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Subject[PATCH RFC tip/core/rcu 2/3] rcu: Add RCU_NONIDLE() for idle-loop RCU read-side critical sections
Date
From: "Paul E. McKenney" <paul.mckenney@linaro.org>

RCU, RCU-bh, and RCU-sched read-side critical sections are forbidden
in the inner idle loop, that is, between the rcu_idle_enter() and the
rcu_idle_exit() -- RCU will happily ignore any such read-side critical
sections. However, things like powertop need tracepoints in the inner
idle loop.

This commit therefore provides an RCU_NONIDLE() macro that can be used to
wrap code in the idle loop that requires RCU read-side critical sections.

Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
include/linux/rcupdate.h | 27 +++++++++++++++++++++++++++
kernel/rcutiny.c | 2 ++
kernel/rcutree.c | 2 ++
3 files changed, 31 insertions(+), 0 deletions(-)
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 6ee663c..9372174 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -190,6 +190,33 @@ extern void rcu_idle_exit(void);
extern void rcu_irq_enter(void);
extern void rcu_irq_exit(void);

+/**
+ * RCU_NONIDLE - Indicate idle-loop code that needs RCU readers
+ * @a: Code that RCU needs to pay attention to.
+ *
+ * RCU, RCU-bh, and RCU-sched read-side critical sections are forbidden
+ * in the inner idle loop, that is, between the rcu_idle_enter() and
+ * the rcu_idle_exit() -- RCU will happily ignore any such read-side
+ * critical sections. However, things like powertop need tracepoints
+ * in the inner idle loop.
+ *
+ * This macro provides the way out: RCU_NONIDLE(do_something_with_RCU())
+ * will tell RCU that it needs to pay attending, invoke its argument
+ * (in this example, a call to the do_something_with_RCU() function),
+ * and then tell RCU to go back to ignoring this CPU. It is permissible
+ * to nest RCU_NONIDLE() wrappers, but the nesting level is currently
+ * quite limited. If deeper nesting is required, it will be necessary
+ * to adjust DYNTICK_TASK_NESTING_VALUE accordingly.
+ *
+ * This macro may be used from process-level code only.
+ */
+#define RCU_NONIDLE(a) \
+ do { \
+ rcu_idle_exit(); \
+ do { a; } while (0); \
+ rcu_idle_enter(); \
+ } while (0)
+
/*
* Infrastructure to implement the synchronize_() primitives in
* TREE_RCU and rcu_barrier_() primitives in TINY_RCU.
diff --git a/kernel/rcutiny.c b/kernel/rcutiny.c
index 5098aea..b4142a9 100644
--- a/kernel/rcutiny.c
+++ b/kernel/rcutiny.c
@@ -97,6 +97,7 @@ void rcu_idle_enter(void)
rcu_idle_enter_common(oldval);
local_irq_restore(flags);
}
+EXPORT_SYMBOL_GPL(rcu_idle_enter);

/*
* Exit an interrupt handler towards idle.
@@ -153,6 +154,7 @@ void rcu_idle_exit(void)
rcu_idle_exit_common(oldval);
local_irq_restore(flags);
}
+EXPORT_SYMBOL_GPL(rcu_idle_exit);

/*
* Enter an interrupt handler, moving away from idle.
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index e81298a..25111d3 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -402,6 +402,7 @@ void rcu_idle_enter(void)
rcu_idle_enter_common(rdtp, oldval);
local_irq_restore(flags);
}
+EXPORT_SYMBOL_GPL(rcu_idle_enter);

/**
* rcu_irq_exit - inform RCU that current CPU is exiting irq towards idle
@@ -493,6 +494,7 @@ void rcu_idle_exit(void)
rcu_idle_exit_common(rdtp, oldval);
local_irq_restore(flags);
}
+EXPORT_SYMBOL_GPL(rcu_idle_exit);

/**
* rcu_irq_enter - inform RCU that current CPU is entering irq away from idle
--
1.7.8


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