// SPDX-License-Identifier: GPL-2.0-only /* * 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> DEFINE_STATIC_KEY_FALSE(context_tracking_key); EXPORT_SYMBOL_GPL(context_tracking_key); DEFINE_PER_CPU(struct context_tracking, context_tracking); EXPORT_SYMBOL_GPL(context_tracking); static noinstr bool context_tracking_recursion_enter(void) { int recursion; recursion = __this_cpu_inc_return(context_tracking.recursion); if (recursion == 1) return true; WARN_ONCE((recursion < 1), "Invalid context tracking recursion value %d\n", recursion); __this_cpu_dec(context_tracking.recursion); return false; } static __always_inline void context_tracking_recursion_exit(void) { __this_cpu_dec(context_tracking.recursion); } /** * 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 noinstr __context_tracking_enter(enum ctx_state state) { /* Kernel threads aren't supposed to go to userspace */ WARN_ON_ONCE(!current->mm); if (!context_tracking_recursion_enter()) return; 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) { instrumentation_begin(); trace_user_enter(0); vtime_user_enter(current); instrumentation_end(); } 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); } context_tracking_recursion_exit(); } EXPORT_SYMBOL_GPL(__context_tracking_enter); void context_tracking_enter(enum ctx_state state) { unsigned long flags; /* * 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; local_irq_save(flags); __context_tracking_enter(state); local_irq_restore(flags); } NOKPROBE_SYMBOL(context_tracking_enter); EXPORT_SYMBOL_GPL(context_tracking_enter); void context_tracking_user_enter(void) { user_enter(); } 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 noinstr __context_tracking_exit(enum ctx_state state) { if (!context_tracking_recursion_enter()) return; 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) { instrumentation_begin(); vtime_user_exit(current); trace_user_exit(0); instrumentation_end(); } } __this_cpu_write(context_tracking.state, CONTEXT_KERNEL); } context_tracking_recursion_exit(); } EXPORT_SYMBOL_GPL(__context_tracking_exit); void context_tracking_exit(enum ctx_state state) { unsigned long flags; if (in_interrupt()) return; local_irq_save(flags); __context_tracking_exit(state); local_irq_restore(flags); } NOKPROBE_SYMBOL(context_tracking_exit); EXPORT_SYMBOL_GPL(context_tracking_exit); void context_tracking_user_exit(void) { user_exit(); } NOKPROBE_SYMBOL(context_tracking_user_exit); void __init context_tracking_cpu_set(int cpu) { static __initdata bool initialized = false; if (!per_cpu(context_tracking.active, cpu)) { per_cpu(context_tracking.active, cpu) = true; static_branch_inc(&context_tracking_key); } if (initialized) return; #ifdef CONFIG_HAVE_TIF_NOHZ /* * Set TIF_NOHZ to init/0 and let it propagate to all tasks through fork * This assumes that init is the only task at this early boot stage. */ set_tsk_thread_flag(&init_task, TIF_NOHZ); #endif WARN_ON_ONCE(!tasklist_empty()); initialized = true; } #ifdef CONFIG_CONTEXT_TRACKING_FORCE void __init context_tracking_init(void) { int cpu; for_each_possible_cpu(cpu) context_tracking_cpu_set(cpu); } #endif