--- /dev/null
+/*
+ * Context tracking: Probe on high level context boundaries such as kernel
+ * and userspace. This includes syscalls and exceptions entry/exit.
+ *
+ * This is used by RCU to remove its dependency on the timer tick while a CPU
+ * runs in userspace.
+ *
+ * Started by Frederic Weisbecker:
+ *
+ * Copyright (C) 2012 Red Hat, Inc., Frederic Weisbecker <fweisbec@redhat.com>
+ *
+ * Many thanks to Gilad Ben-Yossef, Paul McKenney, Ingo Molnar, Andrew Morton,
+ * Steven Rostedt, Peter Zijlstra for suggestions and improvements.
+ *
+ */
+
+#include <linux/context_tracking.h>
+#include <linux/rcupdate.h>
+#include <linux/sched.h>
+#include <linux/hardirq.h>
+#include <linux/export.h>
+#include <linux/kprobes.h>
+
+#define CREATE_TRACE_POINTS
+#include <trace/events/context_tracking.h>
+
+struct static_key context_tracking_enabled = STATIC_KEY_INIT_FALSE;
+EXPORT_SYMBOL_GPL(context_tracking_enabled);
+
+DEFINE_PER_CPU(struct context_tracking, context_tracking);
+EXPORT_SYMBOL_GPL(context_tracking);
+
+void context_tracking_cpu_set(int cpu)
+{
+ if (!per_cpu(context_tracking.active, cpu)) {
+ per_cpu(context_tracking.active, cpu) = true;
+ static_key_slow_inc(&context_tracking_enabled);
+ }
+}
+
+/**
+ * context_tracking_enter - Inform the context tracking that the CPU is going
+ * enter user or guest space mode.
+ *
+ * This function must be called right before we switch from the kernel
+ * to user or guest space, when it's guaranteed the remaining kernel
+ * instructions to execute won't use any RCU read side critical section
+ * because this function sets RCU in extended quiescent state.
+ */
+void context_tracking_enter(enum ctx_state state)
+{
+ unsigned long flags;
+
+ /*
+ * Repeat the user_enter() check here because some archs may be calling
+ * this from asm and if no CPU needs context tracking, they shouldn't
+ * go further. Repeat the check here until they support the inline static
+ * key check.
+ */
+ if (!context_tracking_is_enabled())
+ return;
+
+ /*
+ * Some contexts may involve an exception occuring in an irq,
+ * leading to that nesting:
+ * rcu_irq_enter() rcu_user_exit() rcu_user_exit() rcu_irq_exit()
+ * This would mess up the dyntick_nesting count though. And rcu_irq_*()
+ * helpers are enough to protect RCU uses inside the exception. So
+ * just return immediately if we detect we are in an IRQ.
+ */
+ if (in_interrupt())
+ return;
+
+ /* Kernel threads aren't supposed to go to userspace */
+ WARN_ON_ONCE(!current->mm);
+
+ local_irq_save(flags);
+ if ( __this_cpu_read(context_tracking.state) != state) {
+ if (__this_cpu_read(context_tracking.active)) {
+ /*
+ * At this stage, only low level arch entry code remains and
+ * then we'll run in userspace. We can assume there won't be
+ * any RCU read-side critical section until the next call to
+ * user_exit() or rcu_irq_enter(). Let's remove RCU's dependency
+ * on the tick.
+ */
+ if (state == CONTEXT_USER) {
+ trace_user_enter(0);
+ vtime_user_enter(current);
+ }
+ rcu_user_enter();
+ }
+ /*
+ * Even if context tracking is disabled on this CPU, because it's outside
+ * the full dynticks mask for example, we still have to keep track of the
+ * context transitions and states to prevent inconsistency on those of
+ * other CPUs.
+ * If a task triggers an exception in userspace, sleep on the exception
+ * handler and then migrate to another CPU, that new CPU must know where
+ * the exception returns by the time we call exception_exit().
+ * This information can only be provided by the previous CPU when it called
+ * exception_enter().
+ * OTOH we can spare the calls to vtime and RCU when context_tracking.active
+ * is false because we know that CPU is not tickless.
+ */
+ __this_cpu_write(context_tracking.state, state);
+ }
+ local_irq_restore(flags);
+}
+NOKPROBE_SYMBOL(context_tracking_enter);
+EXPORT_SYMBOL_GPL(context_tracking_enter);
+
+void context_tracking_user_enter(void)
+{
+ context_tracking_enter(CONTEXT_USER);
+}
+NOKPROBE_SYMBOL(context_tracking_user_enter);
+
+/**
+ * context_tracking_exit - Inform the context tracking that the CPU is
+ * exiting user or guest mode and entering the kernel.
+ *
+ * This function must be called after we entered the kernel from user or
+ * guest space before any use of RCU read side critical section. This
+ * potentially include any high level kernel code like syscalls, exceptions,
+ * signal handling, etc...
+ *
+ * This call supports re-entrancy. This way it can be called from any exception
+ * handler without needing to know if we came from userspace or not.
+ */
+void context_tracking_exit(enum ctx_state state)
+{
+ unsigned long flags;
+
+ if (!context_tracking_is_enabled())
+ return;
+
+ if (in_interrupt())
+ return;
+
+ local_irq_save(flags);
+ if (__this_cpu_read(context_tracking.state) == state) {
+ if (__this_cpu_read(context_tracking.active)) {
+ /*
+ * We are going to run code that may use RCU. Inform
+ * RCU core about that (ie: we may need the tick again).
+ */
+ rcu_user_exit();
+ if (state == CONTEXT_USER) {
+ vtime_user_exit(current);
+ trace_user_exit(0);
+ }
+ }
+ __this_cpu_write(context_tracking.state, CONTEXT_KERNEL);
+ }
+ local_irq_restore(flags);
+}
+NOKPROBE_SYMBOL(context_tracking_exit);
+EXPORT_SYMBOL_GPL(context_tracking_exit);
+
+void context_tracking_user_exit(void)
+{
+ context_tracking_exit(CONTEXT_USER);
+}
+NOKPROBE_SYMBOL(context_tracking_user_exit);
+
+/**
+ * __context_tracking_task_switch - context switch the syscall callbacks
+ * @prev: the task that is being switched out
+ * @next: the task that is being switched in
+ *
+ * The context tracking uses the syscall slow path to implement its user-kernel
+ * boundaries probes on syscalls. This way it doesn't impact the syscall fast
+ * path on CPUs that don't do context tracking.
+ *
+ * But we need to clear the flag on the previous task because it may later
+ * migrate to some CPU that doesn't do the context tracking. As such the TIF
+ * flag may not be desired there.
+ */
+void __context_tracking_task_switch(struct task_struct *prev,
+ struct task_struct *next)
+{
+ clear_tsk_thread_flag(prev, TIF_NOHZ);
+ set_tsk_thread_flag(next, TIF_NOHZ);
+}
+
+#ifdef CONFIG_CONTEXT_TRACKING_FORCE
+void __init context_tracking_init(void)
+{
+ int cpu;
+
+ for_each_possible_cpu(cpu)
+ context_tracking_cpu_set(cpu);
+}
+#endif
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