[PATCH 1/3] srcu: use cpu_online() instead custom check

[Sebastian Andrzej Siewior]

The current check via srcu_online is slightly racy because after looking
at srcu_online there could be an interrupt that interrupted us long
enough until the CPU we checked against went offline.
An alternative would be to hold the hotplug rwsem (so the CPUs don't
change their state) and then check based on cpu_online() if we queue it
on a specific CPU or not. queue_work_on() itself can handle if something
is enqueued on an offline CPU but a timer which is enqueued on an offline
CPU won't fire until the CPU is back online.

I am not sure if the removal in rcu_init() is okay or not. I assume that
SRCU won't enqueue a work item before SRCU is up and ready.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
 kernel/rcu/srcutree.c | 22 ++++------------------
 kernel/rcu/tree.c     |  6 ------
 2 files changed, 4 insertions(+), 24 deletions(-)

diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
index 729a8706751d..d190af0e56f8 100644
--- a/kernel/rcu/srcutree.c
+++ b/kernel/rcu/srcutree.c
@@ -36,6 +36,7 @@
 #include <linux/delay.h>
 #include <linux/module.h>
 #include <linux/srcu.h>
+#include <linux/cpu.h>
 
 #include "rcu.h"
 #include "rcu_segcblist.h"
@@ -424,21 +425,6 @@ static void srcu_gp_start(struct srcu_struct *sp)
 	WARN_ON_ONCE(state != SRCU_STATE_SCAN1);
 }
 
-/*
- * Track online CPUs to guide callback workqueue placement.
- */
-DEFINE_PER_CPU(bool, srcu_online);
-
-void srcu_online_cpu(unsigned int cpu)
-{
-	WRITE_ONCE(per_cpu(srcu_online, cpu), true);
-}
-
-void srcu_offline_cpu(unsigned int cpu)
-{
-	WRITE_ONCE(per_cpu(srcu_online, cpu), false);
-}
-
 /*
  * Place the workqueue handler on the specified CPU if online, otherwise
  * just run it whereever.  This is useful for placing workqueue handlers
@@ -450,12 +436,12 @@ static bool srcu_queue_delayed_work_on(int cpu, struct workqueue_struct *wq,
 {
 	bool ret;
 
-	preempt_disable();
-	if (READ_ONCE(per_cpu(srcu_online, cpu)))
+	cpus_read_lock();
+	if (cpu_online(cpu))
 		ret = queue_delayed_work_on(cpu, wq, dwork, delay);
 	else
 		ret = queue_delayed_work(wq, dwork, delay);
-	preempt_enable();
+	cpus_read_unlock();
 	return ret;
 }
 
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 1250e4bd4b85..a3cb562955c9 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3729,8 +3729,6 @@ int rcutree_online_cpu(unsigned int cpu)
 {
 	sync_sched_exp_online_cleanup(cpu);
 	rcutree_affinity_setting(cpu, -1);
-	if (IS_ENABLED(CONFIG_TREE_SRCU))
-		srcu_online_cpu(cpu);
 	return 0;
 }
 
@@ -3741,8 +3739,6 @@ int rcutree_online_cpu(unsigned int cpu)
 int rcutree_offline_cpu(unsigned int cpu)
 {
 	rcutree_affinity_setting(cpu, cpu);
-	if (IS_ENABLED(CONFIG_TREE_SRCU))
-		srcu_offline_cpu(cpu);
 	return 0;
 }
 
@@ -4188,8 +4184,6 @@ void __init rcu_init(void)
 	for_each_online_cpu(cpu) {
 		rcutree_prepare_cpu(cpu);
 		rcu_cpu_starting(cpu);
-		if (IS_ENABLED(CONFIG_TREE_SRCU))
-			srcu_online_cpu(cpu);
 	}
 }
 
-- 
2.14.1