INIT_FALSE;

  EXPORT_SYMBOL_GPL(context_tracking_enabled);

  
  DEFINE_PER_CPU(struct context_tracking, context_tracking);

  EXPORT_SYMBOL_GPL(context_tracking);

  void context_tracking_cpu_set(int cpu)
  {

  	if (!per_cpu(context_tracking.active, cpu)) {
  		per_cpu(context_tracking.active, cpu) = true;
  		static_key_slow_inc(&context_tracking_enabled);
  	}

  }

  /**

   * context_tracking_user_enter - Inform the context tracking that the CPU is going to
   *                               enter userspace mode.

   *
   * This function must be called right before we switch from the kernel
   * to userspace, when it's guaranteed the remaining kernel instructions
   * to execute won't use any RCU read side critical section because this
   * function sets RCU in extended quiescent state.
   */

  void context_tracking_user_enter(void)

  {
  	unsigned long flags;
  
  	/*

  	 * Repeat the user_enter() check here because some archs may be calling
  	 * this from asm and if no CPU needs context tracking, they shouldn't

  	 * go further. Repeat the check here until they support the inline static
  	 * key check.

  	 */

  	if (!context_tracking_is_enabled())

  		return;
  
  	/*

  	 * Some contexts may involve an exception occuring in an irq,
  	 * leading to that nesting:
  	 * rcu_irq_enter() rcu_user_exit() rcu_user_exit() rcu_irq_exit()
  	 * This would mess up the dyntick_nesting count though. And rcu_irq_*()
  	 * helpers are enough to protect RCU uses inside the exception. So
  	 * just return immediately if we detect we are in an IRQ.
  	 */
  	if (in_interrupt())
  		return;

  	/* Kernel threads aren't supposed to go to userspace */

  	WARN_ON_ONCE(!current->mm);
  
  	local_irq_save(flags);

  	if ( __this_cpu_read(context_tracking.state) != IN_USER) {
  		if (__this_cpu_read(context_tracking.active)) {

  			trace_user_enter(0);

  			/*
  			 * At this stage, only low level arch entry code remains and
  			 * then we'll run in userspace. We can assume there won't be
  			 * any RCU read-side critical section until the next call to
  			 * user_exit() or rcu_irq_enter(). Let's remove RCU's dependency
  			 * on the tick.
  			 */
  			vtime_user_enter(current);
  			rcu_user_enter();
  		}

  		/*

  		 * Even if context tracking is disabled on this CPU, because it's outside
  		 * the full dynticks mask for example, we still have to keep track of the
  		 * context transitions and states to prevent inconsistency on those of
  		 * other CPUs.
  		 * If a task triggers an exception in userspace, sleep on the exception
  		 * handler and then migrate to another CPU, that new CPU must know where
  		 * the exception returns by the time we call exception_exit().
  		 * This information can only be provided by the previous CPU when it called
  		 * exception_enter().
  		 * OTOH we can spare the calls to vtime and RCU when context_tracking.active
  		 * is false because we know that CPU is not tickless.

  		 */

  		__this_cpu_write(context_tracking.state, IN_USER);

  	}
  	local_irq_restore(flags);
  }

  NOKPROBE_SYMBOL(context_tracking_user_enter);

  /**

   * context_tracking_user_exit - Inform the context tracking that the CPU is
   *                              exiting userspace mode and entering the kernel.

   *
   * This function must be called after we entered the kernel from userspace
   * before any use of RCU read side critical section. This potentially include
   * any high level kernel code like syscalls, exceptions, signal handling, etc...
   *
   * This call supports re-entrancy. This way it can be called from any exception
   * handler without needing to know if we came from userspace or not.
   */

  void context_tracking_user_exit(void)

  {
  	unsigned long flags;

  	if (!context_tracking_is_enabled())

  		return;

  	if (in_interrupt())
  		return;
  
  	local_irq_save(flags);
  	if (__this_cpu_read(context_tracking.state) == IN_USER) {

  		if (__this_cpu_read(context_tracking.active)) {
  			/*
  			 * We are going to run code that may use RCU. Inform
  			 * RCU core about that (ie: we may need the tick again).
  			 */
  			rcu_user_exit();
  			vtime_user_exit(current);

  			trace_user_exit(0);

  		}

  		__this_cpu_write(context_tracking.state, IN_KERNEL);

  	}
  	local_irq_restore(flags);
  }

  NOKPROBE_SYMBOL(context_tracking_user_exit);

  /**

   * __context_tracking_task_switch - context switch the syscall callbacks

   * @prev: the task that is being switched out
   * @next: the task that is being switched in
   *
   * The context tracking uses the syscall slow path to implement its user-kernel
   * boundaries probes on syscalls. This way it doesn't impact the syscall fast
   * path on CPUs that don't do context tracking.
   *
   * But we need to clear the flag on the previous task because it may later
   * migrate to some CPU that doesn't do the context tracking. As such the TIF
   * flag may not be desired there.
   */

  void __context_tracking_task_switch(struct task_struct *prev,
  				    struct task_struct *next)

  {

  	clear_tsk_thread_flag(prev, TIF_NOHZ);
  	set_tsk_thread_flag(next, TIF_NOHZ);

  }

  
  #ifdef CONFIG_CONTEXT_TRACKING_FORCE
  void __init context_tracking_init(void)
  {
  	int cpu;
  
  	for_each_possible_cpu(cpu)
  		context_tracking_cpu_set(cpu);
  }
  #endif