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

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 <akpm@linux-foundation.org>
Signed-off-by: default avatarFrederic Weisbecker <fweisbec@gmail.com>
Cc: Alessio Igor Bogani <abogani@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Chris Metcalf <cmetcalf@tilera.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Geoff Levand <geoff@infradead.org>
Cc: Gilad Ben Yossef <gilad@benyossef.com>
Cc: Hakan Akkan <hakanakkan@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Li Zhong <zhong@linux.vnet.ibm.com>
Cc: Namhyung Kim <namhyung.kim@lge.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
parent 90f45e4e

kernel/context_tracking.c

+65 −10
Original line number Diff line number Diff line 
/*

 * 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>
@@ -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) = { 
#endif

};



/**

 * 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())

		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.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 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 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())

		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 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)

{