Date:	Thu, 08 Aug 2013 13:27:55 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	Steven Rostedt <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, C.Emde@...dl.org
Subject: Re: [PATCH 0/8] rcu: Ensure rcu read site is deadlock-immunity

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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