locking/rtmutex: Handle non enqueued waiters gracefully in remove_waiter()

In -RT task_blocks_on_rt_mutex() may return with -EAGAIN due to
(->pi_blocked_on == PI_WAKEUP_INPROGRESS) before it added itself as a
waiter. In such a case remove_waiter() must not be called because without a
waiter it will trigger the BUG_ON() statement.

This was initially reported by Yimin Deng. Thomas Gleixner fixed it then
with an explicit check for waiters before calling remove_waiter().

Instead of an explicit NULL check before calling rt_mutex_top_waiter() make
the function return NULL if there are no waiters. With that fixed the now
pointless NULL check is removed from rt_mutex_slowlock().

Reported-and-debugged-by: Yimin Deng <yimin11.deng@gmail.com>
Suggested-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/CAAh1qt=DCL9aUXNxanP5BKtiPp3m+qj4yB+gDohhXPVFCxWwzg@mail.gmail.com
Link: https://lkml.kernel.org/r/20180327121438.sss7hxg3crqy4ecd@linutronix.de

1 files changed, 6 insertions, 5 deletions

diff --git a/kernel/locking/rtmutex_common.h b/kernel/locking/rtmutex_common.h
index 68686b3ec3c1..d1d62f942be2 100644
--- a/kernel/locking/rtmutex_common.h
+++ b/kernel/locking/rtmutex_common.h
@@ -52,12 +52,13 @@ static inline int rt_mutex_has_waiters(struct rt_mutex *lock)
 static inline struct rt_mutex_waiter *
 rt_mutex_top_waiter(struct rt_mutex *lock)
 {
-        struct rt_mutex_waiter *w;
-
-        w = rb_entry(lock->waiters.rb_leftmost,
-                     struct rt_mutex_waiter, tree_entry);
-        BUG_ON(w->lock != lock);
+        struct rb_node *leftmost = rb_first_cached(&lock->waiters);
+        struct rt_mutex_waiter *w = NULL;
 
+        if (leftmost) {
+                w = rb_entry(leftmost, struct rt_mutex_waiter, tree_entry);
+                BUG_ON(w->lock != lock);
+        }
         return w;
 }