From: Peter Zijlstra <pet...@infradead.org> 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 we must not call remove_waiter() 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 check before calling rt_mutex_top_waiter() we could make it return NULL if there are no waiters. Now that it is possible to call remove_waiter() unconditionally I also remove that check from rt_mutex_slowlock(). Link: https://lkml.kernel.org/CAAh1qt=dcl9auxnxanp5bktipp3m+qj4yb+gdohhxpvfcxw...@mail.gmail.com Reported-and-debugged-by: Yimin Deng <yimin11.d...@gmail.com> Suggested-by: Thomas Gleixner <t...@linutronix.de> Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org> Signed-off-by: Sebastian Andrzej Siewior <bige...@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 65cc0cb984e6..355716d03b1a 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; } -- 2.16.3
