[tip:locking/core] locking/rtmutex: Handle non enqueued waiters gracefully in remove_waiter()

[tip-bot for Peter Zijlstra <tipbot@zytor.com>]

Commit-ID:  c28d62cf52d791ba5f6db7ce525ed06b86291c82
Gitweb:     https://git.kernel.org/tip/c28d62cf52d791ba5f6db7ce525ed06b86291c82
Author:     Peter Zijlstra <peterz@infradead.org>
AuthorDate: Tue, 27 Mar 2018 14:14:38 +0200
Committer:  Thomas Gleixner <tglx@linutronix.de>
CommitDate: Wed, 28 Mar 2018 23:01:30 +0200

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
---
 kernel/locking/rtmutex.c        |  3 +--
 kernel/locking/rtmutex_common.h | 11 ++++++-----
 2 files changed, 7 insertions(+), 7 deletions(-)

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 940633c63254..4f014be7a4b8 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1268,8 +1268,7 @@ rt_mutex_slowlock(struct rt_mutex *lock, int state,
 
 	if (unlikely(ret)) {
 		__set_current_state(TASK_RUNNING);
-		if (rt_mutex_has_waiters(lock))
-			remove_waiter(lock, &waiter);
+		remove_waiter(lock, &waiter);
 		rt_mutex_handle_deadlock(ret, chwalk, &waiter);
 	}
 
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;
 }