/* * 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> #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);} #ifdef CONFIG_PREEMPT/** * preempt_schedule_context - preempt_schedule called by tracing * * The tracing infrastructure uses preempt_enable_notrace to prevent * recursion and tracing preempt enabling caused by the tracing * infrastructure itself. But as tracing can happen in areas coming * from userspace or just about to enter userspace, a preempt enable * can occur before user_exit() is called. This will cause the scheduler * to be called when the system is still in usermode. * * To prevent this, the preempt_enable_notrace will use this function * instead of preempt_schedule() to exit user context if needed before * calling the scheduler. */asmlinkage void __sched notrace preempt_schedule_context(void){	enum ctx_state prev_ctx; 	if (likely(!preemptible()))		return; 	/*	 * Need to disable preemption in case user_exit() is traced	 * and the tracer calls preempt_enable_notrace() causing	 * an infinite recursion.	 */	preempt_disable_notrace();	prev_ctx = exception_enter();	preempt_enable_no_resched_notrace(); 	preempt_schedule(); 	preempt_disable_notrace();	exception_exit(prev_ctx);	preempt_enable_notrace();}EXPORT_SYMBOL_GPL(preempt_schedule_context);#endif /* CONFIG_PREEMPT */ /** * 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);} /** * __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_FORCEvoid __init context_tracking_init(void){	int cpu; 	for_each_possible_cpu(cpu)		context_tracking_cpu_set(cpu);}#endif