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 <[email protected]>
Suggested-by: Thomas Gleixner <[email protected]>
Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Link: https://lkml.kernel.org/r/CAAh1qt=DCL9aUXNxanP5BKtiPp3m+qj4yB+gDohhXPVFCxWwzg@mail.gmail.com
Link: https://lkml.kernel.org/r/[email protected]

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 940633c632541d7aa31eb68ef7c271f17bb86369..4f014be7a4b8bf9a4de974377592374c75031cab 100644 (file)
--- 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 68686b3ec3c17cb199a72300f06e2904640ff189..d1d62f942be2285a7e11588201ac199dc700a805 100644 (file)
--- 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;
 }