xref: /openbmc/linux/kernel/context_tracking.c (revision b830f94f)
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_enabled);
29 EXPORT_SYMBOL_GPL(context_tracking_enabled);
30 
31 DEFINE_PER_CPU(struct context_tracking, context_tracking);
32 EXPORT_SYMBOL_GPL(context_tracking);
33 
34 static 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 
48 static 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  */
62 void __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 }
102 NOKPROBE_SYMBOL(__context_tracking_enter);
103 EXPORT_SYMBOL_GPL(__context_tracking_enter);
104 
105 void 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 }
124 NOKPROBE_SYMBOL(context_tracking_enter);
125 EXPORT_SYMBOL_GPL(context_tracking_enter);
126 
127 void context_tracking_user_enter(void)
128 {
129 	user_enter();
130 }
131 NOKPROBE_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  */
145 void __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 }
166 NOKPROBE_SYMBOL(__context_tracking_exit);
167 EXPORT_SYMBOL_GPL(__context_tracking_exit);
168 
169 void 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 }
180 NOKPROBE_SYMBOL(context_tracking_exit);
181 EXPORT_SYMBOL_GPL(context_tracking_exit);
182 
183 void context_tracking_user_exit(void)
184 {
185 	user_exit();
186 }
187 NOKPROBE_SYMBOL(context_tracking_user_exit);
188 
189 void __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
212 void __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
220