From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757031AbcK3VHs (ORCPT ); Wed, 30 Nov 2016 16:07:48 -0500 Received: from Galois.linutronix.de ([146.0.238.70]:55434 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754355AbcK3VHZ (ORCPT ); Wed, 30 Nov 2016 16:07:25 -0500 Message-Id: <20161130210030.591941927@linutronix.de> User-Agent: quilt/0.63-1 Date: Wed, 30 Nov 2016 21:04:45 -0000 From: Thomas Gleixner To: LKML Cc: David Daney , Peter Zijlstra , Ingo Molnar , Steven Rostedt , Sebastian Siewior , Will Deacon , Mark Rutland Subject: [patch 4/4] rtmutex: Explain locking rules for rt_mutex_proxy_unlock()/init_proxy_locked() References: <20161130205431.629977871@linutronix.de> MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-15 Content-Disposition: inline; filename=rtmutex--Explain-locking-rules-for-rt_mutex_proxy_unlock.patch Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org While debugging the unlock vs. dequeue race which resulted in state corruption of futexes the lockless nature of rt_mutex_proxy_unlock() caused some confusion. Add commentry to explain why it is safe to do this lockless. Add matching comments to rt_mutex_init_proxy_locked() for completeness sake. Signed-off-by: Thomas Gleixner --- kernel/locking/rtmutex.c | 16 ++++++++++++---- 1 file changed, 12 insertions(+), 4 deletions(-) --- a/kernel/locking/rtmutex.c +++ b/kernel/locking/rtmutex.c @@ -1617,11 +1617,15 @@ EXPORT_SYMBOL_GPL(__rt_mutex_init); * rt_mutex_init_proxy_locked - initialize and lock a rt_mutex on behalf of a * proxy owner * - * @lock: the rt_mutex to be locked + * @lock: the rt_mutex to be locked * @proxy_owner:the task to set as owner * * No locking. Caller has to do serializing itself - * Special API call for PI-futex support + * + * Special API call for PI-futex support. This initializes the rtmutex and + * assigns it to @proxy_owner. Concurrent operations on the rtmutex are not + * possible at this point because the pi_state which contains the rtmutex + * is not yet visible to other tasks. */ void rt_mutex_init_proxy_locked(struct rt_mutex *lock, struct task_struct *proxy_owner) @@ -1635,10 +1639,14 @@ void rt_mutex_init_proxy_locked(struct r /** * rt_mutex_proxy_unlock - release a lock on behalf of owner * - * @lock: the rt_mutex to be locked + * @lock: the rt_mutex to be locked * * No locking. Caller has to do serializing itself - * Special API call for PI-futex support + * + * Special API call for PI-futex support. This merily cleans up the rtmutex + * (debugging) state. Concurrent operations on this rt_mutex are not + * possible because it belongs to the pi_state which is about to be freed + * and it is not longer visible to other tasks. */ void rt_mutex_proxy_unlock(struct rt_mutex *lock, struct task_struct *proxy_owner)