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Date: Wed, 30 Jul 2008 21:09:49 +0400
From: Oleg Nesterov <oleg@...sign.ru>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Ingo Molnar <mingo@...e.hu>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Roland McGrath <roland@...hat.com>,
linux-kernel@...r.kernel.org
Subject: [PATCH 3/3] wait_task_inactive: don't use the dummy version when !SMP && PREEMPT
wait_task_inactive() must return success only when/if the task leaves
the runqueue, it doesn't work correctly when !SMP && PREEMPT because
in that case we use the "dummy" version which doesn't check .on_rq.
Change the "#if" around the full-blown version from SMP to SMP || PREEMPT.
The patch looks monstrous because it moves the (unchanged) definition
of wait_task_inactive() outside of "#ifdef CONFIG_SMP", but it is quite
trivial.
Signed-off-by: Oleg Nesterov <oleg@...sign.ru>
include/linux/sched.h | 2
kernel/sched.c | 210 +++++++++++++++++++++++++-------------------------
2 files changed, 107 insertions(+), 105 deletions(-)
--- 27/include/linux/sched.h~3_WTI_PREEMPT 2008-07-30 20:28:31.000000000 +0400
+++ 27/include/linux/sched.h 2008-07-30 20:47:46.000000000 +0400
@@ -1874,7 +1874,7 @@ struct task_struct *fork_idle(int);
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
+#if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT)
extern unsigned long wait_task_inactive(struct task_struct *, long match_state);
#else
static inline unsigned long wait_task_inactive(struct task_struct *p,
--- 27/kernel/sched.c~3_WTI_PREEMPT 2008-07-30 19:47:56.000000000 +0400
+++ 27/kernel/sched.c 2008-07-30 20:43:41.000000000 +0400
@@ -1864,110 +1864,6 @@ migrate_task(struct task_struct *p, int
return 1;
}
-/*
- * wait_task_inactive - wait for a thread to unschedule.
- *
- * If @match_state is nonzero, it's the @p->state value just checked and
- * not expected to change. If it changes, i.e. @p might have woken up,
- * then return zero. When we succeed in waiting for @p to be off its CPU,
- * we return a positive number (its total switch count). If a second call
- * a short while later returns the same number, the caller can be sure that
- * @p has remained unscheduled the whole time.
- *
- * The caller must ensure that the task *will* unschedule sometime soon,
- * else this function might spin for a *long* time. This function can't
- * be called with interrupts off, or it may introduce deadlock with
- * smp_call_function() if an IPI is sent by the same process we are
- * waiting to become inactive.
- */
-unsigned long wait_task_inactive(struct task_struct *p, long match_state)
-{
- unsigned long flags;
- int running, on_rq;
- unsigned long ncsw;
- struct rq *rq;
-
- for (;;) {
- /*
- * We do the initial early heuristics without holding
- * any task-queue locks at all. We'll only try to get
- * the runqueue lock when things look like they will
- * work out!
- */
- rq = task_rq(p);
-
- /*
- * If the task is actively running on another CPU
- * still, just relax and busy-wait without holding
- * any locks.
- *
- * NOTE! Since we don't hold any locks, it's not
- * even sure that "rq" stays as the right runqueue!
- * But we don't care, since "task_running()" will
- * return false if the runqueue has changed and p
- * is actually now running somewhere else!
- */
- while (task_running(rq, p)) {
- if (match_state && unlikely(p->state != match_state))
- return 0;
- cpu_relax();
- }
-
- /*
- * Ok, time to look more closely! We need the rq
- * lock now, to be *sure*. If we're wrong, we'll
- * just go back and repeat.
- */
- rq = task_rq_lock(p, &flags);
- running = task_running(rq, p);
- on_rq = p->se.on_rq;
- ncsw = 0;
- if (!match_state || p->state == match_state)
- ncsw = p->nvcsw | LONG_MIN; /* sets MSB */
- task_rq_unlock(rq, &flags);
-
- /*
- * If it changed from the expected state, bail out now.
- */
- if (unlikely(!ncsw))
- break;
-
- /*
- * Was it really running after all now that we
- * checked with the proper locks actually held?
- *
- * Oops. Go back and try again..
- */
- if (unlikely(running)) {
- cpu_relax();
- continue;
- }
-
- /*
- * It's not enough that it's not actively running,
- * it must be off the runqueue _entirely_, and not
- * preempted!
- *
- * So if it wa still runnable (but just not actively
- * running right now), it's preempted, and we should
- * yield - it could be a while.
- */
- if (unlikely(on_rq)) {
- schedule_timeout_uninterruptible(1);
- continue;
- }
-
- /*
- * Ahh, all good. It wasn't running, and it wasn't
- * runnable, which means that it will never become
- * running in the future either. We're all done!
- */
- break;
- }
-
- return ncsw;
-}
-
/***
* kick_process - kick a running thread to enter/exit the kernel
* @p: the to-be-kicked thread
@@ -2176,6 +2072,112 @@ static int sched_balance_self(int cpu, i
#endif /* CONFIG_SMP */
+#if defined(CONFIG_SMP) || defined(CONFIG_PREEMPT)
+/*
+ * wait_task_inactive - wait for a thread to unschedule.
+ *
+ * If @match_state is nonzero, it's the @p->state value just checked and
+ * not expected to change. If it changes, i.e. @p might have woken up,
+ * then return zero. When we succeed in waiting for @p to be off its CPU,
+ * we return a positive number (its total switch count). If a second call
+ * a short while later returns the same number, the caller can be sure that
+ * @p has remained unscheduled the whole time.
+ *
+ * The caller must ensure that the task *will* unschedule sometime soon,
+ * else this function might spin for a *long* time. This function can't
+ * be called with interrupts off, or it may introduce deadlock with
+ * smp_call_function() if an IPI is sent by the same process we are
+ * waiting to become inactive.
+ */
+unsigned long wait_task_inactive(struct task_struct *p, long match_state)
+{
+ unsigned long flags;
+ int running, on_rq;
+ unsigned long ncsw;
+ struct rq *rq;
+
+ for (;;) {
+ /*
+ * We do the initial early heuristics without holding
+ * any task-queue locks at all. We'll only try to get
+ * the runqueue lock when things look like they will
+ * work out!
+ */
+ rq = task_rq(p);
+
+ /*
+ * If the task is actively running on another CPU
+ * still, just relax and busy-wait without holding
+ * any locks.
+ *
+ * NOTE! Since we don't hold any locks, it's not
+ * even sure that "rq" stays as the right runqueue!
+ * But we don't care, since "task_running()" will
+ * return false if the runqueue has changed and p
+ * is actually now running somewhere else!
+ */
+ while (task_running(rq, p)) {
+ if (match_state && unlikely(p->state != match_state))
+ return 0;
+ cpu_relax();
+ }
+
+ /*
+ * Ok, time to look more closely! We need the rq
+ * lock now, to be *sure*. If we're wrong, we'll
+ * just go back and repeat.
+ */
+ rq = task_rq_lock(p, &flags);
+ running = task_running(rq, p);
+ on_rq = p->se.on_rq;
+ ncsw = 0;
+ if (!match_state || p->state == match_state)
+ ncsw = p->nvcsw | LONG_MIN; /* sets MSB */
+ task_rq_unlock(rq, &flags);
+
+ /*
+ * If it changed from the expected state, bail out now.
+ */
+ if (unlikely(!ncsw))
+ break;
+
+ /*
+ * Was it really running after all now that we
+ * checked with the proper locks actually held?
+ *
+ * Oops. Go back and try again..
+ */
+ if (unlikely(running)) {
+ cpu_relax();
+ continue;
+ }
+
+ /*
+ * It's not enough that it's not actively running,
+ * it must be off the runqueue _entirely_, and not
+ * preempted!
+ *
+ * So if it wa still runnable (but just not actively
+ * running right now), it's preempted, and we should
+ * yield - it could be a while.
+ */
+ if (unlikely(on_rq)) {
+ schedule_timeout_uninterruptible(1);
+ continue;
+ }
+
+ /*
+ * Ahh, all good. It wasn't running, and it wasn't
+ * runnable, which means that it will never become
+ * running in the future either. We're all done!
+ */
+ break;
+ }
+
+ return ncsw;
+}
+#endif
+
/***
* try_to_wake_up - wake up a thread
* @p: the to-be-woken-up thread
--
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