[patch 02/13] workqueue: Provide work_on_cpu_safe()

[Thomas Gleixner]

work_on_cpu() is not protected against CPU hotplug. For code which requires
to be either executed on an online CPU or to fail if the CPU is not
available the callsite would have to protect against CPU hotplug.

Provide a function which does get/put_online_cpus() around the call to
work_on_cpu() and fails the call with -ENODEV if the target CPU is not
online.

Preparatory patch to convert several racy task affinity manipulations.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
---
 include/linux/workqueue.h |    5 +++++
 kernel/workqueue.c        |   23 +++++++++++++++++++++++
 2 files changed, 28 insertions(+)

--- a/include/linux/workqueue.h
+++ b/include/linux/workqueue.h
@@ -608,8 +608,13 @@ static inline long work_on_cpu(int cpu,
 {
 	return fn(arg);
 }
+static inline long work_on_cpu_safe(int cpu, long (*fn)(void *), void *arg)
+{
+	return fn(arg);
+}
 #else
 long work_on_cpu(int cpu, long (*fn)(void *), void *arg);
+long work_on_cpu_safe(int cpu, long (*fn)(void *), void *arg);
 #endif /* CONFIG_SMP */
 
 #ifdef CONFIG_FREEZER
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -4735,6 +4735,29 @@ long work_on_cpu(int cpu, long (*fn)(voi
 	return wfc.ret;
 }
 EXPORT_SYMBOL_GPL(work_on_cpu);
+
+/**
+ * work_on_cpu_safe - run a function in thread context on a particular cpu
+ * @cpu: the cpu to run on
+ * @fn:  the function to run
+ * @arg: the function argument
+ *
+ * Disables CPU hotplug and calls work_on_cpu(). The caller must not hold
+ * any locks which would prevent @fn from completing.
+ *
+ * Return: The value @fn returns.
+ */
+long work_on_cpu_safe(int cpu, long (*fn)(void *), void *arg)
+{
+	long ret = -ENODEV;
+
+	get_online_cpus();
+	if (cpu_online(cpu))
+		ret = work_on_cpu(cpu, fn, arg);
+	put_online_cpus();
+	return ret;
+}
+EXPORT_SYMBOL_GPL(work_on_cpu_safe);
 #endif /* CONFIG_SMP */
 
 #ifdef CONFIG_FREEZER