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