// SPDX-License-Identifier: GPL-2.0-only/* * 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_key);EXPORT_SYMBOL_GPL(context_tracking_key); 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\n", 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_key);	} 	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_FORCEvoid __init context_tracking_init(void){	int cpu; 	for_each_possible_cpu(cpu)		context_tracking_cpu_set(cpu);}#endif