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
28 DEFINE_STATIC_KEY_FALSE(context_tracking_key);
29 EXPORT_SYMBOL_GPL(context_tracking_key);
30
31 DEFINE_PER_CPU(struct context_tracking, context_tracking);
32 EXPORT_SYMBOL_GPL(context_tracking);
33
context_tracking_recursion_enter(void)34 static 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
context_tracking_recursion_exit(void)48 static __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 */
__context_tracking_enter(enum ctx_state state)62 void 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 }
104 EXPORT_SYMBOL_GPL(__context_tracking_enter);
105
context_tracking_enter(enum ctx_state state)106 void 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 }
125 NOKPROBE_SYMBOL(context_tracking_enter);
126 EXPORT_SYMBOL_GPL(context_tracking_enter);
127
context_tracking_user_enter(void)128 void context_tracking_user_enter(void)
129 {
130 user_enter();
131 }
132 NOKPROBE_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 */
__context_tracking_exit(enum ctx_state state)146 void 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 }
169 EXPORT_SYMBOL_GPL(__context_tracking_exit);
170
context_tracking_exit(enum ctx_state state)171 void 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 }
182 NOKPROBE_SYMBOL(context_tracking_exit);
183 EXPORT_SYMBOL_GPL(context_tracking_exit);
184
context_tracking_user_exit(void)185 void context_tracking_user_exit(void)
186 {
187 user_exit();
188 }
189 NOKPROBE_SYMBOL(context_tracking_user_exit);
190
context_tracking_cpu_set(int cpu)191 void __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
context_tracking_init(void)216 void __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
224