From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756749AbdEKPVT convert rfc822-to-8bit (ORCPT ); Thu, 11 May 2017 11:21:19 -0400 Received: from Galois.linutronix.de ([146.0.238.70]:49737 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753147AbdEKPVF (ORCPT ); Thu, 11 May 2017 11:21:05 -0400 Date: Thu, 11 May 2017 17:20:54 +0200 From: Sebastian Sewior To: Thomas Gleixner Cc: Wanpeng Li , LKML , Peter Zijlstra , Steven Rostedt , linux-rt-users , Engleder Gerhard Subject: [PATCH RT v2] futex/rtmutex: Cure RT double blocking issue Message-ID: <20170511152054.bpgik7etss7umxzq@linutronix.de> References: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8BIT In-Reply-To: User-Agent: NeoMutt/20170306 (1.8.0) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org RT has a problem when the wait on a futex/rtmutex got interrupted by a timeout or a signal. task->pi_blocked_on is still set when returning from rt_mutex_wait_proxy_lock(). The task must acquire the hash bucket lock after this. If the hash bucket lock is contended then the BUG_ON(rt_mutex_real_waiter(task->pi_blocked_on)) in task_blocks_on_rt_mutex() will trigger. This can be avoided by clearing task->pi_blocked_on in the return path of rt_mutex_wait_proxy_lock() which removes the task from the boosting chain of the rtmutex. That's correct because the task is not longer blocked on it. Signed-off-by: Thomas Gleixner Reported-by: Engleder Gerhard Signed-off-by: Sebastian Andrzej Siewior --- v1…v2: reset ->pi_blocked_on only in the error case. kernel/locking/rtmutex.c | 19 +++++++++++++++++++ 1 file changed, 19 insertions(+) diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c index 314fc65a35b1..4675f1197f33 100644 --- a/kernel/locking/rtmutex.c +++ b/kernel/locking/rtmutex.c @@ -2400,6 +2400,7 @@ int rt_mutex_wait_proxy_lock(struct rt_mutex *lock, struct hrtimer_sleeper *to, struct rt_mutex_waiter *waiter) { + struct task_struct *tsk = current; int ret; raw_spin_lock_irq(&lock->wait_lock); @@ -2409,6 +2410,24 @@ int rt_mutex_wait_proxy_lock(struct rt_mutex *lock, /* sleep on the mutex */ ret = __rt_mutex_slowlock(lock, TASK_INTERRUPTIBLE, to, waiter, NULL); + /* + * RT has a problem here when the wait got interrupted by a timeout + * or a signal. task->pi_blocked_on is still set. The task must + * acquire the hash bucket lock when returning from this function. + * + * If the hash bucket lock is contended then the + * BUG_ON(rt_mutex_real_waiter(task->pi_blocked_on)) in + * task_blocks_on_rt_mutex() will trigger. This can be avoided by + * clearing task->pi_blocked_on which removes the task from the + * boosting chain of the rtmutex. That's correct because the task + * is not longer blocked on it. + */ + if (ret) { + raw_spin_lock(&tsk->pi_lock); + tsk->pi_blocked_on = NULL; + raw_spin_unlock(&tsk->pi_lock); + } + raw_spin_unlock_irq(&lock->wait_lock); return ret; -- 2.11.0