From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933592AbaDVRsI (ORCPT ); Tue, 22 Apr 2014 13:48:08 -0400 Received: from cdptpa-outbound-snat.email.rr.com ([107.14.166.227]:14530 "EHLO cdptpa-oedge-vip.email.rr.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S932927AbaDVRsG convert rfc822-to-8bit (ORCPT ); Tue, 22 Apr 2014 13:48:06 -0400 Date: Tue, 22 Apr 2014 13:48:02 -0400 From: Steven Rostedt To: Sebastian Andrzej Siewior Cc: Stanislav Meduna , "linux-rt-users@vger.kernel.org" , Linux ARM Kernel , "linux-kernel@vger.kernel.org" , Thomas Gleixner , Peter Zijlstra Subject: Re: BUG: spinlock trylock failure on UP, i.MX28 3.12.15-rt25 Message-ID: <20140422134802.73fc1fa4@gandalf.local.home> In-Reply-To: <53569E05.8010600@linutronix.de> References: <534C3606.7010206@meduna.org> <534C731F.1050406@meduna.org> <534DADF1.6060608@meduna.org> <20140422115439.GA20669@linutronix.de> <20140422094657.5b6ca1e2@gandalf.local.home> <53569E05.8010600@linutronix.de> X-Mailer: Claws Mail 3.9.3 (GTK+ 2.24.22; x86_64-pc-linux-gnu) MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT X-RR-Connecting-IP: 107.14.168.142:25 X-Cloudmark-Score: 0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, 22 Apr 2014 18:51:17 +0200 Sebastian Andrzej Siewior wrote: > On 04/22/2014 03:46 PM, Steven Rostedt wrote: > > [ added Peter ] > > > > On Tue, 22 Apr 2014 13:54:39 +0200 > > Sebastian Andrzej Siewior wrote: > > > >> this is, erm, harmless. We grab the timer lock via trylock in hardirq > >> context. If the lock is already taken then we fail to get it we go for > >> plan B. According to lockdep a trylock should not fail on UP. This is > >> true in general except for this timer case. I was thinking abour > >> disabling this lockdep check… > > > > trylock not failing on UP, can that be an issue? I mean, if a hardirq > > does a trylock to see if it can grab a lock that is not protected by > > disabling irqs, and will go to plan B if it fails, on UP, it will > > always get it. But the issue is still there. That would mean that a > > hardirq could have preempted a critical section and doing a trylock > > here would succeed when it really should have failed. > > If you take a lock with irqs enabled and disabled then lockdep should > complain about it. There's nothing wrong with taking locks with irqs enabled and disabled. It's only wrong if that lock (or a lock that is held when the lock is taken) is also taking in interrupt *context*. > > This is the ->wait_lock of the timer base lock. This (sleeping) lock is > usually taken with interrupts enabled. Except here, in the timer > callback, we check if the lock is available or not. And this lock may > be a) taken (and the ->wait_lock unlocked) or b) in process to be taken > but the caller only succeeded to acquire the ->wait_lock before the > interrupt occurred. This is the case here and we can't acquire the > ->wait_lock a second time the check if the lock is really taken. But > since the wait_lock is occupied it is likely that the lock itself is > occupied as well. I need to take a deeper look into the actual code. But as trylocks on UP are nops (always succeed), and if it expects to be able to do something in a critical section that is protected by spinlocks (again nops on UP), this would be broken for UP. -- Steve