Messages in this thread Patch in this message | | | Date | Fri, 16 Mar 2018 13:20:41 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] kernel/rtmutex: Handle non enqueued waiters gracefully in remove_waiter() |
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On Mon, Mar 12, 2018 at 03:28:45PM +0100, Sebastian Andrzej Siewior wrote: > 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() with > the following note: > > | Guard it with rt_mutex_has_waiters(). This is a quick fix which is > | easy to backport. The proper fix is to have a central check in > | remove_waiter() so we can call it unconditionally. > > This is the suggested change. > Now that it is possible to call remove_waiter() unconditionally I also > remove that check from rt_mutex_slowlock(). > > Link: https://lkml.kernel.org/r/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: Sebastian Andrzej Siewior <bigeasy@linutronix.de> > --- > kernel/locking/rtmutex.c | 8 +++++--- > 1 file changed, 5 insertions(+), 3 deletions(-) > > diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c > index 65cc0cb984e6..57d28d8f5280 100644 > --- a/kernel/locking/rtmutex.c > +++ b/kernel/locking/rtmutex.c > @@ -1068,12 +1068,15 @@ static void mark_wakeup_next_waiter(struct wake_q_head *wake_q, > static void remove_waiter(struct rt_mutex *lock, > struct rt_mutex_waiter *waiter) > { > - bool is_top_waiter = (waiter == rt_mutex_top_waiter(lock));
I'm a little confused, but isn't it easier to make rt_mutex_top_waiter() return NULL if there aren't in fact any waiters?
diff --git a/kernel/locking/rtmutex_common.h b/kernel/locking/rtmutex_common.h index 68686b3ec3c1..70bcafc385c4 100644 --- a/kernel/locking/rtmutex_common.h +++ b/kernel/locking/rtmutex_common.h @@ -52,11 +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); - BUG_ON(w->lock != lock); + if (leftmost) { + w = rb_entry(leftmost, struct rt_mutex_waiter, tree_entry); + BUG_ON(w->lock != lock); + } return w; }
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