From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933090AbeCSPLf (ORCPT ); Mon, 19 Mar 2018 11:11:35 -0400 Received: from Galois.linutronix.de ([146.0.238.70]:60492 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932496AbeCSPLe (ORCPT ); Mon, 19 Mar 2018 11:11:34 -0400 Date: Mon, 19 Mar 2018 16:11:31 +0100 (CET) From: Thomas Gleixner To: Peter Zijlstra cc: Sebastian Andrzej Siewior , linux-kernel@vger.kernel.org, Ingo Molnar Subject: Re: [PATCH] kernel/rtmutex: Handle non enqueued waiters gracefully in remove_waiter() In-Reply-To: <20180316122041.GE4064@hirez.programming.kicks-ass.net> Message-ID: References: <20180312142845.m4ohqyg3mak34umf@linutronix.de> <20180316122041.GE4064@hirez.programming.kicks-ass.net> User-Agent: Alpine 2.21 (DEB 202 2017-01-01) MIME-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, 16 Mar 2018, Peter Zijlstra wrote: > 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 > > Suggested-by: Thomas Gleixner > > Signed-off-by: Sebastian Andrzej Siewior > > --- > > 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? That works as well. Thanks, tglx > 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; > } >