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

----- hutao(a)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.