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/*
* 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>
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_user_enter - Inform the context tracking that the CPU is going to
* enter userspace mode.
*
* This function must be called right before we switch from the kernel
* to userspace, 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_user_enter(void)
{
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;
/* 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) != IN_USER) {
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.
*/
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, IN_USER);
}
local_irq_restore(flags);
}
#ifdef CONFIG_PREEMPT
/**
* preempt_schedule_context - preempt_schedule called by tracing
*
* The tracing infrastructure uses preempt_enable_notrace to prevent
* recursion and tracing preempt enabling caused by the tracing
* infrastructure itself. But as tracing can happen in areas coming
* from userspace or just about to enter userspace, a preempt enable
* can occur before user_exit() is called. This will cause the scheduler
* to be called when the system is still in usermode.
*
* To prevent this, the preempt_enable_notrace will use this function
* instead of preempt_schedule() to exit user context if needed before
* calling the scheduler.
*/
void __sched notrace preempt_schedule_context(void)
{
enum ctx_state prev_ctx;
if (likely(!preemptible()))
return;
/*
* Need to disable preemption in case user_exit() is traced
* and the tracer calls preempt_enable_notrace() causing
* an infinite recursion.
*/
preempt_disable_notrace();
prev_ctx = exception_enter();
preempt_enable_no_resched_notrace();
preempt_schedule();
preempt_disable_notrace();
exception_exit(prev_ctx);
preempt_enable_notrace();
}
EXPORT_SYMBOL_GPL(preempt_schedule_context);
#endif /* CONFIG_PREEMPT */
/**
* context_tracking_user_exit - Inform the context tracking that the CPU is
* exiting userspace mode and entering the kernel.
*
* This function must be called after we entered the kernel from userspace
* 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_user_exit(void)
{
unsigned long flags;
if (in_interrupt())
return;
local_irq_save(flags);
if (__this_cpu_read(context_tracking.state) == IN_USER) {
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();
vtime_user_exit(current);
}
__this_cpu_write(context_tracking.state, IN_KERNEL);
}
local_irq_restore(flags);
}
/**
* __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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