Messages in this thread | | | From | "Rafael J. Wysocki" <> | Subject | Re: [Query] Preemption (hogging) of the work handler | Date | Thu, 14 Jul 2016 16:47:11 +0200 |
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On Thursday, July 14, 2016 04:39:39 PM Jan Kara wrote: > On Thu 14-07-16 16:33:38, Rafael J. Wysocki wrote: > > On Thursday, July 14, 2016 04:12:16 PM Jan Kara wrote: > > > On Wed 13-07-16 14:45:07, Sergey Senozhatsky wrote: > > > > Cc Petr Mladek. > > > > > > > > On (07/12/16 16:19), Viresh Kumar wrote: > > > > [..] > > > > > Okay, we have tracked this BUG and its really interesting. > > > > > > > > good find! > > > > > > > > > I hacked the platform's serial driver to implement a putchar() routine > > > > > that simply writes to the FIFO in polling mode, that helped us in > > > > > tracing on where we are going wrong. > > > > > > > > > > The problem is that we are running asynchronous printks and we call > > > > > wake_up_process() from the last running CPU which has disabled > > > > > interrupts. That takes us to: try_to_wake_up(). > > > > > > > > > > In our case the CPU gets deadlocked on this line in try_to_wake_up(). > > > > > > > > > > raw_spin_lock_irqsave(&p->pi_lock, flags); > > > > > > > > yeah, printk() can't handle these types of recursion. it can prevent > > > > printk() calls issued from within the logbuf_lock spinlock section, > > > > with some limitations: > > > > > > > > if (unlikely(logbuf_cpu == smp_processor_id())) { > > > > recursion_bug = true; > > > > return; > > > > } > > > > > > > > raw_spin_lock(&logbuf_lock); > > > > logbuf_cpu = this_cpu; > > > > ... > > > > logbuf_cpu = UINT_MAX; > > > > raw_spin_unlock(&logbuf_lock); > > > > > > > > so should, for instance, raw_spin_unlock() generate spin_dump(), printk() > > > > will blow up (both sync and async), because logbuf_cpu is already reset. > > > > it may look that async printk added another source of recursion - wake_up(). > > > > but, apparently, this is not exactly correct. because there is already a > > > > wake_up() call in console_unlock() - up(). > > > > > > > > printk() > > > > if (logbuf_cpu == smp_processor_id()) > > > > return; > > > > > > > > raw_spin_lock(&logbuf_lock); > > > > logbuf_cpu = this_cpu; > > > > ... > > > > logbuf_cpu = UINT_MAX; > > > > raw_spin_unlock(&logbuf_lock); > > > > > > > > console_trylock() > > > > raw_spin_lock_irqsave(&sem->lock) << *** > > > > raw_spin_unlock_irqsave(&sem->lock) << *** > > > > > > > > console_unlock() > > > > up() > > > > raw_spin_lock_irqsave(&sem->lock) << *** > > > > __up() > > > > wake_up_process() > > > > try_to_wake_up() << *** in may places > > > > > > > > > > > > *** a printk() call from here will kill the system. either it will > > > > recurse printk(), or spin forever in 'nested' printk() on one of > > > > the already taken spin locks. > > > > > > Exactly. Calling printk() from certain parts of the kernel (like scheduler > > > code or timer code) has been always unsafe because printk itself uses these > > > parts and so it can lead to deadlocks. That's why printk_deffered() has > > > been introduced as you mention below. > > > > > > And with sync printk the above deadlock doesn't trigger only by chance - if > > > there happened to be a waiter on console_sem while we suspend, the same > > > deadlock would trigger because up(&console_sem) will try to wake him up and > > > the warning in timekeeping code will cause recursive printk. > > > > > > So I think your patch doesn't really address the real issue - it only > > > works around the particular WARN_ON(timekeeping_enabled) warning but if > > > there was a different warning in timekeeping code which would trigger, it > > > has a potential for causing recursive printk deadlock (and indeed we had > > > such issues previously - see e.g. 504d58745c9c "timer: Fix lock inversion > > > between hrtimer_bases.lock and scheduler locks"). > > > > > > So there are IMHO two issues here worth looking at: > > > > > > 1) I didn't find how a wakeup would would lead to calling to ktime_get() in > > > the current upstream kernel or even current RT kernel. Maybe this is a > > > problem specific to the 3.10 kernel you are using? If yes, we don't have to > > > do anything for current upstream AFAIU. > > > > > > If I just missed how wakeup can call into ktime_get() in current upstream, > > > there is another question: > > > > > > 2) Is it OK that printk calls wakeup so late during suspend? I believe it > > > is but I'm neither scheduler nor suspend expert. > > > > I don't think it really is OK. Nothing will wake up for sure at this point, > > so why to do that in the first place? > > So that the process is put into a runnable state (currently it is in > uninterruptible sleep) and may run after the system resumes?
Fair enough.
But calling ktime_get() with suspended timekeeping is dumb at best which is why the warning is there.
Thanks, Rafael
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