Date:	Thu, 8 Aug 2013 00:05:51 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Lai Jiangshan <laijs@...fujitsu.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 Thu, Aug 08, 2013 at 01:27:55PM +0800, Lai Jiangshan wrote:
> 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().

Hmmm...  And the case where it rcu_read_unlock_special() is
called from rcu_preempt_note_context_switch()?

							Thanx, Paul

> > 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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