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Bug#899369: general: system freezes when sending SysRq-c signal instead of dumping the crashdump when on iSCSI root

Package: general
Severity: normal

Dear Maintainer,

*** Reporter, please consider answering these questions, where appropriate ***

   * What led up to the situation?
When on iSCSI root, Debian freezes after sending the SysRq-c signal. SysRq-c signal works in a frozen state just fine. I am aware about the fact that Linux cannot save the crashdump on iSCSI, so I configured the NFS resource for a crashdump. However, I don't see on an NFS server any network activity indicating that Debian tries to access the NFS share.

   * What exactly did you do (or not do) that was effective (or
I have configured the kdump as follows:

# kdump-config show
DUMP_MODE:        kdump
USE_KDUMP:        1
KDUMP_SYSCTL:     kernel.panic_on_oops=1
KDUMP_COREDIR:    /var/crash
crashkernel addr: 0x15000000
   /var/lib/kdump/vmlinuz: symbolic link to /boot/vmlinuz-4.16.0-1-amd64
kdump initrd: 
   /var/lib/kdump/initrd.img: symbolic link to /var/lib/kdump/initrd.img-4.16.0-1-amd64
NFS_TIMEO:        600
HOSTTAG:          hv22
current state:    ready to kdump

kexec command:
  /sbin/kexec -p --command-line="BOOT_IMAGE=/boot/vmlinuz-4.16.0-1-amd64 root=UUID=4e04292f-93d9-4020-acc3-fa7c3bb1ab88 intel_iommu=on iommu=pt modprobe.blacklist=nouveau quiet nr_cpus=1 systemd.unit=kdump-tools.service irqpoll nousb ata_piix.prefer_ms_hyperv=0" --initrd=/var/lib/kdump/initrd.img /var/lib/kdump/vmlinuz

NFS share is accessible from the freezing host when just mounting it.

   * What was the outcome of this action?
Actual outcome is that Debian freezes as soon as I send the SysRq-c.

   * What outcome did you expect instead?
Expected output would be: system dumps kernel core on an NFS share.

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-- System Information:
Debian Release: buster/sid
  APT prefers testing
  APT policy: (500, 'testing')
Architecture: amd64 (x86_64)

Kernel: Linux 4.16.0-1-amd64 (SMP w/56 CPU cores)
Locale: LANG=C, LC_CTYPE=C (charmap=ANSI_X3.4-1968), LANGUAGE=C (charmap=ANSI_X3.4-1968)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled