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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 | #include <linux/kprobes.h> | |
23 | ||
24 | #define CREATE_TRACE_POINTS | |
25 | #include <trace/events/context_tracking.h> | |
26 | ||
27 | struct static_key context_tracking_enabled = STATIC_KEY_INIT_FALSE; | |
28 | EXPORT_SYMBOL_GPL(context_tracking_enabled); | |
29 | ||
30 | DEFINE_PER_CPU(struct context_tracking, context_tracking); | |
31 | EXPORT_SYMBOL_GPL(context_tracking); | |
32 | ||
33 | void context_tracking_cpu_set(int cpu) | |
34 | { | |
35 | if (!per_cpu(context_tracking.active, cpu)) { | |
36 | per_cpu(context_tracking.active, cpu) = true; | |
37 | static_key_slow_inc(&context_tracking_enabled); | |
38 | } | |
39 | } | |
40 | ||
41 | /** | |
42 | * context_tracking_user_enter - Inform the context tracking that the CPU is going to | |
43 | * enter userspace mode. | |
44 | * | |
45 | * This function must be called right before we switch from the kernel | |
46 | * to userspace, when it's guaranteed the remaining kernel instructions | |
47 | * to execute won't use any RCU read side critical section because this | |
48 | * function sets RCU in extended quiescent state. | |
49 | */ | |
50 | void context_tracking_user_enter(void) | |
51 | { | |
52 | unsigned long flags; | |
53 | ||
54 | /* | |
55 | * Repeat the user_enter() check here because some archs may be calling | |
56 | * this from asm and if no CPU needs context tracking, they shouldn't | |
57 | * go further. Repeat the check here until they support the inline static | |
58 | * key check. | |
59 | */ | |
60 | if (!context_tracking_is_enabled()) | |
61 | return; | |
62 | ||
63 | /* | |
64 | * Some contexts may involve an exception occuring in an irq, | |
65 | * leading to that nesting: | |
66 | * rcu_irq_enter() rcu_user_exit() rcu_user_exit() rcu_irq_exit() | |
67 | * This would mess up the dyntick_nesting count though. And rcu_irq_*() | |
68 | * helpers are enough to protect RCU uses inside the exception. So | |
69 | * just return immediately if we detect we are in an IRQ. | |
70 | */ | |
71 | if (in_interrupt()) | |
72 | return; | |
73 | ||
74 | /* Kernel threads aren't supposed to go to userspace */ | |
75 | WARN_ON_ONCE(!current->mm); | |
76 | ||
77 | local_irq_save(flags); | |
78 | if ( __this_cpu_read(context_tracking.state) != IN_USER) { | |
79 | if (__this_cpu_read(context_tracking.active)) { | |
80 | trace_user_enter(0); | |
81 | /* | |
82 | * At this stage, only low level arch entry code remains and | |
83 | * then we'll run in userspace. We can assume there won't be | |
84 | * any RCU read-side critical section until the next call to | |
85 | * user_exit() or rcu_irq_enter(). Let's remove RCU's dependency | |
86 | * on the tick. | |
87 | */ | |
88 | vtime_user_enter(current); | |
89 | rcu_user_enter(); | |
90 | } | |
91 | /* | |
92 | * Even if context tracking is disabled on this CPU, because it's outside | |
93 | * the full dynticks mask for example, we still have to keep track of the | |
94 | * context transitions and states to prevent inconsistency on those of | |
95 | * other CPUs. | |
96 | * If a task triggers an exception in userspace, sleep on the exception | |
97 | * handler and then migrate to another CPU, that new CPU must know where | |
98 | * the exception returns by the time we call exception_exit(). | |
99 | * This information can only be provided by the previous CPU when it called | |
100 | * exception_enter(). | |
101 | * OTOH we can spare the calls to vtime and RCU when context_tracking.active | |
102 | * is false because we know that CPU is not tickless. | |
103 | */ | |
104 | __this_cpu_write(context_tracking.state, IN_USER); | |
105 | } | |
106 | local_irq_restore(flags); | |
107 | } | |
108 | NOKPROBE_SYMBOL(context_tracking_user_enter); | |
109 | ||
110 | /** | |
111 | * context_tracking_user_exit - Inform the context tracking that the CPU is | |
112 | * exiting userspace mode and entering the kernel. | |
113 | * | |
114 | * This function must be called after we entered the kernel from userspace | |
115 | * before any use of RCU read side critical section. This potentially include | |
116 | * any high level kernel code like syscalls, exceptions, signal handling, etc... | |
117 | * | |
118 | * This call supports re-entrancy. This way it can be called from any exception | |
119 | * handler without needing to know if we came from userspace or not. | |
120 | */ | |
121 | void context_tracking_user_exit(void) | |
122 | { | |
123 | unsigned long flags; | |
124 | ||
125 | if (!context_tracking_is_enabled()) | |
126 | return; | |
127 | ||
128 | if (in_interrupt()) | |
129 | return; | |
130 | ||
131 | local_irq_save(flags); | |
132 | if (__this_cpu_read(context_tracking.state) == IN_USER) { | |
133 | if (__this_cpu_read(context_tracking.active)) { | |
134 | /* | |
135 | * We are going to run code that may use RCU. Inform | |
136 | * RCU core about that (ie: we may need the tick again). | |
137 | */ | |
138 | rcu_user_exit(); | |
139 | vtime_user_exit(current); | |
140 | trace_user_exit(0); | |
141 | } | |
142 | __this_cpu_write(context_tracking.state, IN_KERNEL); | |
143 | } | |
144 | local_irq_restore(flags); | |
145 | } | |
146 | NOKPROBE_SYMBOL(context_tracking_user_exit); | |
147 | ||
148 | /** | |
149 | * __context_tracking_task_switch - context switch the syscall callbacks | |
150 | * @prev: the task that is being switched out | |
151 | * @next: the task that is being switched in | |
152 | * | |
153 | * The context tracking uses the syscall slow path to implement its user-kernel | |
154 | * boundaries probes on syscalls. This way it doesn't impact the syscall fast | |
155 | * path on CPUs that don't do context tracking. | |
156 | * | |
157 | * But we need to clear the flag on the previous task because it may later | |
158 | * migrate to some CPU that doesn't do the context tracking. As such the TIF | |
159 | * flag may not be desired there. | |
160 | */ | |
161 | void __context_tracking_task_switch(struct task_struct *prev, | |
162 | struct task_struct *next) | |
163 | { | |
164 | clear_tsk_thread_flag(prev, TIF_NOHZ); | |
165 | set_tsk_thread_flag(next, TIF_NOHZ); | |
166 | } | |
167 | ||
168 | #ifdef CONFIG_CONTEXT_TRACKING_FORCE | |
169 | void __init context_tracking_init(void) | |
170 | { | |
171 | int cpu; | |
172 | ||
173 | for_each_possible_cpu(cpu) | |
174 | context_tracking_cpu_set(cpu); | |
175 | } | |
176 | #endif |