patches/srcu-use-cpu_online-instead-custom-check.patch

From: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Date: Wed, 13 Sep 2017 14:43:41 +0200
Subject: [PATCH] srcu: use cpu_online() instead custom check

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@linutronix.de>
---
 kernel/rcu/srcutree.c |   22 ++++------------------
 kernel/rcu/tree.c     |    4 ----
 2 files changed, 4 insertions(+), 22 deletions(-)

--- 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"
@@ -452,21 +453,6 @@ static void srcu_gp_start(struct srcu_st
 }
 
 /*
- * 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
  * that are to invoke the specified CPU's callbacks.
@@ -477,12 +463,12 @@ static bool srcu_queue_delayed_work_on(i
 {
 	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;
 }
 
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -3775,8 +3775,6 @@ int rcutree_online_cpu(unsigned int cpu)
 		rnp->ffmask |= rdp->grpmask;
 		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 	}
-	if (IS_ENABLED(CONFIG_TREE_SRCU))
-		srcu_online_cpu(cpu);
 	if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
 		return 0; /* Too early in boot for scheduler work. */
 	sync_sched_exp_online_cleanup(cpu);
@@ -3804,8 +3802,6 @@ int rcutree_offline_cpu(unsigned int cpu
 	}
 
 	rcutree_affinity_setting(cpu, cpu);
-	if (IS_ENABLED(CONFIG_TREE_SRCU))
-		srcu_offline_cpu(cpu);
 	return 0;
 }