Re: [BUG] "sched: Remove rq->lock from the first half of ttwu()" locks up on ARM

Peter Zijlstra
2011-05-25T21:16:07Z

On Wed, 2011-05-25 at 19:08 +0200, Peter Zijlstra wrote:
> Ooh, shiny, whilst typing this I got an NMI-watchdog error reporting me
> that CPU1 got stuck in try_to_wake_up(), so it looks like I can indeed
> reproduce some funnies.
>
> /me goes dig in.

Does the below make your ARM box happy again?

It restores the old ttwu behaviour for this case and seems to not mess
up my x86 with __ARCH_WANT_INTERRUPTS_ON_CTXSW.

Figuring out why the existing condition failed and writing a proper
changelog requires a mind that is slightly less deprived of sleep and I
shall attempt that tomorrow -- provided this does indeed work for you.

---
diff --git a/kernel/sched.c b/kernel/sched.c
index 2d12893..6976eac 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2573,7 +2573,19 @@ static void ttwu_queue_remote(struct task_struct *p, int cpu)
if (!next)
smp_send_reschedule(cpu);
}
-#endif
+
+#ifdef __ARCH_WANT_INTERRUPTS_ON_CTXSW
+static void ttwu_activate_remote(struct task_struct *p, int wake_flags)
+{
+ struct rq *rq = __task_rq_lock(p);
+
+ ttwu_activate(rq, p, ENQUEUE_WAKEUP | ENQUEUE_WAKING);
+ ttwu_do_wakeup(rq, p, wake_flags);
+
+ __task_rq_unlock(rq);
+}
+#endif /* __ARCH_WANT_INTERRUPTS_ON_CTXSW */
+#endif /* CONFIG_SMP */

static void ttwu_queue(struct task_struct *p, int cpu)
{
@@ -2630,18 +2642,11 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
*/
while (p->on_cpu) {
#ifdef __ARCH_WANT_INTERRUPTS_ON_CTXSW
- /*
- * If called from interrupt context we could have landed in the
- * middle of schedule(), in this case we should take care not
- * to spin on ->on_cpu if p is current, since that would
- * deadlock.
- */
- if (p == current) {
- ttwu_queue(p, cpu);
- goto stat;
- }
-#endif
+ ttwu_activate_remote(p, wake_flags);
+ goto stat;
+#else
cpu_relax();
+#endif
}
/*
* Pairs with the smp_wmb() in finish_lock_switch().