From mboxrd@z Thu Jan 1 00:00:00 1970 From: Steven Rostedt Subject: Re: BUG: spinlock trylock failure on UP, i.MX28 3.12.15-rt25 Date: Tue, 22 Apr 2014 13:48:02 -0400 Message-ID: <20140422134802.73fc1fa4@gandalf.local.home> 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> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: Stanislav Meduna , "linux-rt-users@vger.kernel.org" , Linux ARM Kernel , "linux-kernel@vger.kernel.org" , Thomas Gleixner , Peter Zijlstra To: Sebastian Andrzej Siewior Return-path: In-Reply-To: <53569E05.8010600@linutronix.de> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-rt-users.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 ] > >=20 > > On Tue, 22 Apr 2014 13:54:39 +0200 > > Sebastian Andrzej Siewior wrote: > > =20 > >> this is, erm, harmless. We grab the timer lock via trylock in hard= irq > >> 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=E2=80=A6 > >=20 > > trylock not failing on UP, can that be an issue? I mean, if a hardi= rq > > does a trylock to see if it can grab a lock that is not protected b= y > > 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. >=20 > 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*. >=20 > 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 tak= en > 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