[PATCH 1/7] work_on_cpu: helper for doing task on a CPU.

[Rusty Russell <rusty@rustcorp.com.au>]

Several places in the kernel do the following:

	saved_mask = current->cpus_allowed;
	set_cpus_allowed_ptr(current, &cpumask_of_cpu(pr->id));
	somefunc();
	/* restore the previous state */
	set_cpus_allowed_ptr(current, &saved_mask);

This is bad, because a process's cpumask is observable and
manipulatable by userspace and should not be toyed with.

We have the infrastructure, this just creates a nice wrapper to
encourage its use.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Ingo Molnar <mingo@elte.hu>
---
 include/linux/workqueue.h |    8 +++++++
 kernel/workqueue.c        |   48 ++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 56 insertions(+)

diff -r b72e0cbdd249 include/linux/workqueue.h
--- a/include/linux/workqueue.h	Thu Oct 23 00:39:36 2008 +1100
+++ b/include/linux/workqueue.h	Thu Oct 23 10:53:51 2008 +1100
@@ -240,4 +240,12 @@ void cancel_rearming_delayed_work(struct
 	cancel_delayed_work_sync(work);
 }
 
+#ifndef CONFIG_SMP
+static inline long work_on_cpu(unsigned int cpu, long (*fn)(void *), void *arg)
+{
+	return fn(arg);
+}
+#else
+long work_on_cpu(unsigned int cpu, long (*fn)(void *), void *arg);
+#endif /* CONFIG_SMP */
 #endif
diff -r b72e0cbdd249 kernel/workqueue.c
--- a/kernel/workqueue.c	Thu Oct 23 00:39:36 2008 +1100
+++ b/kernel/workqueue.c	Thu Oct 23 10:53:51 2008 +1100
@@ -970,6 +970,54 @@ undo:
 	return ret;
 }
 
+#ifdef CONFIG_SMP
+struct work_for_cpu {
+	struct work_struct work;
+	long (*fn)(void *);
+	void *arg;
+	long ret;
+	struct completion done;
+};
+
+static void do_work_for_cpu(struct work_struct *w)
+{
+	struct work_for_cpu *wfc = container_of(w, struct work_for_cpu, work);
+
+	wfc->ret = wfc->fn(wfc->arg);
+	complete(&wfc->done);
+}
+
+/**
+ * work_on_cpu - run a function in user context on a particular cpu
+ * @cpu: the cpu to run on
+ * @fn: the function to run
+ * @arg: the function arg
+ *
+ * This will return -EINVAL in the cpu is not online, or the return value
+ * of @fn otherwise.
+ */
+long work_on_cpu(unsigned int cpu, long (*fn)(void *), void *arg)
+{
+	struct work_for_cpu wfc;
+
+	INIT_WORK(&wfc.work, do_work_for_cpu);
+	init_completion(&wfc.done);
+	wfc.fn = fn;
+	wfc.arg = arg;
+	get_online_cpus();
+	if (unlikely(!cpu_online(cpu))) {
+		wfc.ret = -EINVAL;
+		complete(&wfc.done);
+	} else
+		schedule_work_on(cpu, &wfc.work);
+	put_online_cpus();
+	wait_for_completion(&wfc.done);
+
+	return wfc.ret;
+}
+EXPORT_SYMBOL_GPL(work_on_cpu);
+#endif /* CONFIG_SMP */
+
 void __init init_workqueues(void)
 {
 	cpu_populated_map = cpu_online_map;