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

[peterz@infradead.org (Peter Zijlstra)]

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().