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

[Jack Steiner]

>> > 
>> > -tip auto-testing found a new boot failure on x86 which happens if 
>> > NR_CPUS is changed from 8 to 4096. The hang goes like this:
>> > 
>> 
>> Still looking but here is what I have found so far.
>> 
>> The most obvious change was to revert the patch that changed MAX_APICS to
>> 32k. With this patch reverted, the system still hangs at the same spot.
>> 
>> I also noticed that the hang is random. It usually occurs  at acpi_event_init()
>> but sometimes it hangs at a different place.
>> 
>> I also observed that the failure sometimes does not occur. The system
>> boots to the point that mounting /root fails, then panics because the mount
>> fails. I expect that this is a different failure due to missing drivers.
>> I'll chase that down later.
>> 
>> 
>> 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?


--- jack
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