On Tue, 22 Apr 2014 18:51:17 +0200 Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> 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 <bigeasy@xxxxxxxxxxxxx> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
