[PATCH] kernel: locking: rtmutex: Fix a possible sleep-in-atomic-context bug in rt_mutex_handle_deadlock()

[Jia-Ju Bai <baijiaju1990@gmail.com>]

The driver may sleep with holding a spinlock.

The function call paths (from bottom to top) in Linux-4.16 are:

[FUNC] schedule
kernel/locking/rtmutex.c, 1223: 
	schedule in rt_mutex_handle_deadlock
kernel/locking/rtmutex.c, 1273: 
	rt_mutex_handle_deadlock in rt_mutex_slowlock
kernel/locking/rtmutex.c, 1249: 
	_raw_spin_lock_irqsave in rt_mutex_slowlock

To fix the bug, the spinlock is released before schedule() and then acquired again.
This is found by my static analysis tool (DSAC).

Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
---
 kernel/locking/rtmutex.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 2823d4163a37..af03e162f812 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1205,7 +1205,7 @@ __rt_mutex_slowlock(struct rt_mutex *lock, int state,
 }
 
 static void rt_mutex_handle_deadlock(int res, int detect_deadlock,
-				     struct rt_mutex_waiter *w)
+				     struct rt_mutex_waiter *w, struct rt_mutex *lock)
 {
 	/*
 	 * If the result is not -EDEADLOCK or the caller requested
@@ -1219,8 +1219,10 @@ static void rt_mutex_handle_deadlock(int res, int detect_deadlock,
 	 */
 	rt_mutex_print_deadlock(w);
 	while (1) {
+		raw_spin_unlock_irq(&lock->wait_lock);
 		set_current_state(TASK_INTERRUPTIBLE);
 		schedule();
+		raw_spin_lock_irq(&lock->wait_lock);
 	}
 }
 
@@ -1269,7 +1271,7 @@ rt_mutex_slowlock(struct rt_mutex *lock, int state,
 	if (unlikely(ret)) {
 		__set_current_state(TASK_RUNNING);
 		remove_waiter(lock, &waiter);
-		rt_mutex_handle_deadlock(ret, chwalk, &waiter);
+		rt_mutex_handle_deadlock(ret, chwalk, &waiter, lock);
 	}
 
 	/*
-- 
2.17.0