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