From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753358AbdEIPLT (ORCPT ); Tue, 9 May 2017 11:11:19 -0400 Received: from Galois.linutronix.de ([146.0.238.70]:41303 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751304AbdEIPLR (ORCPT ); Tue, 9 May 2017 11:11:17 -0400 Date: Tue, 9 May 2017 17:11:10 +0200 (CEST) From: Thomas Gleixner To: LKML cc: Sebastian Sewior , Peter Zijlstra , Steven Rostedt , linux-rt-users , Engleder Gerhard Subject: [PATCH RT] futex/rtmutex: Cure RT double blocking issue Message-ID: User-Agent: Alpine 2.20 (DEB 67 2015-01-07) 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 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 --- kernel/locking/rtmutex.c | 17 +++++++++++++++++ 1 file changed, 17 insertions(+) --- a/kernel/locking/rtmutex.c +++ b/kernel/locking/rtmutex.c @@ -2380,6 +2380,7 @@ int rt_mutex_wait_proxy_lock(struct rt_m struct hrtimer_sleeper *to, struct rt_mutex_waiter *waiter) { + struct task_struct *tsk = current; int ret; raw_spin_lock_irq(&lock->wait_lock); @@ -2389,6 +2390,22 @@ int rt_mutex_wait_proxy_lock(struct rt_m /* 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. + */ + 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;