[Crash-utility] crash does not get proper backtrace?

[Dave Anderson]

----- "KAMEZAWA Hiroyuki" <kamezawa.hiroyu at jp.fujitsu.com> wrote:

> On Thu, 2 Sep 2010 08:44:12 -0400 (EDT)
> Dave Anderson <anderson at redhat.com> wrote:
> 
> > 
> > ----- hutao at cn.fujitsu.com wrote:
> > 
> > > Hi,
> > > 
> > > I got a problem where it seemed crash got a bad backtrace.
> > > The problem occurred under the following conditions:
> > > On a qemu guest system loading a module that stuck at
> > > the init function(say, call a function that did deadlooping),
> > > then dumped the guest by `virsh dump vm dumpfile', and run
> > > crash on the dumpfile.
> > > 
> > > The module is:
> > > 
> > > ---
> > > #include <linux/module.h>
> > > 
> > > int endless_loop(void)
> > > {
> > > 	printk("endless loop\n");
> > > 	while (1);
> > > 
> > > 	return 0;
> > > }
> > > 
> > > int __init endless_init(void)
> > > {
> > > 	endless_loop();
> > > 
> > > 	return 0;
> > > }
> > > module_init(endless_init);
> > > 
> > > MODULE_LICENSE("GPL");
> > > ---
> > > 
> > > crash bt command got:
> > > 
> > > crash> bt -a
> > > PID: 0      TASK: ffffffff81648020  CPU: 0   COMMAND: "swapper"
> > >  #0 [ffffffff81601e08] schedule at ffffffff813e8a49
> > >  #1 [ffffffff81601e18] apic_timer_interrupt at ffffffff8100344e
> > >  #2 [ffffffff81601ea0] need_resched at ffffffff8100970c
> > >  #3 [ffffffff81601eb0] default_idle at ffffffff81009f6b
> > >  #4 [ffffffff81601ec0] cpu_idle at ffffffff81001bf5
> > > 
> > > PID: 1088   TASK: ffff88001dda2d60  CPU: 1   COMMAND: "insmod"
> > >  #0 [ffff88001e751dc8] schedule at ffffffff813e8a49
> > >  #1 [ffff88001e751dd0] schedule at ffffffff813e8aec
> > >  #2 [ffff88001e751e80] preempt_schedule_irq at ffffffff813e8c90
> > >  #3 [ffff88001e751e90] retint_kernel at ffffffff813eab86
> > >  #4 [ffff88001e751f20] do_one_initcall at ffffffff81000210
> > >  #5 [ffff88001e751f50] sys_init_module at ffffffff8106b7ca
> > >  #6 [ffff88001e751f80] system_call_fastpath at ffffffff81002a82
> > >     RIP: 00007f761bb58b7a  RSP: 00007fff67a43120  RFLAGS: 00010206
> > >     RAX: 00000000000000af  RBX: ffffffff81002a82  RCX: 0000000000020010
> > >     RDX: 0000000000b96010  RSI: 00000000000163da  RDI: 0000000000b96030
> > >     RBP: 0000000000b96010   R8: 0000000000010011   R9: 0000000000080000
> > >     R10: 00007f761bb4b140  R11: 0000000000000202  R12: 00000000000163da
> > >     R13: 00007fff67a44985  R14: 00000000000163da  R15: 0000000000b96010
> > >     ORIG_RAX: 00000000000000af  CS: 0033  SS: 002b
> > > 
> > > Does it lose some function calls between do_one_initcall and retint_kernel?
> > > (endless_loop <- endless_init)
> > 
> > Your best bet is to use "bt -t" in a case such as that.
> > 
> > If there are no "starting hooks" for the backtrace code to use, then
> > it simply defaults to the RSP value left in the task->thread_struct->rsp,
> > and the RIP of the instruction following "__switch_to".  Those will be
> > stale, because they represent the last time that the task blocked in
> > kernel space.  In the case of your endless loop inside the kernel, there
> > is nothing for the crash utility to grab onto as the starting points because
> > the task is essentially "active".  It's somewhat similar in nature to 
> > using "bt -a" on a live system -- the tasks are running either in
> > kernel or user space, but do not have any "starting points" for the
> > backtrace code to latch onto, so it's not even allowed as a command.
> > 
> 
> Hmm. but, IIUC, vmcore on the real host (not on virtual machine) taken by kdump
> can show endless_loop(). Is it because kdump() reads paniced-host-image ? And
> we should take vmcore generated by "virsh dump" as 
>  - "it's just a live dump image and there are no guarantee of synchronous register
>     inforamtion. If you take care, please freeze kernel by some switch". 
> 
> Can SIGSTOP or somethig to qemu will help us to take synchronous snapshot of registers ?

Kdump works because the shutdown sends an NMI to each cpu, leaving an obvious
shutdown trail that can be tracked from the NMI stack back to the process stack.

You could also try using alt-sysrq-c on the guest prior to taking the virsh dump
from the host.  

Dave