Re: [bug] Re: [PATCH] - Fix stack overflow for large values of MAX_APICS

[Paul E. McKenney]

On Thu, Jun 26, 2008 at 07:41:25AM -0500, Jack Steiner wrote:
> On Thu, Jun 26, 2008 at 02:32:31PM +0200, Ingo Molnar wrote:
> > 
> > * Jack Steiner <steiner@xxxxxxx> wrote:
> > 
> > > >> I added trace code & isolated the hang to a call to 
> > > >> synchronize_rcu(). Usually from netlink_change_ngroups().
> > > >> 
> > > >> If I boot with "maxcpus=1, it never hangs (obviously) but always fails
> > > >> to mount /root.
> > > >> 
> > > >> Next I changed NR_CPUS to 128. I still see random hangs.
> > > >> 
> > > >> 
> > > >> I'll chase this more tomorrow. Has anyone else seen any failures that might be
> > > >> related???
> > > >> 
> > > >> 
> > > 
> > > Is this already fixed? I see a number of patches to this area have been merged
> > > since the failure occurred.
> > > 
> > > I added enough hacks to get backtraces on threads at the time a hang occurs.
> > > show_state() shows 79 "kstopmachine" tasks. Most have one of the following backtraces:
> > > 
> > > 
> > > 	<6>kstopmachine  R  running task     6400   375    369
> > > 	 ffff8101ad28bd80 ffffffff8068c5c6 ffff8101ad28bb20 0000000000000002
> > > 	 0000000000000046 0000000000000000 0000000000002f42 ffff8101ad28c8b8
> > > 	 ffff8101ad28bb90 ffffffff80254fac 0000000100000000 0000000000000000
> > > 	Call Trace:
> > > 	   [<ffffffff8068c5c6>] ? thread_return+0x4d/0xbd
> > > 	   [<ffffffff80254fac>] ? __lock_acquire+0x643/0x6ad
> > > 	   [<ffffffff80212507>] ? sched_clock+0x9/0xc
> > > 	   [<ffffffff8022cf41>] ? update_curr_rt+0x111/0x11a
> > > 	   [<ffffffff80212507>] ? sched_clock+0x9/0xc
> > > 	   [<ffffffff8068c5f2>] ? thread_return+0x79/0xbd
> > > 	   [<ffffffff8068ef48>] ? _spin_unlock_irq+0x2b/0x37
> > > 	   [<ffffffff8068c5f2>] ? thread_return+0x79/0xbd
> > > 	   [<ffffffff80254fac>] ? __lock_acquire+0x643/0x6ad
> > > 	   [<ffffffff80212507>] ? sched_clock+0x9/0xc
> > > 	   [<ffffffff8068c806>] wait_for_common+0x150/0x160
> > > 	   [<ffffffff8068ef48>] ? _spin_unlock_irq+0x2b/0x37
> > > 	   [<ffffffff80254fac>] ? __lock_acquire+0x643/0x6ad
> > > 	   [<ffffffff80212507>] ? sched_clock+0x9/0xc
> > > 	   [<ffffffff8023422b>] ? sys_sched_yield+0x0/0x6e
> > > 	   [<ffffffff8026736d>] ? stopmachine+0xaf/0xda
> > > 	   [<ffffffff8020d558>] ? child_rip+0xa/0x12
> > > 	   [<ffffffff802672be>] ? stopmachine+0x0/0xda
> > > 	   [<ffffffff8020d54e>] ? child_rip+0x0/0x12
> > > 
> > > 	<6>kstopmachine  ? 0000000000000000  6400   367      1
> > > 	  ffff8101af9b9ee0 0000000000000046 0000000000000000 0000000000000000
> > > 	  0000000000000000 ffff8101af9b4000 ffff8101afdc0000 ffff8101af9b4540
> > > 	  0000000600000000 00000000ffff909f ffffffffffffffff ffffffffffffffff
> > > 	Call Trace:
> > > 	     [<ffffffff8023be98>] do_exit+0x6fe/0x702
> > > 	     [<ffffffff8020d55f>] child_rip+0x11/0x12
> > > 	     [<ffffffff802672be>] ? stopmachine+0x0/0xda
> > > 	     [<ffffffff8020d54e>] ? child_rip+0x0/0x12
> > > 
> > > 
> > > The boot thread shows:
> > > 	 <6>swapper       D 0000000000000002  2640     1      0
> > > 	  ffff8101afc3fcd0 0000000000000046 ffffffff807d8341 0000000000000200
> > > 	  ffffffff807d8335 ffff8101afc40000 ffff8101ad284000 ffff8101afc40540
> > > 	  00000005afc3faa0 ffffffff8021e837 ffff8101afc3fab0 ffff8101afc3fd50
> > > 
> > > 	 [<ffffffff8068c961>] schedule_timeout+0x27/0xb9
> > > 	 [<ffffffff8068ef48>] ? _spin_unlock_irq+0x2b/0x37
> > > 	 [<ffffffff8068c79c>] wait_for_common+0xe6/0x160
> > > 	 [<ffffffff8022d88a>] ? default_wake_function+0x0/0xf
> > > 	 [<ffffffff8068c8a0>] wait_for_completion+0x18/0x1a
> > > 	 [<ffffffff8024981a>] synchronize_rcu+0x3a/0x41
> > > 	 [<ffffffff802498a3>] ? wakeme_after_rcu+0x0/0x15
> > > 	 [<ffffffff805d8e1b>] netlink_change_ngroups+0xce/0xfc
> > > 	 [<ffffffff805da2c9>] genl_register_mc_group+0xfd/0x160
> > > 	 [<ffffffff80ac6d5d>] ? acpi_event_init+0x0/0x57
> > > 	 [<ffffffff80ac6d92>] acpi_event_init+0x35/0x57
> > > 	 [<ffffffff80aaca8c>] kernel_init+0x1c5/0x31f
> > > 
> > > 
> > > Is this hang already fixed or should I dig deeper?
> > 
> > there's no known hang in tip/master. I.e. removing your MAX_APICS patch 
> > clearly resolved that crash.
> 
> Hmmm.  I'm puzzled. With the tip/master tree that I built earlier this week, I was
> able to get hangs both with & without the MAX_APICS patch. Although less frequent,
> I also got hangs with NR_CPUS=128 & without the MAX_APICS patch. I'm not certain
> that all hangs were identical to the above backtraces, but they all hung at
> about the same spot.
> 
> I'll build a new tip/master tree, apply the MAX_APICS patch and retest using
> your random config & boot options that caused the problem.

Is the kstopmachine related to the synchronize_rcu()?  A task that is
running in a tight loop with irqs disabled (as stopmachine() seems to do,
judging from the source code) will absolutely prevent RCU grace periods
from completing.

So if the stop_machine() sequence gets stuck for whatever reason, RCU
grace periods will get stuck as well.

							Thanx, Paul
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