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SubjectRe: [PATCH] context_tracking: Add comments on interface and internals
On Thu, 13 Dec 2012 21:57:05 +0100
Frederic Weisbecker <fweisbec@gmail.com> wrote:

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

Thanks, it helps.

> --- 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.

"on the timer tick"

> + *
> + * 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>
> @@ -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.

s/in //

> + *
> + * This function must be called right before we switch from the kernel
> + * to the user space, when the last remaining kernel instructions to execute

s/the user space/userspace/

> + * are low level arch code that perform the resuming to userspace.

This is a bit vague - what is "right before"? What happens if this is
done a few instructions early? I mean, what exactly is the requirement
here? Might it be something like "after the last rcu_foo operation"?

IOW, if the call to user_enter() were moved earlier and earlier, at
what point would the kernel gain a bug? What caused that bug?

> + * This call supports re-entrancy.

Presumably the explanation for user_exit() applies here.

> + */
> 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 */

s/thread/threads/

> 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

s/we/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.
> + */
> 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.

Also a very vague spec.

> + * 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.

s/bothering/needing/

s/come/came/

> + */
> 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)
> {

It's mainly this bit which makes me wonder why the code is in lib/. Is
there any conceivable prospect that any other subsystem will use this
code for anything?




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