[PATCH] Make idle loop (mostly) safe for RCU read-side critical sections.

From: Paul E . McKenney
Date: Sun Apr 05 2009 - 13:09:54 EST


Not for inclusion, demo only. Untested, probably fails to compile.

This patch is for demonstration purposes only. It adds a facility to
rcutree.c to allow RCU read-side critical sections to be used in
idle loops, as long as those RCU read-side critical sections do not
lap over the call to rcu_idle().

If this were a real patch, it would have the following:

o A config variable to allow architectures to opt out of this
sort of behavior. (But then again, maybe not.)

o Follow-up patches that added a call to rcu_idle() to each
idle loop in the kernel, probably grouped by architecture.

o Documentation updates to explain the new loosened restrictions
regarding RCU read-side critical sections and idle loops.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
---
arch/x86/kernel/process.c | 1 +
include/linux/rcupdate.h | 1 +
kernel/rcutree.c | 21 ++++++++++++++-------
3 files changed, 16 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 156f875..adbaf13 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -310,6 +310,7 @@ void default_idle(void)
current_thread_info()->status |= TS_POLLING;
trace_power_end(&it);
} else {
+ rcu_idle();
local_irq_enable();
/* loop is done by the caller */
cpu_relax();
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 528343e..3905f54 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -265,6 +265,7 @@ extern void synchronize_rcu(void);
extern void rcu_barrier(void);
extern void rcu_barrier_bh(void);
extern void rcu_barrier_sched(void);
+extern void rcu_idle(void);

/* Internal to kernel */
extern void rcu_init(void);
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 97ce315..4c61b71 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -937,6 +937,17 @@ static void rcu_do_batch(struct rcu_data *rdp)
}

/*
+ * Called from each idle loop to enable RCU to treat the idle loop as
+ * a quiescent state. Note that this code assumes that idle CPUs continue
+ * executing instructions until they enter nohz mode.
+ */
+void rcu_idle(void)
+{
+ rcu_qsctr_inc(cpu);
+ rcu_bh_qsctr_inc(cpu);
+}
+
+/*
* Check to see if this CPU is in a non-context-switch quiescent state
* (user mode or idle loop for rcu, non-softirq execution for rcu_bh).
* Also schedule the RCU softirq handler.
@@ -947,15 +958,11 @@ static void rcu_do_batch(struct rcu_data *rdp)
*/
void rcu_check_callbacks(int cpu, int user)
{
- if (user ||
- (idle_cpu(cpu) && rcu_scheduler_active &&
- !in_softirq() && hardirq_count() <= (1 << HARDIRQ_SHIFT))) {
+ if (user) {

/*
- * Get here if this CPU took its interrupt from user
- * mode or from the idle loop, and if this is not a
- * nested interrupt. In this case, the CPU is in
- * a quiescent state, so count it.
+ * Get here if this CPU took its interrupt from user mode.
+ * In this case, the CPU is in a quiescent state, so count it.
*
* No memory barrier is required here because both
* rcu_qsctr_inc() and rcu_bh_qsctr_inc() reference
--
1.5.2.5

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/