Merge branch 'kconfig' of git://git.kernel.org/pub/scm/linux/kernel/git/mmarek/kbuild
[firefly-linux-kernel-4.4.55.git] / kernel / context_tracking.c
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 static bool context_tracking_recursion_enter(void)
34 {
35         int recursion;
36
37         recursion = __this_cpu_inc_return(context_tracking.recursion);
38         if (recursion == 1)
39                 return true;
40
41         WARN_ONCE((recursion < 1), "Invalid context tracking recursion value %d\n", recursion);
42         __this_cpu_dec(context_tracking.recursion);
43
44         return false;
45 }
46
47 static void context_tracking_recursion_exit(void)
48 {
49         __this_cpu_dec(context_tracking.recursion);
50 }
51
52 /**
53  * context_tracking_enter - Inform the context tracking that the CPU is going
54  *                          enter user or guest space mode.
55  *
56  * This function must be called right before we switch from the kernel
57  * to user or guest space, when it's guaranteed the remaining kernel
58  * instructions to execute won't use any RCU read side critical section
59  * because this function sets RCU in extended quiescent state.
60  */
61 void context_tracking_enter(enum ctx_state state)
62 {
63         unsigned long flags;
64
65         /*
66          * Repeat the user_enter() check here because some archs may be calling
67          * this from asm and if no CPU needs context tracking, they shouldn't
68          * go further. Repeat the check here until they support the inline static
69          * key check.
70          */
71         if (!context_tracking_is_enabled())
72                 return;
73
74         /*
75          * Some contexts may involve an exception occuring in an irq,
76          * leading to that nesting:
77          * rcu_irq_enter() rcu_user_exit() rcu_user_exit() rcu_irq_exit()
78          * This would mess up the dyntick_nesting count though. And rcu_irq_*()
79          * helpers are enough to protect RCU uses inside the exception. So
80          * just return immediately if we detect we are in an IRQ.
81          */
82         if (in_interrupt())
83                 return;
84
85         /* Kernel threads aren't supposed to go to userspace */
86         WARN_ON_ONCE(!current->mm);
87
88         local_irq_save(flags);
89         if (!context_tracking_recursion_enter())
90                 goto out_irq_restore;
91
92         if ( __this_cpu_read(context_tracking.state) != state) {
93                 if (__this_cpu_read(context_tracking.active)) {
94                         /*
95                          * At this stage, only low level arch entry code remains and
96                          * then we'll run in userspace. We can assume there won't be
97                          * any RCU read-side critical section until the next call to
98                          * user_exit() or rcu_irq_enter(). Let's remove RCU's dependency
99                          * on the tick.
100                          */
101                         if (state == CONTEXT_USER) {
102                                 trace_user_enter(0);
103                                 vtime_user_enter(current);
104                         }
105                         rcu_user_enter();
106                 }
107                 /*
108                  * Even if context tracking is disabled on this CPU, because it's outside
109                  * the full dynticks mask for example, we still have to keep track of the
110                  * context transitions and states to prevent inconsistency on those of
111                  * other CPUs.
112                  * If a task triggers an exception in userspace, sleep on the exception
113                  * handler and then migrate to another CPU, that new CPU must know where
114                  * the exception returns by the time we call exception_exit().
115                  * This information can only be provided by the previous CPU when it called
116                  * exception_enter().
117                  * OTOH we can spare the calls to vtime and RCU when context_tracking.active
118                  * is false because we know that CPU is not tickless.
119                  */
120                 __this_cpu_write(context_tracking.state, state);
121         }
122         context_tracking_recursion_exit();
123 out_irq_restore:
124         local_irq_restore(flags);
125 }
126 NOKPROBE_SYMBOL(context_tracking_enter);
127 EXPORT_SYMBOL_GPL(context_tracking_enter);
128
129 void context_tracking_user_enter(void)
130 {
131         context_tracking_enter(CONTEXT_USER);
132 }
133 NOKPROBE_SYMBOL(context_tracking_user_enter);
134
135 /**
136  * context_tracking_exit - Inform the context tracking that the CPU is
137  *                         exiting user or guest mode and entering the kernel.
138  *
139  * This function must be called after we entered the kernel from user or
140  * guest space before any use of RCU read side critical section. This
141  * potentially include any high level kernel code like syscalls, exceptions,
142  * signal handling, etc...
143  *
144  * This call supports re-entrancy. This way it can be called from any exception
145  * handler without needing to know if we came from userspace or not.
146  */
147 void context_tracking_exit(enum ctx_state state)
148 {
149         unsigned long flags;
150
151         if (!context_tracking_is_enabled())
152                 return;
153
154         if (in_interrupt())
155                 return;
156
157         local_irq_save(flags);
158         if (!context_tracking_recursion_enter())
159                 goto out_irq_restore;
160
161         if (__this_cpu_read(context_tracking.state) == state) {
162                 if (__this_cpu_read(context_tracking.active)) {
163                         /*
164                          * We are going to run code that may use RCU. Inform
165                          * RCU core about that (ie: we may need the tick again).
166                          */
167                         rcu_user_exit();
168                         if (state == CONTEXT_USER) {
169                                 vtime_user_exit(current);
170                                 trace_user_exit(0);
171                         }
172                 }
173                 __this_cpu_write(context_tracking.state, CONTEXT_KERNEL);
174         }
175         context_tracking_recursion_exit();
176 out_irq_restore:
177         local_irq_restore(flags);
178 }
179 NOKPROBE_SYMBOL(context_tracking_exit);
180 EXPORT_SYMBOL_GPL(context_tracking_exit);
181
182 void context_tracking_user_exit(void)
183 {
184         context_tracking_exit(CONTEXT_USER);
185 }
186 NOKPROBE_SYMBOL(context_tracking_user_exit);
187
188 void __init context_tracking_cpu_set(int cpu)
189 {
190         static __initdata bool initialized = false;
191
192         if (!per_cpu(context_tracking.active, cpu)) {
193                 per_cpu(context_tracking.active, cpu) = true;
194                 static_key_slow_inc(&context_tracking_enabled);
195         }
196
197         if (initialized)
198                 return;
199
200         /*
201          * Set TIF_NOHZ to init/0 and let it propagate to all tasks through fork
202          * This assumes that init is the only task at this early boot stage.
203          */
204         set_tsk_thread_flag(&init_task, TIF_NOHZ);
205         WARN_ON_ONCE(!tasklist_empty());
206
207         initialized = true;
208 }
209
210 #ifdef CONFIG_CONTEXT_TRACKING_FORCE
211 void __init context_tracking_init(void)
212 {
213         int cpu;
214
215         for_each_possible_cpu(cpu)
216                 context_tracking_cpu_set(cpu);
217 }
218 #endif