Merge tag 'drm-next-2019-12-06' of git://anongit.freedesktop.org/drm/drm
[platform/kernel/linux-rpi.git] / kernel / context_tracking.c
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_key);
29 EXPORT_SYMBOL_GPL(context_tracking_key);
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_key);
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