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实例演示 | 用Kdump分析内核奔溃原因

刘盼

发布于 2022-07-27 01:50:42

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文章被收录于专栏：人人都是极客人人都是极客

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代码可运行

本文主要介绍kdump服务和crash的使用，并结合一个简单的实例演示如何分析内核奔溃的原因。本文基于linux kernel 4.19, 体系结构为aarch64。

kdump概述

kdump

kdump 是一种先进的基于 kexec 的内核崩溃转储机制，用来捕获kernel crash（内核崩溃）的时候产生的crash dump。当内核产生错误时，kdump会将内存导出为vmcore保存到磁盘。

kdump流程

当系统崩溃时，kdump 使用 kexec 启动到第二个内核。第二个内核通常叫做捕获内核，以很小内存启动以捕获转储镜像。第一个内核启动时会保留一段内存给kdump用。

kdump的配置

系统启动时为crashkernel保留内存

可以在kernel command line中加入如下参数：crashkernel=size[@offset]。保留内存是否预留成功，可以通过cat /proc/meminfo查看。。

cat /proc/meminfo | grep Crash

安装kexec-toools

yum install kexec-tools

kexec-tool推荐使用rpm方式安装，使用时需要和内核版本配套。

启动kdump服务

systemctl start kdump.service // 启动kdump服务 service kdump status // 查看kdump状态

测试kdump是否可以正常dump

echo c > /proc/sysrq-trigger

如果没有问题，系统会自动重启，重启后可以看到在/var/crash/目录下生成了coredump文件。

qemu使用kdump

我们经常会使用qemu去启动虚拟机。qemu启动的内核发生错误也可以用kdump生成vmcore文件。

首先先将qemu的panic重启关闭，防止coredump的时候发生了reboot

echo 0 > /proc/sys/kernel/panic

触发kernel panic

echo c > /proc/sysrq-trigger

kernel panic后，使得qemu进入monitor模式

ctrl + A, ---> c, qemu进入monitor模式

进入monitor模式后，进行coredump

dump-guest-memory -z xxx-vmcore

如下图所示，成功在qemu 的kernel panic后，获得了coredump文件。

使用crash分析内核奔溃转储文件

在内核奔溃后，如果部署了kdump, 会在/var/crash目录中找到vmcore转储文件，vmcore文件可以配合crash工具进行分析。

crash的版本要和内核的版本保持一致，比如上面成功dump了qemu arm64的coredump文件，就需要配套的arm64的crash工具进行分析，否则会报兼容性错误。

编译arm64 crash工具：

下载：https://github.com/crash-utility/crash/releases 编译安装： $ tar -xf crash-7.2.8.tar.gz $ cd crash-7.2.8/ $ make target=arm64

安装完成后，使用crash工具分析vmcore文件， vmlinux在编译内核时会在根目录下生成。

crash vmcore vmlinux

crash常用命令

bt: 查看函数调用栈

crash> bt
PID: 1452   TASK: ffff80007b0f1a80  CPU: 1   COMMAND: "sh"
 #0 [ffff00000aeb3900] __delay at ffff000008af2528
 #1 [ffff00000aeb3930] __const_udelay at ffff000008af2488
 #2 [ffff00000aeb3940] panic at ffff0000080d7f04
 #3 [ffff00000aeb3a20] die at ffff00000808cb18
 #4 [ffff00000aeb3a60] die_kernel_fault at ffff00000809f7e8
 #5 [ffff00000aeb3a90] __do_kernel_fault at ffff00000809f07c
 #6 [ffff00000aeb3ac0] do_page_fault at ffff00000809f12c
 #7 [ffff00000aeb3b30] do_translation_fault at ffff00000809f574
 #8 [ffff00000aeb3b40] do_mem_abort at ffff000008081448
 #9 [ffff00000aeb3ca0] el1_ia at ffff00000808318c
     PC: ffff0000085dc0d0  [sysrq_handle_crash+32]
     LR: ffff0000085dc0bc  [sysrq_handle_crash+12]
     SP: ffff00000aeb3cb0  PSTATE: 40000005
    X29: ffff00000aeb3cb0  X28: ffff80007b0f1a80  X27: 0000000000000000
    X26: 0000000000000000  X25: 0000000056000000  X24: 0000000000000000
    X23: 0000000000000007  X22: ffff000009289000  X21: ffff000009289400
    X20: 0000000000000063  X19: ffff0000091a1000  X18: ffffffffffffffff
    X17: 0000000000000000  X16: 0000000000000000  X15: ffff0000091896c8
    X14: ffff0000892ed70f  X13: ffff0000092ed71d  X12: ffff0000091a1000
    X11: 0000000005f5e0ff  X10: ffff000009189940   X9: 00000000ffffffd0
     X8: ffff000008602b08   X7: 54203a2071527379   X6: 00000000000000d2
     X5: 0000000000000000   X4: 0000000000000000   X3: ffffffffffffffff
     X2: 2c501196acfc7700   X1: 0000000000000000   X0: 0000000000000001
#10 [ffff00000aeb3cb0] sysrq_handle_crash at ffff0000085dc0cc
#11 [ffff00000aeb3cc0] __handle_sysrq at ffff0000085dc6cc
#12 [ffff00000aeb3d00] write_sysrq_trigger at ffff0000085dcc60
#13 [ffff00000aeb3d20] proc_reg_write at ffff0000082ac7e4
#14 [ffff00000aeb3d40] __vfs_write at ffff00000823a9cc
#15 [ffff00000aeb3de0] vfs_write at ffff00000823ace0
#16 [ffff00000aeb3e20] ksys_write at ffff00000823afd4
#17 [ffff00000aeb3e70] __arm64_sys_write at ffff00000823b064
#18 [ffff00000aeb3e80] el0_svc_common at ffff000008094ef4
#19 [ffff00000aeb3eb0] el0_svc_handler at ffff000008094fa8
#20 [ffff00000aeb3ff0] el0_svc at ffff000008084044
     PC: 0000000000401a58   LR: 00000000004b2be4   SP: 0000ffffe68f8e10
    X29: 0000ffffe68f9500  X28: 0000ffffe68f9fba  X27: 000000000056f9c0
    X26: 00000000005ed000  X25: 0000000000000000  X24: 0000000000000020
    X23: 0000000011710110  X22: 00000000005ed000  X21: 0000000000000002
    X20: 0000000011710110  X19: 0000000000000001  X18: 0000000000000001
    X17: 0000000000000000  X16: 0000000000000000  X15: 0000000000000008
    X14: 0000000000000012  X13: 726567676972742d  X12: 0101010101010101
    X11: 0000005000564818  X10: 0101010101010101   X9: fffffffffffffff0
     X8: 0000000000000040   X7: 0000000011710120   X6: 0080808080808080
     X5: 0000000000000000   X4: 0000000000000063   X3: 0000000011710111
     X2: 0000000000000002   X1: 0000000011710110   X0: 0000000000000001
    ORIG_X0: 0000000000000001  SYSCALLNO: 40  PSTATE: 80000000

log: 查看内核dmesg日志

crash> log
[    0.000000] Booting Linux on physical CPU 0x0000000000 [0x411fd070]
[    0.000000] Linux version 4.20.0-rc4-00007-gef78e5e (root@localhost.localdomain) (gcc version 7.3.1 20180425 [linaro-7.3-2018.05 revision d29120a424ecfbc167ef90065c0eeb7f91977701] (Linaro GCC 7.3-2018.05)) #3 SMP PREEMPT Wed Jan 15 07:52:10 PST 2020
[    0.000000] Machine model: linux,dummy-virt
[    0.000000] efi: Getting EFI parameters from FDT:
[    0.000000] efi: UEFI not found.
[    0.000000] cma: Reserved 32 MiB at 0x00000000be000000
[    0.000000] NUMA: No NUMA configuration found
[    0.000000] NUMA: Faking a node at [mem 0x0000000040000000-0x00000000bfffffff]
[    0.000000] NUMA: NODE_DATA [mem 0xbdfea840-0xbdfebfff]
[    0.000000] Zone ranges:
[    0.000000]   DMA32    [mem 0x0000000040000000-0x00000000bfffffff]
[    0.000000]   Normal   empty
[    0.000000] Movable zone start for each node
[    0.000000] Early memory node ranges
[    0.000000]   node   0: [mem 0x0000000040000000-0x00000000bfffffff]
[    0.000000] Initmem setup node 0 [mem 0x0000000040000000-0x00000000bfffffff]
[    0.000000] On node 0 totalpages: 524288
[    0.000000]   DMA32 zone: 8192 pages used for memmap
[    0.000000]   DMA32 zone: 0 pages reserved
[    0.000000]   DMA32 zone: 524288 pages, LIFO batch:63
[    0.000000] psci: probing for conduit method from DT.
[    0.000000] psci: PSCIv0.2 detected in firmware.
[    0.000000] psci: Using standard PSCI v0.2 function IDs
[    0.000000] psci: Trusted OS migration not required
[    0.000000] random: get_random_bytes called from start_kernel+0xa8/0x418 with crng_init=0
[    0.000000] percpu: Embedded 23 pages/cpu @(____ptrval____) s55704 r8192 d30312 u94208
[    0.000000] pcpu-alloc: s55704 r8192 d30312 u94208 alloc=23*4096
[    0.000000] pcpu-alloc: [0] 0 [0] 1 
[    0.000000] Detected PIPT I-cache on CPU0
[    0.000000] CPU features: enabling workaround for ARM erratum 832075
[    0.000000] CPU features: enabling workaround for ARM erratum 834220
[    0.000000] CPU features: enabling workaround for EL2 vector hardening
[    0.000000] CPU features: detected: Kernel page table isolation (KPTI)
[    0.000000] Built 1 zonelists, mobility grouping on.  Total pages: 516096
[    0.000000] Policy zone: DMA32
[    0.000000] Kernel command line: rdinit=/linuxrc console=ttyAMA0
[    0.000000] Memory: 2009884K/2097152K available (10876K kernel code, 1414K rwdata, 5100K rodata, 1344K init, 380K bss, 54500K reserved, 32768K cma-reserved)
[    0.000000] SLUB: HWalign=64, Order=0-3, MinObjects=0, CPUs=2, Nodes=1
[    0.000000] rcu: Preemptible hierarchical RCU implementation.
[    0.000000] rcu:     RCU restricting CPUs from NR_CPUS=64 to nr_cpu_ids=2.
[    0.000000]  Tasks RCU enabled.
[    0.000000] rcu: RCU calculated value of scheduler-enlistment delay is 25 jiffies.
[    0.000000] rcu: Adjusting geometry for rcu_fanout_leaf=16, nr_cpu_ids=2
[    0.000000] NR_IRQS: 64, nr_irqs: 64, preallocated irqs: 0
[    0.000000] GICv2m: range[mem 0x08020000-0x08020fff], SPI[80:143]
[    0.000000] arch_timer: cp15 timer(s) running at 62.50MHz (virt).
[    0.000000] clocksource: arch_sys_counter: mask: 0xffffffffffffff max_cycles: 0x1cd42e208c, max_idle_ns: 881590405314 ns

struct: 查看数据结构

crash> struct task_struct ffff0000085dc0d0 -x
struct task_struct {
  thread_info = {
    flags = 0xa8c17bfd39000020, 
    addr_limit = 0xd503201fd65f03c0, 
    preempt_count = 0xa9bf7bfd
  }, 
  state = 0x97ec827fd50342ff, 
  stack = 0xd65f03c0a8c17bfd, 
  usage = {
    counter = 0xa9bd7bfd
  }, 
  flags = 0x910003fd, 
  ptrace = 0xa90153f3, 
  wake_entry = {
    next = 0xaa0103f4911b2262
  }, 
  on_cpu = 0xf9400041, 
  cpu = 0xf90017a1, 
  wakee_flips = 0xd2800001, 
  wakee_flip_decay_ts = 0x37f8018097f909a2, 
  last_wakee = 0xf10bfc7ff94013a3, 
  recent_used_cpu = 0x54000228, 
  wake_cpu = 0xb0006561, 
  on_rq = 0x91018021, 
  prio = 0xf9401284, 
  static_prio = 0x52800000, 
  normal_prio = 0xb9404022, 
  rt_priority = 0x79000083, 
  sched_class = 0x911b2273b9004022, 
  se = {
    load = {
      weight = 0x940b05adf9400013, 
      inv_weight = 0x91012260
    }, 
    runnable_weight = 0x97edbddd91052260, 
    run_node = {
      __rb_parent_color = 0x940b05c5aa1403e0, 
      rb_right = 0x97f0ec85aa1303e0, 
      rb_left = 0xa8c27bfda94153f3
    }, 
    group_node = {
      next = 0xd503201fd65f03c0, 
      prev = 0x52800021a9bf7bfd
    }, 
    on_rq = 0x910003fd, 
    exec_start = 0xd280000097f251d0, 
    sum_exec_runtime = 0xa8c17bfd97ec8318, 
    vruntime = 0xd503201fd65f03c0, 
    prev_sum_exec_runtime = 0x910003fda9be7bfd, 
    nr_migrations = 0xd1012013f9000bf3, 
    statistics = {<No data fields>}, 
    depth = 0x39434660, 
    parent = 0x52800020f9000fb4, 
    cfs_rq = 0xb940ce7439034a60, 
    my_q = 0x52800023d5033f9f, 
    avg = {
      last_update_time = 0x940b0cf552800001, 
      load_sum = 0x52800003aa1303e0, 
      runnable_load_sum = 0x5280002152800c62, 
      util_sum = 0x940b0cf0,

struct -o [struct] : 显示结构体中成员的偏移 struct [struct] [address] : 显示对应地址结构体的值 [struct] [address] ：简化形式显示对应地址结构体的值 [struct] [address] -xo: 打印结构体定义和大小 [struct].member[address]: 显示某个成员的值

rd: 读取内存内容

crash> rd ffff0000085dc0d0 32
ffff0000085dc0d0:  a8c17bfd39000020 d503201fd65f03c0    ..9.{...._.. ..
ffff0000085dc0e0:  910003fda9bf7bfd 97ec827fd50342ff   .{.......B......
ffff0000085dc0f0:  d65f03c0a8c17bfd 910003fda9bd7bfd   .{...._..{......
ffff0000085dc100:  b0005d73a90153f3 aa0103f4911b2262   .S..s]..b"......
ffff0000085dc110:  f90017a1f9400041 910083a2d2800001   A.@.............
ffff0000085dc120:  37f8018097f909a2 f10bfc7ff94013a3   .......7..@.....
ffff0000085dc130:  b000656154000228 f940128491018021   (..Tae..!.....@.
ffff0000085dc140:  b940402252800000 1100044279000083   ...R"@@....yB...
ffff0000085dc150:  911b2273b9004022 f9400261f94017a2   "@..s"....@.a.@.
ffff0000085dc160:  b50000c1ca010041 a8c37bfda94153f3   A........SA..{..
ffff0000085dc170:  128002a0d65f03c0 97ebee0b17fffff7   .._.............
ffff0000085dc180:  910003fda9be7bfd aa0003f4a90153f3   .{.......S......
ffff0000085dc190:  940b05adf9400013 97ec5fb991012260   ..@.....`"..._..
ffff0000085dc1a0:  97edbddd91052260 940b05c5aa1403e0   `"..............
ffff0000085dc1b0:  97f0ec85aa1303e0 a8c27bfda94153f3   .........SA..{..
ffff0000085dc1c0:  d503201fd65f03c0 52800021a9bf7bfd   .._.. ...{..!..R

rd [addr] [len]: 查看指定地址，长度为len的内存 rd -S [addr][len]: 尝试将地址转换为对应的符号 rd [addr] -e [addr] : 查看指定内存区域内容

dis: 进行返汇编，查看对应地址的代码逻辑

crash> dis -r ffff0000085dc0d0
0xffff0000085dc0b0 <sysrq_handle_crash>:        stp     x29, x30, [sp,#-16]!
0xffff0000085dc0b4 <sysrq_handle_crash+4>:      mov     x29, sp
0xffff0000085dc0b8 <sysrq_handle_crash+8>:      bl      0xffff000008141a48 <__rcu_read_unlock>
0xffff0000085dc0bc <sysrq_handle_crash+12>:     adrp    x1, 0xffff0000092e9000 <xen_dummy_shared_info+984>
0xffff0000085dc0c0 <sysrq_handle_crash+16>:     mov     w0, #0x1                        // #1
0xffff0000085dc0c4 <sysrq_handle_crash+20>:     str     w0, [x1,#1448]
0xffff0000085dc0c8 <sysrq_handle_crash+24>:     dsb     st
0xffff0000085dc0cc <sysrq_handle_crash+28>:     mov     x1, #0x0                        // #0
0xffff0000085dc0d0 <sysrq_handle_crash+32>:     strb    w0, [x1]

crash> dis -f ffff0000085dc0d0
0xffff0000085dc0d0 <sysrq_handle_crash+32>:     strb    w0, [x1]
0xffff0000085dc0d4 <sysrq_handle_crash+36>:     ldp     x29, x30, [sp],#16
0xffff0000085dc0d8 <sysrq_handle_crash+40>:     ret

ps: 查看线程状态

crash> ps
   PID    PPID  CPU       TASK        ST  %MEM     VSZ    RSS  COMM
>     0      0   0  ffff000009192580  RU   0.0       0      0  [swapper/0]
      0      0   1  ffff80007bbc1a80  RU   0.0       0      0  [swapper/1]
      1      0   0  ffff80007bb68000  IN   0.0    2196     60  linuxrc
      2      0   0  ffff80007bb68d40  IN   0.0       0      0  [kthreadd]
      3      2   0  ffff80007bb69a80  ID   0.0       0      0  [rcu_gp]
      4      2   0  ffff80007bb6a7c0  ID   0.0       0      0  [rcu_par_gp]
      5      2   0  ffff80007bb6b500  ID   0.0       0      0  [kworker/0:0]
      6      2   0  ffff80007bb6c240  ID   0.0       0      0  [kworker/0:0H]
      7      2   0  ffff80007bb6cf80  ID   0.0       0      0  [kworker/u4:0]
      8      2   0  ffff80007bb6dcc0  ID   0.0       0      0  [mm_percpu_wq]
      9      2   0  ffff80007bb6ea00  IN   0.0       0      0  [ksoftirqd/0]
     10      2   0  ffff80007bbc0000  ID   0.0       0      0  [rcu_preempt]
     11      2   0  ffff80007bbc0d40  IN   0.0       0      0  [migration/0]
     12      2   0  ffff80007bbc27c0  IN   0.0       0      0  [cpuhp/0]
     13      2   1  ffff80007bbc3500  IN   0.0       0      0  [cpuhp/1]
     14      2   1  ffff80007bbc4240  IN   0.0       0      0  [migration/1]
     15      2   1  ffff80007bbc4f80  IN   0.0       0      0  [ksoftirqd/1]
     16      2   1  ffff80007bbc5cc0  ID   0.0       0      0  [kworker/1:0]
     17      2   1  ffff80007bbc6a00  ID   0.0       0      0  [kworker/1:0H]
     18      2   0  ffff80007bbd0000  IN   0.0       0      0  [kdevtmpfs]
     19      2   0  ffff80007bbd0d40  ID   0.0       0      0  [netns]
     20      2   0  ffff80007b040000  ID   0.0       0      0  [kworker/u4:1]
     21      2   1  ffff80007b040d40  IN   0.0       0      0  [rcu_tasks_kthre]
     42      2   1  ffff80007b0f3500  ID   0.0       0      0  [kworker/1:1]
     43      2   0  ffff80007b0f4240  ID   0.0       0      0  [kworker/0:1]
     49      2   1  ffff80007b0f4f80  ID   0.0       0      0  [kworker/u4:2]
     56      2   1  ffff80007b140000  IN   0.0       0      0  [kauditd]
    212      2   0  ffff80007b26ea00  ID   0.0       0      0  [kworker/u4:3]
    256      2   0  ffff80007b336a00  ID   0.0       0      0  [kworker/u4:4]
    471      2   1  ffff80007b2d6a00  IN   0.0       0      0  [oom_reaper]
    472      2   1  ffff80007b2d5cc0  ID   0.0       0      0  [writeback]
    474      2   0  ffff80007b330d40  IN   0.0       0      0  [kcompactd0]
    475      2   0  ffff80007b3327c0  IN   0.0       0      0  [ksmd]
    476      2   0  ffff80007b2d1a80  IN   0.0       0      0  [khugepaged]
    477      2   0  ffff80007b2d0000  ID   0.0       0      0  [crypto]
    478      2   1  ffff80007b2d0d40  ID   0.0       0      0  [kintegrityd]
    480      2   1  ffff80007b2d27c0  ID   0.0       0      0  [kblockd]
    501      2   1  ffff80007b2d3500  ID   0.0       0      0  [tpm_dev_wq]
    508      2   1  ffff80007b2d4240  ID   0.0       0      0  [ata_sff]
    541      2   0  ffff80007ac98000  ID   0.0       0      0  [edac-poller]
    551      2   1  ffff80007b044240  ID   0.0       0      0  [devfreq_wq]
    561      2   1  ffff80007b268000  IN   0.0       0      0  [watchdogd]
    647      2   0  ffff80007b268d40  ID   0.0       0      0  [rpciod]
    648      2   1  ffff80007b26c240  ID   0.0       0      0  [kworker/u5:0]
    649      2   0  ffff80007ad04f80  ID   0.0       0      0  [xprtiod]
    718      2   1  ffff80007bbd3500  IN   0.0       0      0  [kswapd0]
    815      2   1  ffff80007ad00000  ID   0.0       0      0  [nfsiod]
   1250      2   0  ffff80007b26dcc0  ID   0.0       0      0  [vfio-irqfd-clea]
>  1452      1   1  ffff80007b0f1a80  RU   0.0    2196     76  sh

ps -p [pid]: 显示进程父子关系 ps -t [pid]: 显示进程运行时间

kmem: 查看内核内存使用情况

crash> kmem -i
                 PAGES        TOTAL      PERCENTAGE
    TOTAL MEM   511276         2 GB         ----
         FREE   506631       1.9 GB   99% of TOTAL MEM
         USED     4645      18.1 MB    0% of TOTAL MEM
       SHARED      353       1.4 MB    0% of TOTAL MEM
      BUFFERS        0            0    0% of TOTAL MEM
       CACHED      480       1.9 MB    0% of TOTAL MEM
         SLAB     1930       7.5 MB    0% of TOTAL MEM

   TOTAL HUGE        0            0         ----
    HUGE FREE        0            0    0% of TOTAL HUGE

   TOTAL SWAP        0            0         ----
    SWAP USED        0            0    0% of TOTAL SWAP
    SWAP FREE        0            0    0% of TOTAL SWAP

 COMMIT LIMIT   255638     998.6 MB         ----
    COMMITTED      479       1.9 MB    0% of TOTAL LIMIT
crash>

kmem -i: 查看内存整体使用情况 kmem -s: 查看slab使用情况 kmem [addr]: 搜索地址所属的内存结构

更多其它命令通过help查看

内核panic实例

内核访问空指针产生panic。

驱动制作

编写一个驱动，构造一个内核模块访问空指针的异常，演示如何使用crash分析内核奔溃的原因。

include <linux/module.h>
#include <linux/kernel.h>
#include <linux/atomic.h>
#include <linux/slab.h>

struct my_struct {
        unsigned long head;
        spinlock_t lock;
};
int *addr = 0; //null pointer
void panic_foo(struct my_struct *ms)
{
        int *p = addr;
        spin_lock(&ms->lock);
        if (ms->head == 10) {
                *p = 0xFFFF;
        } else if (ms->head = 0) {
                // do sth
        } else {
                // do sth
        }

        spin_unlock(&ms->lock);
}
int panic_kernel_init(void)
{
        struct my_struct *ms = kzalloc(sizeof(struct my_struct), GFP_KERNEL);
        spin_lock_init(&ms->lock);
        ms->head = 10;
        panic_foo(ms);

        return 0;
}
void panic_kernel_exit(void)
{
}
module_init(panic_kernel_init);
module_exit(panic_kernel_exit);

obj-m := panic-kernel.o
KERNEL_DIR := /home/linux

PWD := $(shell pwd)
all:
        make -C $(KERNEL_DIR) SUBDIRS=$(PWD) modules

clean:
        rm *.o *.ko *.mod.c

.PHONY: clean

将编好的驱动打包进根文件系统，启动后插入内核模块。

panic 分析

内核的call trace如上图所示，将对应的文件反汇编，找到问题出现对应的代码。

aarch64-linux-gnu-objdump -S panic-kernel.o > test.txt

截取部分反汇编如下：

Disassembly of section .text:

0000000000000000 <panic_foo>:

int *addr = 0; //null pointer


void panic_foo(struct my_struct *ms)
{
   0:   a9bd7bfd        stp     x29, x30, [sp, #-48]!
   4:   910003fd        mov     x29, sp
   8:   a90153f3        stp     x19, x20, [sp, #16]
   c:   aa0003f3        mov     x19, x0
        int *p = addr;
  10:   90000000        adrp    x0, 0 <panic_foo>
        raw_spin_lock_init(&(_lock)->rlock);            \
} while (0)

从汇编代码可以看出， panic_foo函数的参数(x0)最终保存在x19寄存器。我们现在想要知道出现问题时，代码走的是哪一个分支。

配合crash进行分析，先导入模块符号表：

crash> mod -S my_module
     MODULE       NAME           SIZE  OBJECT FILE
ffff000000ae2000  panic_kernel  16384  my_module/panic-kernel.o

使用crash 查看出问题时结构体的值，确认函数走的是哪个分支。函数的参数是x19:

crash> struct my_struct ffff8000fa4d9780
struct my_struct {
  head = 10, 
  lock = {
    {
      rlock = {
        raw_lock = {
          {
            val = {
              counter = 1
            }, 
            {
              locked = 1 '\001', 
              pending = 0 '\000'
            }, 
            {
              locked_pending = 1, 
              tail = 0
            }
          }
        }
      }
    }
  }
}

从打印的之来看，head成员的值为10，可以确定代码走的是哪一个分支。

再结合之前的反汇编代码，出错的位置在pc: panic_foo +0x54。pc保存的是栈顶指针，lr保存的是函数返回的地址（x30）

static __always_inline void spin_unlock(spinlock_t *lock)
{
        raw_spin_unlock(&lock->rlock);
  38:   aa1403e0        mov     x0, x20
  3c:   94000000        bl      0 <_raw_spin_unlock>
        } else {
                // do sth
        }

        spin_unlock(&ms->lock);
}
  40:   f94013f5        ldr     x21, [sp, #32]
  44:   a94153f3        ldp     x19, x20, [sp, #16]
  48:   a8c37bfd        ldp     x29, x30, [sp], #48
  4c:   d65f03c0        ret
                *p = 0xFFFF;
  50:   529fffe0        mov     w0, #0xffff                     // #65535
  54:   b90002a0        str     w0, [x21]
  58:   aa1403e0        mov     x0, x20
  5c:   94000000        bl      0 <_raw_spin_unlock>
}