kernel/context_tracking.c

/*
 * 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/percpu.h>
#include <linux/hardirq.h>

struct context_tracking {
	/*
	 * When active is false, probes are unset in order
	 * to minimize overhead: TIF flags are cleared
	 * and calls to user_enter/exit are ignored. This
	 * may be further optimized using static keys.
	 */
	bool active;
	enum {
		IN_KERNEL = 0,
		IN_USER,
	} state;
};

static DEFINE_PER_CPU(struct context_tracking, context_tracking) = {
#ifdef CONFIG_CONTEXT_TRACKING_FORCE
	.active = true,
#endif
};

/**
 * 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 user_enter(void)
{
	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;

	/* Kernel threads aren't supposed to go to userspace */
	WARN_ON_ONCE(!current->mm);

	local_irq_save(flags);
	if (__this_cpu_read(context_tracking.active) &&
	    __this_cpu_read(context_tracking.state) != IN_USER) {
		__this_cpu_write(context_tracking.state, IN_USER);
		/*
		 * 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.
		 */
		rcu_user_enter();
	}
	local_irq_restore(flags);
}


/**
 * 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 user_exit(void)
{
	unsigned long flags;

	if (in_interrupt())
		return;

	local_irq_save(flags);
	if (__this_cpu_read(context_tracking.state) == IN_USER) {
		__this_cpu_write(context_tracking.state, IN_KERNEL);
		/*
		 * 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();
	}
	local_irq_restore(flags);
}


/**
 * 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)
{
	if (__this_cpu_read(context_tracking.active)) {
		clear_tsk_thread_flag(prev, TIF_NOHZ);
		set_tsk_thread_flag(next, TIF_NOHZ);
	}
}