This subsystem lacks many explanations on its purpose and
design. Add these missing comments.
Reported-by: Andrew Morton <a...@linux-foundation.org>
Signed-off-by: Frederic Weisbecker <fweis...@gmail.com>
Cc: Gilad Ben-Yossef <gi...@benyossef.com>
Cc: Thomas Gleixner <t...@linutronix.de>
Cc: Andrew Morton <a...@linux-foundation.org>
Cc: Paul E. McKenney <paul...@linux.vnet.ibm.com>
Cc: Ingo Molnar <mi...@kernel.org>
Cc: Steven Rostedt <rost...@goodmis.org>
Cc: Peter Zijlstra <pet...@infradead.org>
Cc: Li Zhong <zh...@linux.vnet.ibm.com>
---
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 <fweis...@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.
+ *
+ * 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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