[PATCH 1/3 v3] genirq: Prevent from early irq thread spurious wake-ups

[Frederic Weisbecker <frederic@kernel.org>]

From: Thomas Gleixner <tglx@linutronix.de>

During initialization, the IRQ thread is created before the IRQ get a
chance to be enabled. But the IRQ enablement may happen before the first
official kthread wake up point. As a result, the firing IRQ can perform
an early wake-up of the IRQ thread before the first official kthread
wake up point.

Although this has happened to be harmless so far, this uncontrolled
behaviour is a bug waiting to happen at some point in the future with
the threaded handler accessing halfway initialized states.

Prevent from such surprise with performing a wake-up only if the target
is in TASK_INTERRUPTIBLE state. Since the IRQ thread waits in this state
for interrupts to handle only after proper initialization, it is then
guaranteed not to be spuriously woken up while waiting in
TASK_UNINTERRUPTIBLE, right after creation in the kthread code, before
the official first wake up point to be reached.

Not-yet-Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
---
 kernel/irq/handle.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/irq/handle.c b/kernel/irq/handle.c
index e103451243a0..786f5570a640 100644
--- a/kernel/irq/handle.c
+++ b/kernel/irq/handle.c
@@ -133,7 +133,15 @@ void __irq_wake_thread(struct irq_desc *desc, struct irqaction *action)
 	 */
 	atomic_inc(&desc->threads_active);
 
-	wake_up_process(action->thread);
+	/*
+	 * This might be a premature wakeup before the thread reached the
+	 * thread function and set the IRQTF_READY bit. It's waiting in
+	 * kthread code with state UNINTERRUPTIBLE. Once it reaches the
+	 * thread function it waits with INTERRUPTIBLE. The wakeup is not
+	 * lost in that case because the thread is guaranteed to observe
+	 * the RUN flag before it goes to sleep in wait_for_interrupt().
+	 */
+	wake_up_state(action->thread, TASK_INTERRUPTIBLE);
 }
 
 static DEFINE_STATIC_KEY_FALSE(irqhandler_duration_check_enabled);
-- 
2.51.1