[patch 02/20] sched: add wait_task_context_switch() fucntion to sched.h

From: markus . t . metzger
Date: Fri Apr 03 2009 - 10:46:55 EST


Add a function to wait until some other task has been switched out at least
once.


Signed-off-by: Markus Metzger <markus.t.metzger@xxxxxxxxx>

---
include/linux/sched.h | 2 2 + 0 - 0 !
kernel/sched.c | 43 43 + 0 - 0 !
2 files changed, 45 insertions(+)

Index: b/include/linux/sched.h
===================================================================
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1988,8 +1988,10 @@ extern void set_task_comm(struct task_st
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)
{
Index: b/kernel/sched.c
===================================================================
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1933,6 +1933,49 @@ migrate_task(struct task_struct *p, int
}

/*
+ * 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.
*
* If @match_state is nonzero, it's the @p->state value just checked and

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