Commit 4eacdf18 authored by Frederic Weisbecker's avatar Frederic Weisbecker Committed by Paul E. McKenney

context_tracking: Add comments on interface and internals

This subsystem lacks many explanations on its purpose and
design. Add these missing comments.

v4: Document function parameter to be more kernel-doc
friendly, as per Namhyung suggestion.
Reported-by: default avatarAndrew Morton <>
Signed-off-by: default avatarFrederic Weisbecker <>
Cc: Alessio Igor Bogani <>
Cc: Andrew Morton <>
Cc: Chris Metcalf <>
Cc: Christoph Lameter <>
Cc: Geoff Levand <>
Cc: Gilad Ben Yossef <>
Cc: Hakan Akkan <>
Cc: Ingo Molnar <>
Cc: Li Zhong <>
Cc: Namhyung Kim <>
Cc: Paul E. McKenney <>
Cc: Paul Gortmaker <>
Cc: Peter Zijlstra <>
Cc: Steven Rostedt <>
Cc: Thomas Gleixner <>
Signed-off-by: default avatarPaul E. McKenney <>
parent 90f45e4e
* 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 <>
* 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>
......@@ -6,8 +22,8 @@
struct context_tracking {
* When active is false, hooks are not set to
* minimize overhead: TIF flags are cleared
* When active is false, probes are unset in order
* to minimize overhead: TIF flags are cleared
* and calls to user_enter/exit are ignored. This
* may be further optimized using static keys.
......@@ -24,6 +40,15 @@ static DEFINE_PER_CPU(struct context_tracking, 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 user_enter(void)
unsigned long flags;
......@@ -39,40 +64,70 @@ void user_enter(void)
if (in_interrupt())
/* Kernel threads aren't supposed to go to userspace */
if (__this_cpu_read( &&
__this_cpu_read(context_tracking.state) != IN_USER) {
__this_cpu_write(context_tracking.state, IN_USER);
* 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.
* 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 user_exit(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())
if (__this_cpu_read(context_tracking.state) == IN_USER) {
__this_cpu_write(context_tracking.state, IN_KERNEL);
* We are going to run code that may use RCU. Inform
* RCU core about that (ie: we may need the tick again).
* 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)
Markdown is supported
0% or
You are about to add 0 people to the discussion. Proceed with caution.
Finish editing this message first!
Please register or to comment