Commit c28d62cf authored Mar 27, 2018 by Peter Zijlstra Committed by Thomas Gleixner Mar 28, 2018

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

parent ac605bee

kernel/locking/rtmutex.c

+1 −2

Original line number	Diff line number	Diff line
		@@ -1268,7 +1268,6 @@ 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);
		rt_mutex_handle_deadlock(ret, chwalk, &waiter);
		}

kernel/locking/rtmutex_common.h

+6 −5

Original line number	Diff line number	Diff line
		@@ -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;
		struct rb_node *leftmost = rb_first_cached(&lock->waiters);
		struct rt_mutex_waiter *w = NULL;

		w = rb_entry(lock->waiters.rb_leftmost,
		struct rt_mutex_waiter, tree_entry);
		if (leftmost) {
		w = rb_entry(leftmost, struct rt_mutex_waiter, tree_entry);
		BUG_ON(w->lock != lock);

		}
		return w;
		}

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