/*
   * 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>

  DEFINE_STATIC_KEY_FALSE(context_tracking_enabled);

  EXPORT_SYMBOL_GPL(context_tracking_enabled);

  
  DEFINE_PER_CPU(struct context_tracking, context_tracking);

  EXPORT_SYMBOL_GPL(context_tracking);

  static bool context_tracking_recursion_enter(void)
  {
  	int recursion;
  
  	recursion = __this_cpu_inc_return(context_tracking.recursion);
  	if (recursion == 1)
  		return true;
  
  	WARN_ONCE((recursion < 1), "Invalid context tracking recursion value %d
  ", recursion);
  	__this_cpu_dec(context_tracking.recursion);
  
  	return false;
  }
  
  static void context_tracking_recursion_exit(void)
  {
  	__this_cpu_dec(context_tracking.recursion);
  }

  /**

   * context_tracking_enter - Inform the context tracking that the CPU is going
   *                          enter user or guest space mode.

   *
   * This function must be called right before we switch from the kernel

   * to user or guest space, 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_enter(enum ctx_state state)

  {

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

  	WARN_ON_ONCE(!current->mm);

  	if (!context_tracking_recursion_enter())

  		return;

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

  		if (__this_cpu_read(context_tracking.active)) {
  			/*
  			 * 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.
  			 */

  			if (state == CONTEXT_USER) {
  				trace_user_enter(0);
  				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, state);

  	}

  	context_tracking_recursion_exit();

  }
  NOKPROBE_SYMBOL(__context_tracking_enter);
  EXPORT_SYMBOL_GPL(__context_tracking_enter);
  
  void context_tracking_enter(enum ctx_state state)
  {
  	unsigned long flags;
  
  	/*
  	 * 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;
  
  	local_irq_save(flags);
  	__context_tracking_enter(state);

  	local_irq_restore(flags);
  }

  NOKPROBE_SYMBOL(context_tracking_enter);

  EXPORT_SYMBOL_GPL(context_tracking_enter);

  
  void context_tracking_user_enter(void)
  {

  	user_enter();

  }

  NOKPROBE_SYMBOL(context_tracking_user_enter);

  /**

   * context_tracking_exit - Inform the context tracking that the CPU is
   *                         exiting user or guest mode and entering the kernel.

   *

   * This function must be called after we entered the kernel from user or
   * guest space 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_exit(enum ctx_state state)

  {

  	if (!context_tracking_recursion_enter())

  		return;

  	if (__this_cpu_read(context_tracking.state) == state) {

  		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();

  			if (state == CONTEXT_USER) {
  				vtime_user_exit(current);
  				trace_user_exit(0);
  			}

  		}

  		__this_cpu_write(context_tracking.state, CONTEXT_KERNEL);

  	}

  	context_tracking_recursion_exit();

  }
  NOKPROBE_SYMBOL(__context_tracking_exit);
  EXPORT_SYMBOL_GPL(__context_tracking_exit);
  
  void context_tracking_exit(enum ctx_state state)
  {
  	unsigned long flags;
  
  	if (in_interrupt())
  		return;
  
  	local_irq_save(flags);
  	__context_tracking_exit(state);

  	local_irq_restore(flags);
  }

  NOKPROBE_SYMBOL(context_tracking_exit);

  EXPORT_SYMBOL_GPL(context_tracking_exit);

  
  void context_tracking_user_exit(void)
  {

  	user_exit();

  }

  NOKPROBE_SYMBOL(context_tracking_user_exit);

  void __init context_tracking_cpu_set(int cpu)

  {

  	static __initdata bool initialized = false;
  
  	if (!per_cpu(context_tracking.active, cpu)) {
  		per_cpu(context_tracking.active, cpu) = true;

  		static_branch_inc(&context_tracking_enabled);

  	}
  
  	if (initialized)
  		return;
  
  	/*
  	 * Set TIF_NOHZ to init/0 and let it propagate to all tasks through fork
  	 * This assumes that init is the only task at this early boot stage.
  	 */
  	set_tsk_thread_flag(&init_task, TIF_NOHZ);
  	WARN_ON_ONCE(!tasklist_empty());
  
  	initialized = true;

  }

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