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Subject[question] panic() during reboot -f (reboot syscall)
Hello,

I wonder if it is "normal" to get panic() when the system is rebooted
using "reboot -f". I looks a bit weird to me.

In our case, the panic() was triggered from ext4 filesystem code
that was mounted with "errors=panic"

crash> bt
PID: 3984 TASK: ffff887db1f6c180 CPU: 32 COMMAND: "bash"
#0 [ffff887e637bf9a8] machine_kexec at ffffffff81059c5c
#1 [ffff887e637bf9f8] __crash_kexec at ffffffff81119e0a
#2 [ffff887e637bfab8] panic at ffffffff81193c31
#3 [ffff887e637bfb30] ext4_handle_error at ffffffffa0229faa [ext4]
#4 [ffff887e637bfb40] __ext4_error_inode at ffffffffa022a12a [ext4]
#5 [ffff887e637bfbe0] __ext4_get_inode_loc at ffffffffa02096a5 [ext4]
#6 [ffff887e637bfc40] ext4_iget at ffffffffa020c028 [ext4]
#7 [ffff887e637bfcc0] ext4_lookup at ffffffffa0216ca0 [ext4]
#8 [ffff887e637bfce8] lookup_real at ffffffff81218e3f
#9 [ffff887e637bfd00] __lookup_hash at ffffffff8121916f
#10 [ffff887e637bfd20] walk_component at ffffffff8121b50f
#11 [ffff887e637bfd70] path_lookupat at ffffffff8121ca30
#12 [ffff887e637bfd98] filename_lookup at ffffffff8121e58c
#13 [ffff887e637bfe98] vfs_fstatat at ffffffff81214549
#14 [ffff887e637bfed8] SYSC_newstat at ffffffff812149ca
#15 [ffff887e637bff50] entry_SYSCALL_64_fastpath at ffffffff8161de61
RIP: 00007f9db8d3ebe5 RSP: 00007ffda081cf68 RFLAGS: 00000246
RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f9db8d3ebe5
RDX: 00000000013c7fa0 RSI: 00000000013c7fa0 RDI: 00000000013c7f40
RBP: 00007f9db943bee0 R8: 00000000013c7f40 R9: 00000000000b0000
R10: 000000007af2c337 R11: 0000000000000246 R12: 00000000013c7fa0
R13: 00000000013c7fa0 R14: 0000000000000008 R15: 00000000013c7f80
ORIG_RAX: 0000000000000004 CS: 0033 SS: 002b


Now, "reboot -f" just calls the reboot() syscall. I do not see
anything that would stop processes. It even does not stop
other CPUs by purpose, see the commit cf7df378aa4ff7da
("reboot: rigrate shutdown/reboot to boot cpu").

But it shuts down devices very early, via:

+ kernel_restart()
+ kernel_restart_prepare()
+ blocking_notifier_call_chain(&reboot_notifier_list, SYS_RESTART, cmd);
+ device_shutdown()

As a result, processes are still running. Filesystem code return
errors because the underlaying disk device was removed. It causes
panic() because "errors=panic" mount option.


My undestanding that userspace is reponsible for "clean" reboot.
The "reboot" command normally stops services, kill processes,
sync disks, umount filesystem before it calls the "reboot"
syscall.

By other words. It looks like the panic() is possible by design.
But it looks a bit weird. Any opinion?

Best Regards,
Petr

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