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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/kvm_host.h> |
19 | #include <linux/rcupdate.h> |
20 | #include <linux/sched.h> |
21 | #include <linux/hardirq.h> |
22 | #include <linux/export.h> |
23 | |
24 | DEFINE_PER_CPU(struct context_tracking, context_tracking) = { |
25 | #ifdef CONFIG_CONTEXT_TRACKING_FORCE |
26 | .active = true, |
27 | #endif |
28 | }; |
29 | |
30 | /** |
31 | * user_enter - Inform the context tracking that the CPU is going to |
32 | * enter userspace mode. |
33 | * |
34 | * This function must be called right before we switch from the kernel |
35 | * to userspace, when it's guaranteed the remaining kernel instructions |
36 | * to execute won't use any RCU read side critical section because this |
37 | * function sets RCU in extended quiescent state. |
38 | */ |
39 | void user_enter(void) |
40 | { |
41 | unsigned long flags; |
42 | |
43 | /* |
44 | * Some contexts may involve an exception occuring in an irq, |
45 | * leading to that nesting: |
46 | * rcu_irq_enter() rcu_user_exit() rcu_user_exit() rcu_irq_exit() |
47 | * This would mess up the dyntick_nesting count though. And rcu_irq_*() |
48 | * helpers are enough to protect RCU uses inside the exception. So |
49 | * just return immediately if we detect we are in an IRQ. |
50 | */ |
51 | if (in_interrupt()) |
52 | return; |
53 | |
54 | /* Kernel threads aren't supposed to go to userspace */ |
55 | WARN_ON_ONCE(!current->mm); |
56 | |
57 | local_irq_save(flags); |
58 | if (__this_cpu_read(context_tracking.active) && |
59 | __this_cpu_read(context_tracking.state) != IN_USER) { |
60 | /* |
61 | * At this stage, only low level arch entry code remains and |
62 | * then we'll run in userspace. We can assume there won't be |
63 | * any RCU read-side critical section until the next call to |
64 | * user_exit() or rcu_irq_enter(). Let's remove RCU's dependency |
65 | * on the tick. |
66 | */ |
67 | vtime_user_enter(current); |
68 | rcu_user_enter(); |
69 | __this_cpu_write(context_tracking.state, IN_USER); |
70 | } |
71 | local_irq_restore(flags); |
72 | } |
73 | |
74 | |
75 | /** |
76 | * user_exit - Inform the context tracking that the CPU is |
77 | * exiting userspace mode and entering the kernel. |
78 | * |
79 | * This function must be called after we entered the kernel from userspace |
80 | * before any use of RCU read side critical section. This potentially include |
81 | * any high level kernel code like syscalls, exceptions, signal handling, etc... |
82 | * |
83 | * This call supports re-entrancy. This way it can be called from any exception |
84 | * handler without needing to know if we came from userspace or not. |
85 | */ |
86 | void user_exit(void) |
87 | { |
88 | unsigned long flags; |
89 | |
90 | if (in_interrupt()) |
91 | return; |
92 | |
93 | local_irq_save(flags); |
94 | if (__this_cpu_read(context_tracking.state) == IN_USER) { |
95 | /* |
96 | * We are going to run code that may use RCU. Inform |
97 | * RCU core about that (ie: we may need the tick again). |
98 | */ |
99 | rcu_user_exit(); |
100 | vtime_user_exit(current); |
101 | __this_cpu_write(context_tracking.state, IN_KERNEL); |
102 | } |
103 | local_irq_restore(flags); |
104 | } |
105 | |
106 | void guest_enter(void) |
107 | { |
108 | if (vtime_accounting_enabled()) |
109 | vtime_guest_enter(current); |
110 | else |
111 | __guest_enter(); |
112 | } |
113 | EXPORT_SYMBOL_GPL(guest_enter); |
114 | |
115 | void guest_exit(void) |
116 | { |
117 | if (vtime_accounting_enabled()) |
118 | vtime_guest_exit(current); |
119 | else |
120 | __guest_exit(); |
121 | } |
122 | EXPORT_SYMBOL_GPL(guest_exit); |
123 | |
124 | |
125 | /** |
126 | * context_tracking_task_switch - context switch the syscall callbacks |
127 | * @prev: the task that is being switched out |
128 | * @next: the task that is being switched in |
129 | * |
130 | * The context tracking uses the syscall slow path to implement its user-kernel |
131 | * boundaries probes on syscalls. This way it doesn't impact the syscall fast |
132 | * path on CPUs that don't do context tracking. |
133 | * |
134 | * But we need to clear the flag on the previous task because it may later |
135 | * migrate to some CPU that doesn't do the context tracking. As such the TIF |
136 | * flag may not be desired there. |
137 | */ |
138 | void context_tracking_task_switch(struct task_struct *prev, |
139 | struct task_struct *next) |
140 | { |
141 | if (__this_cpu_read(context_tracking.active)) { |
142 | clear_tsk_thread_flag(prev, TIF_NOHZ); |
143 | set_tsk_thread_flag(next, TIF_NOHZ); |
144 | } |
145 | } |
146 |
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