From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752106AbeCPMUu (ORCPT ); Fri, 16 Mar 2018 08:20:50 -0400 Received: from merlin.infradead.org ([205.233.59.134]:39802 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751236AbeCPMUs (ORCPT ); Fri, 16 Mar 2018 08:20:48 -0400 Date: Fri, 16 Mar 2018 13:20:41 +0100 From: Peter Zijlstra To: Sebastian Andrzej Siewior Cc: linux-kernel@vger.kernel.org, Ingo Molnar , Thomas Gleixner Subject: Re: [PATCH] kernel/rtmutex: Handle non enqueued waiters gracefully in remove_waiter() Message-ID: <20180316122041.GE4064@hirez.programming.kicks-ass.net> References: <20180312142845.m4ohqyg3mak34umf@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180312142845.m4ohqyg3mak34umf@linutronix.de> User-Agent: Mutt/1.9.3 (2018-01-21) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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? 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; }