[PATCH 1/7] locking/rt: Use raw_spin_lock_irqsave() in __rwbase_read_unlock()

[David Woodhouse <dwmw2@infradead.org>]

From: David Woodhouse <dwmw@amazon.co.uk>

__rwbase_read_unlock() uses raw_spin_lock_irq()/raw_spin_unlock_irq()
which unconditionally disables and re-enables interrupts. When
read_unlock() is called from hardirq context (e.g. after a successful
read_trylock() in a timer callback), the raw_spin_unlock_irq()
incorrectly re-enables interrupts within the hardirq handler.

This causes lockdep warnings ('hardirqs_on_prepare' from hardirq
context) and can lead to IRQ state corruption.

Using read_trylock() in hardirq context on PREEMPT_RT is safe because
it does not record the lock owner. The read_unlock() acquires the
wait_lock which is hardirq safe. This change additionally allows
rwlock_t during early boot.

Switch to raw_spin_lock_irqsave()/raw_spin_unlock_irqrestore() to
preserve the caller's IRQ state.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---
 kernel/locking/rwbase_rt.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/locking/rwbase_rt.c b/kernel/locking/rwbase_rt.c
index 82e078c0665a..25744862d627 100644
--- a/kernel/locking/rwbase_rt.c
+++ b/kernel/locking/rwbase_rt.c
@@ -153,8 +153,9 @@ static void __sched __rwbase_read_unlock(struct rwbase_rt *rwb,
 	struct rt_mutex_base *rtm = &rwb->rtmutex;
 	struct task_struct *owner;
 	DEFINE_RT_WAKE_Q(wqh);
+	unsigned long flags;
 
-	raw_spin_lock_irq(&rtm->wait_lock);
+	raw_spin_lock_irqsave(&rtm->wait_lock, flags);
 	/*
 	 * Wake the writer, i.e. the rtmutex owner. It might release the
 	 * rtmutex concurrently in the fast path (due to a signal), but to
@@ -167,7 +168,7 @@ static void __sched __rwbase_read_unlock(struct rwbase_rt *rwb,
 
 	/* Pairs with the preempt_enable in rt_mutex_wake_up_q() */
 	preempt_disable();
-	raw_spin_unlock_irq(&rtm->wait_lock);
+	raw_spin_unlock_irqrestore(&rtm->wait_lock, flags);
 	rt_mutex_wake_up_q(&wqh);
 }
 
-- 
2.51.0