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4eacdf18 FW |
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 | ||
91d1aa43 FW |
17 | #include <linux/context_tracking.h> |
18 | #include <linux/rcupdate.h> | |
19 | #include <linux/sched.h> | |
91d1aa43 | 20 | #include <linux/hardirq.h> |
6a61671b | 21 | #include <linux/export.h> |
91d1aa43 | 22 | |
95a79fd4 | 23 | DEFINE_PER_CPU(struct context_tracking, context_tracking) = { |
91d1aa43 FW |
24 | #ifdef CONFIG_CONTEXT_TRACKING_FORCE |
25 | .active = true, | |
26 | #endif | |
27 | }; | |
28 | ||
4eacdf18 FW |
29 | /** |
30 | * user_enter - Inform the context tracking that the CPU is going to | |
31 | * enter userspace mode. | |
32 | * | |
33 | * This function must be called right before we switch from the kernel | |
34 | * to userspace, when it's guaranteed the remaining kernel instructions | |
35 | * to execute won't use any RCU read side critical section because this | |
36 | * function sets RCU in extended quiescent state. | |
37 | */ | |
91d1aa43 FW |
38 | void user_enter(void) |
39 | { | |
40 | unsigned long flags; | |
41 | ||
42 | /* | |
43 | * Some contexts may involve an exception occuring in an irq, | |
44 | * leading to that nesting: | |
45 | * rcu_irq_enter() rcu_user_exit() rcu_user_exit() rcu_irq_exit() | |
46 | * This would mess up the dyntick_nesting count though. And rcu_irq_*() | |
47 | * helpers are enough to protect RCU uses inside the exception. So | |
48 | * just return immediately if we detect we are in an IRQ. | |
49 | */ | |
50 | if (in_interrupt()) | |
51 | return; | |
52 | ||
4eacdf18 | 53 | /* Kernel threads aren't supposed to go to userspace */ |
91d1aa43 FW |
54 | WARN_ON_ONCE(!current->mm); |
55 | ||
56 | local_irq_save(flags); | |
57 | if (__this_cpu_read(context_tracking.active) && | |
58 | __this_cpu_read(context_tracking.state) != IN_USER) { | |
4eacdf18 FW |
59 | /* |
60 | * At this stage, only low level arch entry code remains and | |
61 | * then we'll run in userspace. We can assume there won't be | |
62 | * any RCU read-side critical section until the next call to | |
63 | * user_exit() or rcu_irq_enter(). Let's remove RCU's dependency | |
64 | * on the tick. | |
65 | */ | |
abf917cd | 66 | vtime_user_enter(current); |
91d1aa43 | 67 | rcu_user_enter(); |
abf917cd | 68 | __this_cpu_write(context_tracking.state, IN_USER); |
91d1aa43 FW |
69 | } |
70 | local_irq_restore(flags); | |
71 | } | |
72 | ||
29bb9e5a SR |
73 | #ifdef CONFIG_PREEMPT |
74 | /** | |
75 | * preempt_schedule_context - preempt_schedule called by tracing | |
76 | * | |
77 | * The tracing infrastructure uses preempt_enable_notrace to prevent | |
78 | * recursion and tracing preempt enabling caused by the tracing | |
79 | * infrastructure itself. But as tracing can happen in areas coming | |
80 | * from userspace or just about to enter userspace, a preempt enable | |
81 | * can occur before user_exit() is called. This will cause the scheduler | |
82 | * to be called when the system is still in usermode. | |
83 | * | |
84 | * To prevent this, the preempt_enable_notrace will use this function | |
85 | * instead of preempt_schedule() to exit user context if needed before | |
86 | * calling the scheduler. | |
87 | */ | |
88 | void __sched notrace preempt_schedule_context(void) | |
89 | { | |
90 | struct thread_info *ti = current_thread_info(); | |
91 | enum ctx_state prev_ctx; | |
92 | ||
93 | if (likely(ti->preempt_count || irqs_disabled())) | |
94 | return; | |
95 | ||
96 | /* | |
97 | * Need to disable preemption in case user_exit() is traced | |
98 | * and the tracer calls preempt_enable_notrace() causing | |
99 | * an infinite recursion. | |
100 | */ | |
101 | preempt_disable_notrace(); | |
102 | prev_ctx = exception_enter(); | |
103 | preempt_enable_no_resched_notrace(); | |
104 | ||
105 | preempt_schedule(); | |
106 | ||
107 | preempt_disable_notrace(); | |
108 | exception_exit(prev_ctx); | |
109 | preempt_enable_notrace(); | |
110 | } | |
111 | EXPORT_SYMBOL_GPL(preempt_schedule_context); | |
112 | #endif /* CONFIG_PREEMPT */ | |
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113 | |
114 | /** | |
115 | * user_exit - Inform the context tracking that the CPU is | |
116 | * exiting userspace mode and entering the kernel. | |
117 | * | |
118 | * This function must be called after we entered the kernel from userspace | |
119 | * before any use of RCU read side critical section. This potentially include | |
120 | * any high level kernel code like syscalls, exceptions, signal handling, etc... | |
121 | * | |
122 | * This call supports re-entrancy. This way it can be called from any exception | |
123 | * handler without needing to know if we came from userspace or not. | |
124 | */ | |
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125 | void user_exit(void) |
126 | { | |
127 | unsigned long flags; | |
128 | ||
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129 | if (in_interrupt()) |
130 | return; | |
131 | ||
132 | local_irq_save(flags); | |
133 | if (__this_cpu_read(context_tracking.state) == IN_USER) { | |
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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 | */ | |
91d1aa43 | 138 | rcu_user_exit(); |
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139 | vtime_user_exit(current); |
140 | __this_cpu_write(context_tracking.state, IN_KERNEL); | |
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141 | } |
142 | local_irq_restore(flags); | |
143 | } | |
144 | ||
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145 | void guest_enter(void) |
146 | { | |
147 | if (vtime_accounting_enabled()) | |
148 | vtime_guest_enter(current); | |
149 | else | |
150 | __guest_enter(); | |
151 | } | |
152 | EXPORT_SYMBOL_GPL(guest_enter); | |
153 | ||
154 | void guest_exit(void) | |
155 | { | |
156 | if (vtime_accounting_enabled()) | |
157 | vtime_guest_exit(current); | |
158 | else | |
159 | __guest_exit(); | |
160 | } | |
161 | EXPORT_SYMBOL_GPL(guest_exit); | |
162 | ||
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163 | |
164 | /** | |
165 | * context_tracking_task_switch - context switch the syscall callbacks | |
166 | * @prev: the task that is being switched out | |
167 | * @next: the task that is being switched in | |
168 | * | |
169 | * The context tracking uses the syscall slow path to implement its user-kernel | |
170 | * boundaries probes on syscalls. This way it doesn't impact the syscall fast | |
171 | * path on CPUs that don't do context tracking. | |
172 | * | |
173 | * But we need to clear the flag on the previous task because it may later | |
174 | * migrate to some CPU that doesn't do the context tracking. As such the TIF | |
175 | * flag may not be desired there. | |
176 | */ | |
91d1aa43 FW |
177 | void context_tracking_task_switch(struct task_struct *prev, |
178 | struct task_struct *next) | |
179 | { | |
180 | if (__this_cpu_read(context_tracking.active)) { | |
181 | clear_tsk_thread_flag(prev, TIF_NOHZ); | |
182 | set_tsk_thread_flag(next, TIF_NOHZ); | |
183 | } | |
184 | } |