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Subject[PATCH v2] locking/rtmutex: Handle non enqueued waiters gracefully in remove_waiter()
From: Peter Zijlstra <peterz@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+gDohhXPVFCxWwzg@mail.gmail.com
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>
---
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
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