From 6530f732939f55f07f0fc9c7e64957b4e71aa774 Mon Sep 17 00:00:00 2001
From: Markus Metzger <markus.t.metzger@intel.com>
Date: Fri, 3 Apr 2009 16:43:34 +0200
Subject: [PATCH] --- yaml --- r: 146121 b: refs/heads/master c:
 a26b89f05d194413c7238e0bea071054f6b5d3c8 h: refs/heads/master i:   146119:
 3b80db9c4c78ff7540db3fe71290f84111113a4c v: v3

---
 [refs]                      |  2 +-
 trunk/include/linux/sched.h |  2 ++
 trunk/kernel/sched.c        | 43 +++++++++++++++++++++++++++++++++++++
 3 files changed, 46 insertions(+), 1 deletion(-)

diff --git a/[refs] b/[refs]
index 8d0fb3538854..9983f5384aad 100644
--- a/[refs]
+++ b/[refs]
@@ -1,2 +1,2 @@
 ---
-refs/heads/master: cac94f979326212831c0ea44ed9ea1622b4f4e93
+refs/heads/master: a26b89f05d194413c7238e0bea071054f6b5d3c8
diff --git a/trunk/include/linux/sched.h b/trunk/include/linux/sched.h
index b94f3541f67b..a5b9a83065fa 100644
--- a/trunk/include/linux/sched.h
+++ b/trunk/include/linux/sched.h
@@ -1993,8 +1993,10 @@ extern void set_task_comm(struct task_struct *tsk, char *from);
 extern char *get_task_comm(char *to, struct task_struct *tsk);
 
 #ifdef CONFIG_SMP
+extern void wait_task_context_switch(struct task_struct *p);
 extern unsigned long wait_task_inactive(struct task_struct *, long match_state);
 #else
+static inline void wait_task_context_switch(struct task_struct *p) {}
 static inline unsigned long wait_task_inactive(struct task_struct *p,
 					       long match_state)
 {
diff --git a/trunk/kernel/sched.c b/trunk/kernel/sched.c
index 6cc1fd5d5072..f91bc8141dc3 100644
--- a/trunk/kernel/sched.c
+++ b/trunk/kernel/sched.c
@@ -2002,6 +2002,49 @@ migrate_task(struct task_struct *p, int dest_cpu, struct migration_req *req)
 	return 1;
 }
 
+/*
+ * wait_task_context_switch -	wait for a thread to complete at least one
+ *				context switch.
+ *
+ * @p must not be current.
+ */
+void wait_task_context_switch(struct task_struct *p)
+{
+	unsigned long nvcsw, nivcsw, flags;
+	int running;
+	struct rq *rq;
+
+	nvcsw	= p->nvcsw;
+	nivcsw	= p->nivcsw;
+	for (;;) {
+		/*
+		 * The runqueue is assigned before the actual context
+		 * switch. We need to take the runqueue lock.
+		 *
+		 * We could check initially without the lock but it is
+		 * very likely that we need to take the lock in every
+		 * iteration.
+		 */
+		rq = task_rq_lock(p, &flags);
+		running = task_running(rq, p);
+		task_rq_unlock(rq, &flags);
+
+		if (likely(!running))
+			break;
+		/*
+		 * The switch count is incremented before the actual
+		 * context switch. We thus wait for two switches to be
+		 * sure at least one completed.
+		 */
+		if ((p->nvcsw - nvcsw) > 1)
+			break;
+		if ((p->nivcsw - nivcsw) > 1)
+			break;
+
+		cpu_relax();
+	}
+}
+
 /*
  * wait_task_inactive - wait for a thread to unschedule.
  *