[PATCH] context_tracking: Add comments on interface and internals

[Frederic Weisbecker]

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

Reported-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: Gilad Ben-Yossef <gilad@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Li Zhong <zhong@xxxxxxxxxxxxxxxxxx>
---
 kernel/context_tracking.c |   74 ++++++++++++++++++++++++++++++++++++++------
 1 files changed, 64 insertions(+), 10 deletions(-)

diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
index e0e07fd..f146b27 100644
--- a/kernel/context_tracking.c
+++ b/kernel/context_tracking.c
@@ -1,3 +1,19 @@
+/*
+ * 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 to the timer tick while a CPU
+ * runs in userspace.
+ *
+ *  Started by Frederic Weisbecker:
+ *
+ * Copyright (C) 2012 Red Hat, Inc., Frederic Weisbecker <fweisbec@xxxxxxxxxx>
+ *
+ * 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, hooks 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 +40,16 @@ static DEFINE_PER_CPU(struct context_tracking, context_tracking) = {
 #endif
 };
 
+/**
+ * user_enter - Inform the context tracking that the CPU is going to
+ *              enter in userspace mode.
+ *
+ * This function must be called right before we switch from the kernel
+ * to the user space, when the last remaining kernel instructions to execute
+ * are low level arch code that perform the resuming to userspace.
+ *
+ * This call supports re-entrancy.
+ */
 void user_enter(void)
 {
 	unsigned long flags;
@@ -39,40 +65,68 @@ void user_enter(void)
 	if (in_interrupt())
 		return;
 
+	/* Kernel thread aren't supposed to go to userspace */
 	WARN_ON_ONCE(!current->mm);
 
 	local_irq_save(flags);
 	if (__this_cpu_read(context_tracking.active) &&
 	    __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 we
+		 * 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.
+		 */
 		rcu_user_enter();
 	}
 	local_irq_restore(flags);
 }
 
+
+/**
+ * user_exit - Inform the context tracking that the CPU is
+ *             exiting userspace mode and entering the kernel.
+ *
+ * This function must be called right before we run any high level kernel
+ * code (ie: anything that is not low level arch entry code) after we entered
+ * the kernel from userspace.
+ *
+ * This call supports re-entrancy. This way it can be called from any exception
+ * handler without bothering to know if we come 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())
 		return;
 
 	local_irq_save(flags);
 	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).
+		 */
 		rcu_user_exit();
 	}
 	local_irq_restore(flags);
 }
 
+
+/**
+ * context_tracking_task_switch - context switch the syscall hooks
+ *
+ * The context tracking uses the syscall slow path to implement its user-kernel
+ * boundaries hooks 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)
 {
-- 
1.7.5.4

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