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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
23#define CREATE_TRACE_POINTS
24#include <trace/events/context_tracking.h>
25
26struct static_key context_tracking_enabled = STATIC_KEY_INIT_FALSE;
27EXPORT_SYMBOL_GPL(context_tracking_enabled);
28
29DEFINE_PER_CPU(struct context_tracking, context_tracking);
30EXPORT_SYMBOL_GPL(context_tracking);
31
32void context_tracking_cpu_set(int cpu)
33{
34 if (!per_cpu(context_tracking.active, cpu)) {
35 per_cpu(context_tracking.active, cpu) = true;
36 static_key_slow_inc(&context_tracking_enabled);
37 }
38}
39
40/**
41 * context_tracking_user_enter - Inform the context tracking that the CPU is going to
42 * enter userspace mode.
43 *
44 * This function must be called right before we switch from the kernel
45 * to userspace, when it's guaranteed the remaining kernel instructions
46 * to execute won't use any RCU read side critical section because this
47 * function sets RCU in extended quiescent state.
48 */
49void context_tracking_user_enter(void)
50{
51 unsigned long flags;
52
53 /*
54 * Repeat the user_enter() check here because some archs may be calling
55 * this from asm and if no CPU needs context tracking, they shouldn't
56 * go further. Repeat the check here until they support the inline static
57 * key check.
58 */
59 if (!context_tracking_is_enabled())
60 return;
61
62 /*
63 * Some contexts may involve an exception occuring in an irq,
64 * leading to that nesting:
65 * rcu_irq_enter() rcu_user_exit() rcu_user_exit() rcu_irq_exit()
66 * This would mess up the dyntick_nesting count though. And rcu_irq_*()
67 * helpers are enough to protect RCU uses inside the exception. So
68 * just return immediately if we detect we are in an IRQ.
69 */
70 if (in_interrupt())
71 return;
72
73 /* Kernel threads aren't supposed to go to userspace */
74 WARN_ON_ONCE(!current->mm);
75
76 local_irq_save(flags);
77 if ( __this_cpu_read(context_tracking.state) != IN_USER) {
78 if (__this_cpu_read(context_tracking.active)) {
79 trace_user_enter(0);
80 /*
81 * At this stage, only low level arch entry code remains and
82 * then we'll run in userspace. We can assume there won't be
83 * any RCU read-side critical section until the next call to
84 * user_exit() or rcu_irq_enter(). Let's remove RCU's dependency
85 * on the tick.
86 */
87 vtime_user_enter(current);
88 rcu_user_enter();
89 }
90 /*
91 * Even if context tracking is disabled on this CPU, because it's outside
92 * the full dynticks mask for example, we still have to keep track of the
93 * context transitions and states to prevent inconsistency on those of
94 * other CPUs.
95 * If a task triggers an exception in userspace, sleep on the exception
96 * handler and then migrate to another CPU, that new CPU must know where
97 * the exception returns by the time we call exception_exit().
98 * This information can only be provided by the previous CPU when it called
99 * exception_enter().
100 * OTOH we can spare the calls to vtime and RCU when context_tracking.active
101 * is false because we know that CPU is not tickless.
102 */
103 __this_cpu_write(context_tracking.state, IN_USER);
104 }
105 local_irq_restore(flags);
106}
107
108#ifdef CONFIG_PREEMPT
109/**
110 * preempt_schedule_context - preempt_schedule called by tracing
111 *
112 * The tracing infrastructure uses preempt_enable_notrace to prevent
113 * recursion and tracing preempt enabling caused by the tracing
114 * infrastructure itself. But as tracing can happen in areas coming
115 * from userspace or just about to enter userspace, a preempt enable
116 * can occur before user_exit() is called. This will cause the scheduler
117 * to be called when the system is still in usermode.
118 *
119 * To prevent this, the preempt_enable_notrace will use this function
120 * instead of preempt_schedule() to exit user context if needed before
121 * calling the scheduler.
122 */
123asmlinkage __visible void __sched notrace preempt_schedule_context(void)
124{
125 enum ctx_state prev_ctx;
126
127 if (likely(!preemptible()))
128 return;
129
130 /*
131 * Need to disable preemption in case user_exit() is traced
132 * and the tracer calls preempt_enable_notrace() causing
133 * an infinite recursion.
134 */
135 preempt_disable_notrace();
136 prev_ctx = exception_enter();
137 preempt_enable_no_resched_notrace();
138
139 preempt_schedule();
140
141 preempt_disable_notrace();
142 exception_exit(prev_ctx);
143 preempt_enable_notrace();
144}
145EXPORT_SYMBOL_GPL(preempt_schedule_context);
146#endif /* CONFIG_PREEMPT */
147
148/**
149 * context_tracking_user_exit - Inform the context tracking that the CPU is
150 * exiting userspace mode and entering the kernel.
151 *
152 * This function must be called after we entered the kernel from userspace
153 * before any use of RCU read side critical section. This potentially include
154 * any high level kernel code like syscalls, exceptions, signal handling, etc...
155 *
156 * This call supports re-entrancy. This way it can be called from any exception
157 * handler without needing to know if we came from userspace or not.
158 */
159void context_tracking_user_exit(void)
160{
161 unsigned long flags;
162
163 if (!context_tracking_is_enabled())
164 return;
165
166 if (in_interrupt())
167 return;
168
169 local_irq_save(flags);
170 if (__this_cpu_read(context_tracking.state) == IN_USER) {
171 if (__this_cpu_read(context_tracking.active)) {
172 /*
173 * We are going to run code that may use RCU. Inform
174 * RCU core about that (ie: we may need the tick again).
175 */
176 rcu_user_exit();
177 vtime_user_exit(current);
178 trace_user_exit(0);
179 }
180 __this_cpu_write(context_tracking.state, IN_KERNEL);
181 }
182 local_irq_restore(flags);
183}
184
185/**
186 * __context_tracking_task_switch - context switch the syscall callbacks
187 * @prev: the task that is being switched out
188 * @next: the task that is being switched in
189 *
190 * The context tracking uses the syscall slow path to implement its user-kernel
191 * boundaries probes on syscalls. This way it doesn't impact the syscall fast
192 * path on CPUs that don't do context tracking.
193 *
194 * But we need to clear the flag on the previous task because it may later
195 * migrate to some CPU that doesn't do the context tracking. As such the TIF
196 * flag may not be desired there.
197 */
198void __context_tracking_task_switch(struct task_struct *prev,
199 struct task_struct *next)
200{
201 clear_tsk_thread_flag(prev, TIF_NOHZ);
202 set_tsk_thread_flag(next, TIF_NOHZ);
203}
204
205#ifdef CONFIG_CONTEXT_TRACKING_FORCE
206void __init context_tracking_init(void)
207{
208 int cpu;
209
210 for_each_possible_cpu(cpu)
211 context_tracking_cpu_set(cpu);
212}
213#endif
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_enabled);
29EXPORT_SYMBOL_GPL(context_tracking_enabled);
30
31DEFINE_PER_CPU(struct context_tracking, context_tracking);
32EXPORT_SYMBOL_GPL(context_tracking);
33
34static 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 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 __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 trace_user_enter(0);
81 vtime_user_enter(current);
82 }
83 rcu_user_enter();
84 }
85 /*
86 * Even if context tracking is disabled on this CPU, because it's outside
87 * the full dynticks mask for example, we still have to keep track of the
88 * context transitions and states to prevent inconsistency on those of
89 * other CPUs.
90 * If a task triggers an exception in userspace, sleep on the exception
91 * handler and then migrate to another CPU, that new CPU must know where
92 * the exception returns by the time we call exception_exit().
93 * This information can only be provided by the previous CPU when it called
94 * exception_enter().
95 * OTOH we can spare the calls to vtime and RCU when context_tracking.active
96 * is false because we know that CPU is not tickless.
97 */
98 __this_cpu_write(context_tracking.state, state);
99 }
100 context_tracking_recursion_exit();
101}
102NOKPROBE_SYMBOL(__context_tracking_enter);
103EXPORT_SYMBOL_GPL(__context_tracking_enter);
104
105void context_tracking_enter(enum ctx_state state)
106{
107 unsigned long flags;
108
109 /*
110 * Some contexts may involve an exception occuring in an irq,
111 * leading to that nesting:
112 * rcu_irq_enter() rcu_user_exit() rcu_user_exit() rcu_irq_exit()
113 * This would mess up the dyntick_nesting count though. And rcu_irq_*()
114 * helpers are enough to protect RCU uses inside the exception. So
115 * just return immediately if we detect we are in an IRQ.
116 */
117 if (in_interrupt())
118 return;
119
120 local_irq_save(flags);
121 __context_tracking_enter(state);
122 local_irq_restore(flags);
123}
124NOKPROBE_SYMBOL(context_tracking_enter);
125EXPORT_SYMBOL_GPL(context_tracking_enter);
126
127void context_tracking_user_enter(void)
128{
129 user_enter();
130}
131NOKPROBE_SYMBOL(context_tracking_user_enter);
132
133/**
134 * context_tracking_exit - Inform the context tracking that the CPU is
135 * exiting user or guest mode and entering the kernel.
136 *
137 * This function must be called after we entered the kernel from user or
138 * guest space before any use of RCU read side critical section. This
139 * potentially include any high level kernel code like syscalls, exceptions,
140 * signal handling, etc...
141 *
142 * This call supports re-entrancy. This way it can be called from any exception
143 * handler without needing to know if we came from userspace or not.
144 */
145void __context_tracking_exit(enum ctx_state state)
146{
147 if (!context_tracking_recursion_enter())
148 return;
149
150 if (__this_cpu_read(context_tracking.state) == state) {
151 if (__this_cpu_read(context_tracking.active)) {
152 /*
153 * We are going to run code that may use RCU. Inform
154 * RCU core about that (ie: we may need the tick again).
155 */
156 rcu_user_exit();
157 if (state == CONTEXT_USER) {
158 vtime_user_exit(current);
159 trace_user_exit(0);
160 }
161 }
162 __this_cpu_write(context_tracking.state, CONTEXT_KERNEL);
163 }
164 context_tracking_recursion_exit();
165}
166NOKPROBE_SYMBOL(__context_tracking_exit);
167EXPORT_SYMBOL_GPL(__context_tracking_exit);
168
169void context_tracking_exit(enum ctx_state state)
170{
171 unsigned long flags;
172
173 if (in_interrupt())
174 return;
175
176 local_irq_save(flags);
177 __context_tracking_exit(state);
178 local_irq_restore(flags);
179}
180NOKPROBE_SYMBOL(context_tracking_exit);
181EXPORT_SYMBOL_GPL(context_tracking_exit);
182
183void context_tracking_user_exit(void)
184{
185 user_exit();
186}
187NOKPROBE_SYMBOL(context_tracking_user_exit);
188
189void __init context_tracking_cpu_set(int cpu)
190{
191 static __initdata bool initialized = false;
192
193 if (!per_cpu(context_tracking.active, cpu)) {
194 per_cpu(context_tracking.active, cpu) = true;
195 static_branch_inc(&context_tracking_enabled);
196 }
197
198 if (initialized)
199 return;
200
201 /*
202 * Set TIF_NOHZ to init/0 and let it propagate to all tasks through fork
203 * This assumes that init is the only task at this early boot stage.
204 */
205 set_tsk_thread_flag(&init_task, TIF_NOHZ);
206 WARN_ON_ONCE(!tasklist_empty());
207
208 initialized = true;
209}
210
211#ifdef CONFIG_CONTEXT_TRACKING_FORCE
212void __init context_tracking_init(void)
213{
214 int cpu;
215
216 for_each_possible_cpu(cpu)
217 context_tracking_cpu_set(cpu);
218}
219#endif