[PATCH] cpu: Document clear_tasks_mm_cpumask()

[Anton Vorontsov <anton.vorontsov@linaro.org>]

This patch adds more comments on clear_tasks_mm_cpumask, plus adds
a runtime check: the function is only suitable for offlined CPUs,
and if called inappropriately, the kernel should scream aloud.

Suggested-by: Andrew Morton <akpm@linux-foundation.org>
Suggested-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
---

On Tue, May 01, 2012 at 12:45:33PM +0200, Peter Zijlstra wrote:
> On Thu, 2012-04-26 at 16:59 -0700, Andrew Morton wrote:
> > > +void clear_tasks_mm_cpumask(int cpu)
> > 
> > The operation of this function was presumably obvious to you at the
> > time you wrote it, but that isn't true of other people at later times.
> > 
> > Please document it?
[...]
> > If someone tries to use this function for a different purpose, or
> > copies-and-modifies it for a different purpose, we just shot them in
> > the foot.
> > 
> > They'd be pretty dumb to do that without reading the local comment,
> > but still...
> 
> Methinks something simple like:
> 
> 	WARN_ON(cpu_online(cpu));
> 
> Ought to cure that worry, no? :-)

Yeah, this is all good ideas, thanks.

How about the following patch?

 kernel/cpu.c |   18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index ecdf499..1bfa26f 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -13,6 +13,7 @@
 #include <linux/oom.h>
 #include <linux/rcupdate.h>
 #include <linux/export.h>
+#include <linux/bug.h>
 #include <linux/kthread.h>
 #include <linux/stop_machine.h>
 #include <linux/mutex.h>
@@ -173,6 +174,18 @@ void __ref unregister_cpu_notifier(struct notifier_block *nb)
 }
 EXPORT_SYMBOL(unregister_cpu_notifier);
 
+/**
+ * clear_tasks_mm_cpumask - Safely clear tasks' mm_cpumask for a CPU
+ * @cpu: a CPU id
+ *
+ * This function walks up all processes, finds a valid mm struct for
+ * each one and then clears a corresponding bit in mm's cpumask. While
+ * this all sounds trivial, there are various non-obvious corner cases,
+ * which this function tries to solve in a safe manner.
+ *
+ * Also note that the function uses a somewhat relaxed locking scheme,
+ * so it may be called only for an already offlined CPU.
+ */
 void clear_tasks_mm_cpumask(int cpu)
 {
 	struct task_struct *p;
@@ -184,10 +197,15 @@ void clear_tasks_mm_cpumask(int cpu)
 	 * Thus, we may use rcu_read_lock() here, instead of grabbing
 	 * full-fledged tasklist_lock.
 	 */
+	WARN_ON(cpu_online(cpu));
 	rcu_read_lock();
 	for_each_process(p) {
 		struct task_struct *t;
 
+		/*
+		 * Main thread might exit, but other threads may still have
+		 * a valid mm. Find one.
+		 */
 		t = find_lock_task_mm(p);
 		if (!t)
 			continue;
-- 
1.7.9.2