1// SPDX-License-Identifier: GPL-2.0-only
  2/*
  3 * Context tracking: Probe on high level context boundaries such as kernel
  4 * and userspace. This includes syscalls and exceptions entry/exit.
  5 *
  6 * This is used by RCU to remove its dependency on the timer tick while a CPU
  7 * runs in userspace.
  8 *
  9 *  Started by Frederic Weisbecker:
 10 *
 11 * Copyright (C) 2012 Red Hat, Inc., Frederic Weisbecker <fweisbec@redhat.com>
 12 *
 13 * Many thanks to Gilad Ben-Yossef, Paul McKenney, Ingo Molnar, Andrew Morton,
 14 * Steven Rostedt, Peter Zijlstra for suggestions and improvements.
 15 *
 16 */
 17
 18#include <linux/context_tracking.h>
 19#include <linux/rcupdate.h>
 20#include <linux/sched.h>
 21#include <linux/hardirq.h>
 22#include <linux/export.h>
 23#include <linux/kprobes.h>
 24
 25#define CREATE_TRACE_POINTS
 26#include <trace/events/context_tracking.h>
 27
 28DEFINE_STATIC_KEY_FALSE(context_tracking_key);
 29EXPORT_SYMBOL_GPL(context_tracking_key);
 30
 31DEFINE_PER_CPU(struct context_tracking, context_tracking);
 32EXPORT_SYMBOL_GPL(context_tracking);
 33
 34static noinstr bool context_tracking_recursion_enter(void)
 35{
 36	int recursion;
 37
 38	recursion = __this_cpu_inc_return(context_tracking.recursion);
 39	if (recursion == 1)
 40		return true;
 41
 42	WARN_ONCE((recursion < 1), "Invalid context tracking recursion value %d\n", recursion);
 43	__this_cpu_dec(context_tracking.recursion);
 44
 45	return false;
 46}
 47
 48static __always_inline void context_tracking_recursion_exit(void)
 49{
 50	__this_cpu_dec(context_tracking.recursion);
 51}
 52
 53/**
 54 * context_tracking_enter - Inform the context tracking that the CPU is going
 55 *                          enter user or guest space mode.
 56 *
 57 * This function must be called right before we switch from the kernel
 58 * to user or guest space, when it's guaranteed the remaining kernel
 59 * instructions to execute won't use any RCU read side critical section
 60 * because this function sets RCU in extended quiescent state.
 61 */
 62void noinstr __context_tracking_enter(enum ctx_state state)
 63{
 64	/* Kernel threads aren't supposed to go to userspace */
 65	WARN_ON_ONCE(!current->mm);
 66
 67	if (!context_tracking_recursion_enter())
 68		return;
 69
 70	if ( __this_cpu_read(context_tracking.state) != state) {
 71		if (__this_cpu_read(context_tracking.active)) {
 72			/*
 73			 * At this stage, only low level arch entry code remains and
 74			 * then we'll run in userspace. We can assume there won't be
 75			 * any RCU read-side critical section until the next call to
 76			 * user_exit() or rcu_irq_enter(). Let's remove RCU's dependency
 77			 * on the tick.
 78			 */
 79			if (state == CONTEXT_USER) {
 80				instrumentation_begin();
 81				trace_user_enter(0);
 82				vtime_user_enter(current);
 83				instrumentation_end();
 84			}
 85			rcu_user_enter();
 86		}
 87		/*
 88		 * Even if context tracking is disabled on this CPU, because it's outside
 89		 * the full dynticks mask for example, we still have to keep track of the
 90		 * context transitions and states to prevent inconsistency on those of
 91		 * other CPUs.
 92		 * If a task triggers an exception in userspace, sleep on the exception
 93		 * handler and then migrate to another CPU, that new CPU must know where
 94		 * the exception returns by the time we call exception_exit().
 95		 * This information can only be provided by the previous CPU when it called
 96		 * exception_enter().
 97		 * OTOH we can spare the calls to vtime and RCU when context_tracking.active
 98		 * is false because we know that CPU is not tickless.
 99		 */
100		__this_cpu_write(context_tracking.state, state);
101	}
102	context_tracking_recursion_exit();
103}
104EXPORT_SYMBOL_GPL(__context_tracking_enter);
105
106void context_tracking_enter(enum ctx_state state)
107{
108	unsigned long flags;
109
110	/*
111	 * Some contexts may involve an exception occuring in an irq,
112	 * leading to that nesting:
113	 * rcu_irq_enter() rcu_user_exit() rcu_user_exit() rcu_irq_exit()
114	 * This would mess up the dyntick_nesting count though. And rcu_irq_*()
115	 * helpers are enough to protect RCU uses inside the exception. So
116	 * just return immediately if we detect we are in an IRQ.
117	 */
118	if (in_interrupt())
119		return;
120
121	local_irq_save(flags);
122	__context_tracking_enter(state);
123	local_irq_restore(flags);
124}
125NOKPROBE_SYMBOL(context_tracking_enter);
126EXPORT_SYMBOL_GPL(context_tracking_enter);
127
128void context_tracking_user_enter(void)
129{
130	user_enter();
131}
132NOKPROBE_SYMBOL(context_tracking_user_enter);
133
134/**
135 * context_tracking_exit - Inform the context tracking that the CPU is
136 *                         exiting user or guest mode and entering the kernel.
137 *
138 * This function must be called after we entered the kernel from user or
139 * guest space before any use of RCU read side critical section. This
140 * potentially include any high level kernel code like syscalls, exceptions,
141 * signal handling, etc...
142 *
143 * This call supports re-entrancy. This way it can be called from any exception
144 * handler without needing to know if we came from userspace or not.
145 */
146void noinstr __context_tracking_exit(enum ctx_state state)
147{
148	if (!context_tracking_recursion_enter())
149		return;
150
151	if (__this_cpu_read(context_tracking.state) == state) {
152		if (__this_cpu_read(context_tracking.active)) {
153			/*
154			 * We are going to run code that may use RCU. Inform
155			 * RCU core about that (ie: we may need the tick again).
156			 */
157			rcu_user_exit();
158			if (state == CONTEXT_USER) {
159				instrumentation_begin();
160				vtime_user_exit(current);
161				trace_user_exit(0);
162				instrumentation_end();
163			}
164		}
165		__this_cpu_write(context_tracking.state, CONTEXT_KERNEL);
166	}
167	context_tracking_recursion_exit();
168}
169EXPORT_SYMBOL_GPL(__context_tracking_exit);
170
171void context_tracking_exit(enum ctx_state state)
172{
173	unsigned long flags;
174
175	if (in_interrupt())
176		return;
177
178	local_irq_save(flags);
179	__context_tracking_exit(state);
180	local_irq_restore(flags);
181}
182NOKPROBE_SYMBOL(context_tracking_exit);
183EXPORT_SYMBOL_GPL(context_tracking_exit);
184
185void context_tracking_user_exit(void)
186{
187	user_exit();
188}
189NOKPROBE_SYMBOL(context_tracking_user_exit);
190
191void __init context_tracking_cpu_set(int cpu)
192{
193	static __initdata bool initialized = false;
194
195	if (!per_cpu(context_tracking.active, cpu)) {
196		per_cpu(context_tracking.active, cpu) = true;
197		static_branch_inc(&context_tracking_key);
198	}
199
200	if (initialized)
201		return;
202
203#ifdef CONFIG_HAVE_TIF_NOHZ
204	/*
205	 * Set TIF_NOHZ to init/0 and let it propagate to all tasks through fork
206	 * This assumes that init is the only task at this early boot stage.
207	 */
208	set_tsk_thread_flag(&init_task, TIF_NOHZ);
209#endif
210	WARN_ON_ONCE(!tasklist_empty());
211
212	initialized = true;
213}
214
215#ifdef CONFIG_CONTEXT_TRACKING_FORCE
216void __init context_tracking_init(void)
217{
218	int cpu;
219
220	for_each_possible_cpu(cpu)
221		context_tracking_cpu_set(cpu);
222}
223#endif