Messages in this thread | | | Date | Thu, 08 Aug 2013 13:27:55 +0800 | From | Lai Jiangshan <> | Subject | Re: [PATCH 0/8] rcu: Ensure rcu read site is deadlock-immunity |
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On 08/08/2013 12:18 PM, Paul E. McKenney wrote: > On Thu, Aug 08, 2013 at 11:10:47AM +0800, Lai Jiangshan wrote: >> On 08/08/2013 10:33 AM, Paul E. McKenney wrote: >>> On Thu, Aug 08, 2013 at 10:33:15AM +0800, Lai Jiangshan wrote: >>>> On 08/08/2013 10:12 AM, Steven Rostedt wrote: >>>>> On Thu, 2013-08-08 at 09:47 +0800, Lai Jiangshan wrote: >>>>> >>>>>>> [ 393.641012] CPU0 >>>>>>> [ 393.641012] ---- >>>>>>> [ 393.641012] lock(&lock->wait_lock); >>>>>>> [ 393.641012] <Interrupt> >>>>>>> [ 393.641012] lock(&lock->wait_lock); >>>>>> >>>>>> Patch2 causes it! >>>>>> When I found all lock which can (chained) nested in rcu_read_unlock_special(), >>>>>> I didn't notice rtmutex's lock->wait_lock is not nested in irq-disabled. >>>>>> >>>>>> Two ways to fix it: >>>>>> 1) change rtmutex's lock->wait_lock, make it alwasys irq-disabled. >>>>>> 2) revert my patch2 >>>>> >>>>> Your patch 2 states: >>>>> >>>>> "After patch 10f39bb1, "special & RCU_READ_UNLOCK_BLOCKED" can't be true >>>>> in irq nor softirq.(due to RCU_READ_UNLOCK_BLOCKED can only be set >>>>> when preemption)" >>>> >>>> Patch5 adds "special & RCU_READ_UNLOCK_BLOCKED" back in irq nor softirq. >>>> This new thing is handle in patch5 if I did not do wrong things in patch5. >>>> (I don't notice rtmutex's lock->wait_lock is not irqs-disabled in patch5) >>>> >>>>> >>>>> But then below we have: >>>>> >>>>> >>>>>> >>>>>>> [ 393.641012] >>>>>>> [ 393.641012] *** DEADLOCK *** >>>>>>> [ 393.641012] >>>>>>> [ 393.641012] no locks held by rcu_torture_rea/697. >>>>>>> [ 393.641012] >>>>>>> [ 393.641012] stack backtrace: >>>>>>> [ 393.641012] CPU: 3 PID: 697 Comm: rcu_torture_rea Not tainted 3.11.0-rc1+ #1 >>>>>>> [ 393.641012] Hardware name: Bochs Bochs, BIOS Bochs 01/01/2007 >>>>>>> [ 393.641012] ffffffff8586fea0 ffff88001fcc3a78 ffffffff8187b4cb ffffffff8104a261 >>>>>>> [ 393.641012] ffff88001e1a20c0 ffff88001fcc3ad8 ffffffff818773e4 0000000000000000 >>>>>>> [ 393.641012] ffff880000000000 ffff880000000001 ffffffff81010a0a 0000000000000001 >>>>>>> [ 393.641012] Call Trace: >>>>>>> [ 393.641012] <IRQ> [<ffffffff8187b4cb>] dump_stack+0x4f/0x84 >>>>>>> [ 393.641012] [<ffffffff8104a261>] ? console_unlock+0x291/0x410 >>>>>>> [ 393.641012] [<ffffffff818773e4>] print_usage_bug+0x1f5/0x206 >>>>>>> [ 393.641012] [<ffffffff81010a0a>] ? save_stack_trace+0x2a/0x50 >>>>>>> [ 393.641012] [<ffffffff810ae603>] mark_lock+0x283/0x2e0 >>>>>>> [ 393.641012] [<ffffffff810ada10>] ? print_irq_inversion_bug.part.40+0x1f0/0x1f0 >>>>>>> [ 393.641012] [<ffffffff810aef66>] __lock_acquire+0x906/0x1d40 >>>>>>> [ 393.641012] [<ffffffff810ae94b>] ? __lock_acquire+0x2eb/0x1d40 >>>>>>> [ 393.641012] [<ffffffff810ae94b>] ? __lock_acquire+0x2eb/0x1d40 >>>>>>> [ 393.641012] [<ffffffff810b0a65>] lock_acquire+0x95/0x210 >>>>>>> [ 393.641012] [<ffffffff818860f3>] ? rt_mutex_unlock+0x53/0x100 >>>>>>> [ 393.641012] [<ffffffff81886d26>] _raw_spin_lock+0x36/0x50 >>>>>>> [ 393.641012] [<ffffffff818860f3>] ? rt_mutex_unlock+0x53/0x100 >>>>>>> [ 393.641012] [<ffffffff818860f3>] rt_mutex_unlock+0x53/0x100 > > The really strange thing here is that I thought that your passing false > in as the new second parameter to rcu_read_unlock_special() was supposed > to prevent rt_mutex_unlock() from being called. > > But then why is the call from rcu_preempt_note_context_switch() also > passing false? I would have expected that one to pass true. Probably > I don't understand your intent with the "unlock" argument. > >>>>>>> [ 393.641012] [<ffffffff810ee3ca>] rcu_read_unlock_special+0x17a/0x2a0 >>>>>>> [ 393.641012] [<ffffffff810ee803>] rcu_check_callbacks+0x313/0x950 >>>>>>> [ 393.641012] [<ffffffff8107a6bd>] ? hrtimer_run_queues+0x1d/0x180 >>>>>>> [ 393.641012] [<ffffffff810abb9d>] ? trace_hardirqs_off+0xd/0x10 >>>>>>> [ 393.641012] [<ffffffff8105bae3>] update_process_times+0x43/0x80 >>>>>>> [ 393.641012] [<ffffffff810a9801>] tick_sched_handle.isra.10+0x31/0x40 >>>>>>> [ 393.641012] [<ffffffff810a98f7>] tick_sched_timer+0x47/0x70 >>>>>>> [ 393.641012] [<ffffffff8107941c>] __run_hrtimer+0x7c/0x490 >>>>>>> [ 393.641012] [<ffffffff810a260d>] ? ktime_get_update_offsets+0x4d/0xe0 >>>>>>> [ 393.641012] [<ffffffff810a98b0>] ? tick_nohz_handler+0xa0/0xa0 >>>>>>> [ 393.641012] [<ffffffff8107a017>] hrtimer_interrupt+0x107/0x260 >>>>> >>>>> The hrtimer_interrupt is calling a rt_mutex_unlock? How did that happen? >>>>> Did it first call a rt_mutex_lock? >>>>> >>>>> If patch two was the culprit, I'm thinking the idea behind patch two is >>>>> wrong. The only option is to remove patch number two! >>>> >>>> removing patch number two can solve the problem found be Paul, but it is not the best. >>>> because I can't declare that rcu is deadlock-immunity >>>> (it will be deadlock if rcu read site overlaps with rtmutex's lock->wait_lock >>>> if I only remove patch2) >>>> I must do more things, but I think it is still better than changing rtmutex's lock->wait_lock. >>> >>> NP, I will remove your current patches and wait for an updated set. >> >> Hi, Paul >> >> Could you agree that moving the rt_mutex_unlock() to rcu_preempt_note_context_switch()? > > My guess is that changing rcu_preempt_note_context_switch()'s call to > rcu_read_unlock_special(t, true) would accomplish this in a nicer way. > Except that I would first need to understand why rcu_check_callbacks()'s > call to rcu_read_unlock_special() resulted in rt_mutex_unlock() being > called. > > Oh! > > In rcu_read_unlock_special, shouldn't the: > > if (unlikely(unlock && irqs_disabled_flags(flags))) {
Sorry. @unlock means it is called from rcu_read_unlock() path.
if rcu_read_unlock() is called from irqs_disabled context, it may be called from suspect lock(scheduler locks, ...) context, so it can't require rnp->lock or invoke wakeup() in rcu_read_unlock_special().
> > Instead be: > > if (unlikely(!unlock || irqs_disabled_flags(flags))) { > > Here I am guessing that the "unlock" parameter means "It is OK for > rcu_read_unlock_special() to invoke rt_mutex_unlock()", so it would be > passed in as false from rcu_check_callbacks() and true everywhere else. > If it means something else, please let me know what that might be. > > Though at this point, it is not clear to me how it helps to call > rcu_read_unlock_special() from rcu_check_callbacks(). After all, > rcu_check_callbacks() has interrupts disabled always, and so it is never > safe for anything that it calls to invoke rt_mutex_unlock(). > > In any case, the opinion that really matters is not mine, but rather > that of the hundreds of millions of computer systems that might soon be > executing this code. As RCU maintainer, I just try my best to predict > what their opinions will be. ;-) > > Thanx, Paul > >> thanks, >> Lai >> >>> >>> Thanx, Paul >>> >>> >> > >
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