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 23 DEFINE_PER_CPU(struct context_tracking, context_tracking) = { 24 #ifdef CONFIG_CONTEXT_TRACKING_FORCE 25 .active = true, 26 #endif 27 }; 28 29 /** 30 * user_enter - Inform the context tracking that the CPU is going to 31 * enter userspace mode. 32 * 33 * This function must be called right before we switch from the kernel 34 * to userspace, when it's guaranteed the remaining kernel instructions 35 * to execute won't use any RCU read side critical section because this 36 * function sets RCU in extended quiescent state. 37 */ 38 void user_enter(void) 39 { 40 unsigned long flags; 41 42 /* 43 * Some contexts may involve an exception occuring in an irq, 44 * leading to that nesting: 45 * rcu_irq_enter() rcu_user_exit() rcu_user_exit() rcu_irq_exit() 46 * This would mess up the dyntick_nesting count though. And rcu_irq_*() 47 * helpers are enough to protect RCU uses inside the exception. So 48 * just return immediately if we detect we are in an IRQ. 49 */ 50 if (in_interrupt()) 51 return; 52 53 /* Kernel threads aren't supposed to go to userspace */ 54 WARN_ON_ONCE(!current->mm); 55 56 local_irq_save(flags); 57 if (__this_cpu_read(context_tracking.active) && 58 __this_cpu_read(context_tracking.state) != IN_USER) { 59 /* 60 * At this stage, only low level arch entry code remains and 61 * then we'll run in userspace. We can assume there won't be 62 * any RCU read-side critical section until the next call to 63 * user_exit() or rcu_irq_enter(). Let's remove RCU's dependency 64 * on the tick. 65 */ 66 vtime_user_enter(current); 67 rcu_user_enter(); 68 __this_cpu_write(context_tracking.state, IN_USER); 69 } 70 local_irq_restore(flags); 71 } 72 73 #ifdef CONFIG_PREEMPT 74 /** 75 * preempt_schedule_context - preempt_schedule called by tracing 76 * 77 * The tracing infrastructure uses preempt_enable_notrace to prevent 78 * recursion and tracing preempt enabling caused by the tracing 79 * infrastructure itself. But as tracing can happen in areas coming 80 * from userspace or just about to enter userspace, a preempt enable 81 * can occur before user_exit() is called. This will cause the scheduler 82 * to be called when the system is still in usermode. 83 * 84 * To prevent this, the preempt_enable_notrace will use this function 85 * instead of preempt_schedule() to exit user context if needed before 86 * calling the scheduler. 87 */ 88 void __sched notrace preempt_schedule_context(void) 89 { 90 struct thread_info *ti = current_thread_info(); 91 enum ctx_state prev_ctx; 92 93 if (likely(ti->preempt_count || irqs_disabled())) 94 return; 95 96 /* 97 * Need to disable preemption in case user_exit() is traced 98 * and the tracer calls preempt_enable_notrace() causing 99 * an infinite recursion. 100 */ 101 preempt_disable_notrace(); 102 prev_ctx = exception_enter(); 103 preempt_enable_no_resched_notrace(); 104 105 preempt_schedule(); 106 107 preempt_disable_notrace(); 108 exception_exit(prev_ctx); 109 preempt_enable_notrace(); 110 } 111 EXPORT_SYMBOL_GPL(preempt_schedule_context); 112 #endif /* CONFIG_PREEMPT */ 113 114 /** 115 * user_exit - Inform the context tracking that the CPU is 116 * exiting userspace mode and entering the kernel. 117 * 118 * This function must be called after we entered the kernel from userspace 119 * before any use of RCU read side critical section. This potentially include 120 * any high level kernel code like syscalls, exceptions, signal handling, etc... 121 * 122 * This call supports re-entrancy. This way it can be called from any exception 123 * handler without needing to know if we came from userspace or not. 124 */ 125 void user_exit(void) 126 { 127 unsigned long flags; 128 129 if (in_interrupt()) 130 return; 131 132 local_irq_save(flags); 133 if (__this_cpu_read(context_tracking.state) == IN_USER) { 134 /* 135 * We are going to run code that may use RCU. Inform 136 * RCU core about that (ie: we may need the tick again). 137 */ 138 rcu_user_exit(); 139 vtime_user_exit(current); 140 __this_cpu_write(context_tracking.state, IN_KERNEL); 141 } 142 local_irq_restore(flags); 143 } 144 145 void guest_enter(void) 146 { 147 if (vtime_accounting_enabled()) 148 vtime_guest_enter(current); 149 else 150 __guest_enter(); 151 } 152 EXPORT_SYMBOL_GPL(guest_enter); 153 154 void guest_exit(void) 155 { 156 if (vtime_accounting_enabled()) 157 vtime_guest_exit(current); 158 else 159 __guest_exit(); 160 } 161 EXPORT_SYMBOL_GPL(guest_exit); 162 163 164 /** 165 * context_tracking_task_switch - context switch the syscall callbacks 166 * @prev: the task that is being switched out 167 * @next: the task that is being switched in 168 * 169 * The context tracking uses the syscall slow path to implement its user-kernel 170 * boundaries probes on syscalls. This way it doesn't impact the syscall fast 171 * path on CPUs that don't do context tracking. 172 * 173 * But we need to clear the flag on the previous task because it may later 174 * migrate to some CPU that doesn't do the context tracking. As such the TIF 175 * flag may not be desired there. 176 */ 177 void context_tracking_task_switch(struct task_struct *prev, 178 struct task_struct *next) 179 { 180 if (__this_cpu_read(context_tracking.active)) { 181 clear_tsk_thread_flag(prev, TIF_NOHZ); 182 set_tsk_thread_flag(next, TIF_NOHZ); 183 } 184 } 185