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Linux/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 DEFINE_STATIC_KEY_FALSE(context_tracking_enabled);
 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         /* Kernel threads aren't supposed to go to userspace */
 64         WARN_ON_ONCE(!current->mm);
 65 
 66         if (!context_tracking_recursion_enter())
 67                 return;
 68 
 69         if ( __this_cpu_read(context_tracking.state) != state) {
 70                 if (__this_cpu_read(context_tracking.active)) {
 71                         /*
 72                          * At this stage, only low level arch entry code remains and
 73                          * then we'll run in userspace. We can assume there won't be
 74                          * any RCU read-side critical section until the next call to
 75                          * user_exit() or rcu_irq_enter(). Let's remove RCU's dependency
 76                          * on the tick.
 77                          */
 78                         if (state == CONTEXT_USER) {
 79                                 trace_user_enter(0);
 80                                 vtime_user_enter(current);
 81                         }
 82                         rcu_user_enter();
 83                 }
 84                 /*
 85                  * Even if context tracking is disabled on this CPU, because it's outside
 86                  * the full dynticks mask for example, we still have to keep track of the
 87                  * context transitions and states to prevent inconsistency on those of
 88                  * other CPUs.
 89                  * If a task triggers an exception in userspace, sleep on the exception
 90                  * handler and then migrate to another CPU, that new CPU must know where
 91                  * the exception returns by the time we call exception_exit().
 92                  * This information can only be provided by the previous CPU when it called
 93                  * exception_enter().
 94                  * OTOH we can spare the calls to vtime and RCU when context_tracking.active
 95                  * is false because we know that CPU is not tickless.
 96                  */
 97                 __this_cpu_write(context_tracking.state, state);
 98         }
 99         context_tracking_recursion_exit();
100 }
101 NOKPROBE_SYMBOL(__context_tracking_enter);
102 EXPORT_SYMBOL_GPL(__context_tracking_enter);
103 
104 void context_tracking_enter(enum ctx_state state)
105 {
106         unsigned long flags;
107 
108         /*
109          * Some contexts may involve an exception occuring in an irq,
110          * leading to that nesting:
111          * rcu_irq_enter() rcu_user_exit() rcu_user_exit() rcu_irq_exit()
112          * This would mess up the dyntick_nesting count though. And rcu_irq_*()
113          * helpers are enough to protect RCU uses inside the exception. So
114          * just return immediately if we detect we are in an IRQ.
115          */
116         if (in_interrupt())
117                 return;
118 
119         local_irq_save(flags);
120         __context_tracking_enter(state);
121         local_irq_restore(flags);
122 }
123 NOKPROBE_SYMBOL(context_tracking_enter);
124 EXPORT_SYMBOL_GPL(context_tracking_enter);
125 
126 void context_tracking_user_enter(void)
127 {
128         user_enter();
129 }
130 NOKPROBE_SYMBOL(context_tracking_user_enter);
131 
132 /**
133  * context_tracking_exit - Inform the context tracking that the CPU is
134  *                         exiting user or guest mode and entering the kernel.
135  *
136  * This function must be called after we entered the kernel from user or
137  * guest space before any use of RCU read side critical section. This
138  * potentially include any high level kernel code like syscalls, exceptions,
139  * signal handling, etc...
140  *
141  * This call supports re-entrancy. This way it can be called from any exception
142  * handler without needing to know if we came from userspace or not.
143  */
144 void __context_tracking_exit(enum ctx_state state)
145 {
146         if (!context_tracking_recursion_enter())
147                 return;
148 
149         if (__this_cpu_read(context_tracking.state) == state) {
150                 if (__this_cpu_read(context_tracking.active)) {
151                         /*
152                          * We are going to run code that may use RCU. Inform
153                          * RCU core about that (ie: we may need the tick again).
154                          */
155                         rcu_user_exit();
156                         if (state == CONTEXT_USER) {
157                                 vtime_user_exit(current);
158                                 trace_user_exit(0);
159                         }
160                 }
161                 __this_cpu_write(context_tracking.state, CONTEXT_KERNEL);
162         }
163         context_tracking_recursion_exit();
164 }
165 NOKPROBE_SYMBOL(__context_tracking_exit);
166 EXPORT_SYMBOL_GPL(__context_tracking_exit);
167 
168 void context_tracking_exit(enum ctx_state state)
169 {
170         unsigned long flags;
171 
172         if (in_interrupt())
173                 return;
174 
175         local_irq_save(flags);
176         __context_tracking_exit(state);
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         user_exit();
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_branch_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
219 

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