Hello James,

crash and kgdb are two completely different debuggers to diagnose the kernel
panic issues. You can not access or start the crash from the context of kgdb.

[**crash utility:**]

There are two ways to invoke crash utility[[1]];

1) Typical postmortem debugging: (after panic)

# crash /path/to/vmcore /path/to/vmlinux

o Kernel object file and memory image are supplied, respectively.

2) Live memory debugging:

# crash

o Pre-defined directories are searched for proper vmlinux
o Version string matched to the running kernel (/proc/version)

**OR**

# crash /path/to/vmlinux

[**kdb/kgdb:**][[2]]

1) You can put the target system in debug mode using SysRq event (g).

# echo g > /proc/sysrq-trigger

Eg:
# echo g > /proc/sysrq-trigger
[  181.300854] SysRq : DEBUG

Entering kdb (current=0xffff8800766ebc60, pid 2347) on processor 1 due to Keyboard Entry

[1]kdb> summary
sysname    Linux
release    3.10.84
version    #1 SMP Tue Jul 28 19:53:37 IST 2015
machine    x86_64
nodename   localhost.localdomain
domainname (none)
ccversion  CCVERSION
date       2015-12-30 13:51:06 tz_minuteswest 0
uptime     00:03
load avg   0.11 0.11 0.04

MemTotal:        2050340 kB
MemFree:         1707332 kB
Buffers:             764 kB

1]kdb> dmesg | grep DMI:
<7>[    0.000000] DMI: Red Hat KVM, BIOS 0.5.1 01/01/2007

[1]kdb> bt
Stack traceback for pid 2347
0xffff8800766ebc60     2347      624  1    1   R  0xffff8800766ec240 *bash
 ffff88007bd61e70 0000000000000018
Call Trace:
 <#DB>  <<EOE>>  [<ffffffff810dd84c>] ? sysrq_handle_dbg+0x2c/0x50
 [<ffffffff8134b312>] ? __handle_sysrq+0xa2/0x170
 [<ffffffff8134b80a>] ? write_sysrq_trigger+0x4a/0x50
 [<ffffffff811fb61d>] ? proc_reg_write+0x3d/0x80
 [<ffffffff811993bd>] ? vfs_write+0xbd/0x1e0
 [<ffffffff81199d89>] ? SyS_write+0x49/0xa0
 [<ffffffff815a27c7>] ? tracesys+0xdd/0xe2

[1]kdb> rd
ax: 0000000000000001  bx: ffffffff818a4560  cx: ffff88007fd0eec0
dx: 0000000000000000  si: 0000000000000000  di: 0000000000000067
bp: ffff88007bd61e70  sp: ffff88007bd61e70  r8: ffffffff81b9069c
r9: 0000000000000248  r10: 0000000000000247  r11: 0000000000000003
r12: 0000000000000067  r13: 0000000000000246  r14: 0000000000000007
r15: 0000000000000000  ip: ffffffff810dd7f4  flags: 00000002  cs: 00000010
ss: 00000018  ds: 00000018  es: 00000018  fs: 00000018  gs: 00000018

[1]kdb> go
[  251.553055] systemd-journald[2372]: File /run/log/journal/71b46828837d4b1cb7f04385c86e7626/system.journal corrupted or uncleanly shut down, renaming and replacing.

#  <<<---{ root user }

[Note:] kdb command "go" is used to continue kernel execution. In above case
it will take you back to bash shell.

2) You can crash the target system using SysRq event (c):

# echo c > /proc/sysrq-trigger

Eg:
# echo c > /proc/sysrq-trigger
[   28.963227] SysRq : Trigger a crash
[   28.964025] BUG: unable to handle kernel NULL pointer dereference at           (null)
[   28.964025] IP: [<ffffffff8134abb6>] sysrq_handle_crash+0x16/0x20
[   28.964025] Oops: 0002 [#1] SMP

Entering kdb (current=0xffff880077275a90, pid 2352) on processor 1 Oops: (null)
due to oops @ 0xffffffff8134abb6
?dCPU: 1 PID: 2352 Comm: bash Not tainted 3.10.84 #1
?dHardware name: Red Hat KVM, BIOS 0.5.1 01/01/2007
?dtask: ffff880077275a90 ti: ffff880076494000 task.ti: ffff880076494000
?dRIP: 0010:[<ffffffff8134abb6>]  [<ffffffff8134abb6>] sysrq_handle_crash+0x16/0x20
?dRSP: 0018:ffff880076495e80  EFLAGS: 00010096
?dRAX: 000000000000000f RBX: ffffffff8190dbc0 RCX: ffff88007fd0eec0
?dRDX: 0000000000000000 RSI: ffff88007fd0d368 RDI: 0000000000000063
?dRBP: ffff880076495e80 R08: ffffffff81b9069c R09: 0000000000000245
?dR10: 0000000000000244 R11: 0000000000000003 R12: 0000000000000063
?dR13: 0000000000000246 R14: 0000000000000007 R15: 0000000000000000
?dFS:  00007f53aec0c740(0000) GS:ffff88007fd00000(0000) knlGS:0000000000000000
?dCS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
?dCR2: 0000000000000000 CR3: 00000000764bd000 CR4: 00000000000006e0
?dDR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
?dDR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
?dStack:
 ffff880076495eb8 ffffffff8134b312 0000000000000002 00007f53aec0a000
 ffff880076495f50 0000000000000002 0000000000000000 ffff880076495ed8
 ffffffff8134b80a 00007f53aec0a000 ffff88007661c000 ffff880076495ef8
?dCall Trace:
more>
Only 'q' or 'Q' are processed at more prompt, input ignored
?d [<ffffffff8134b312>] __handle_sysrq+0xa2/0x170
?d [<ffffffff8134b80a>] write_sysrq_trigger+0x4a/0x50
?d [<ffffffff811fb61d>] proc_reg_write+0x3d/0x80
?d [<ffffffff811993bd>] vfs_write+0xbd/0x1e0
?d [<ffffffff81199d89>] SyS_write+0x49/0xa0
?d [<ffffffff815a27c7>] tracesys+0xdd/0xe2
?dCode: eb 9b 45 01 f4 45 39 65 34 75 e5 4c 89 ef e8 02 f8 ff ff eb db 0f 1f 44 00 00 55 c7 05 20 17 54 00 01 00 00 00 48 89 e5 0f ae f8 <c6> 04 25 00 00 00 00 01 5d c3 0f 1f 44 00 00 55 31 c0 c7 05 7e


[1]kdb> summary
sysname    Linux
release    3.10.84
version    #1 SMP Tue Jul 28 19:53:37 IST 2015
machine    x86_64
nodename   localhost.localdomain
domainname (none)
ccversion  CCVERSION
date       2015-12-30 14:07:49 tz_minuteswest 0
uptime     00:01
load avg   0.95 0.21 0.06

MemTotal:        2050340 kB
MemFree:         1775224 kB
Buffers:             764 kB

[1]kdb> bt
Stack traceback for pid 2352
0xffff880077275a90     2352      629  1    1   R  0xffff880077276070 *bash
 ffff880076495e80 0000000000000018 ffff880076495eb8 ffffffff8134b312
 0000000000000002 00007f53aec0a000 ffff880076495f50 0000000000000002
 0000000000000000 ffff880076495ed8 ffffffff8134b80a 00007f53aec0a000
Call Trace:
 [<ffffffff8134b312>] ? __handle_sysrq+0xa2/0x170
 [<ffffffff8134b80a>] ? write_sysrq_trigger+0x4a/0x50
 [<ffffffff811fb61d>] ? proc_reg_write+0x3d/0x80
 [<ffffffff811993bd>] ? vfs_write+0xbd/0x1e0
 [<ffffffff81199d89>] ? SyS_write+0x49/0xa0
 [<ffffffff815a27c7>] ? tracesys+0xdd/0xe2

[1]kdb> rd
ax: 000000000000000f  bx: ffffffff8190dbc0  cx: ffff88007fd0eec0
dx: 0000000000000000  si: 0000000000000000  di: 0000000000000063
bp: ffff880076495e80  sp: ffff880076495e80  r8: ffffffff81b9069c
r9: 0000000000000245  r10: 0000000000000244  r11: 0000000000000003
r12: 0000000000000063  r13: 0000000000000246  r14: 0000000000000007
r15: 0000000000000000  ip: ffffffff8134abb6  flags: 00010096  cs: 00000010
ss: 00000018  ds: 00000018  es: 00000018  fs: 00000018  gs: 00000018

[1]kdb> go
Catastrophic error detected
kdb_continue_catastrophic=0, type go a second time if you really want to continue

[1]kdb> go
Catastrophic error detected
kdb_continue_catastrophic=0, attempting to continue
[   28.964025] Modules linked in: ip6t_rpfilter ip6t_REJECT ipt_REJECT xt_conntrack ebtable_nat ebtable_broute bridge stp llc ebtable_filter ebtables ip6table_nat nf_conntrack_ipv6 nf_defrag_ipv6 nf_nat_ipv6 ip6table_mangle ip6table_security ip6table_raw ip6table_filter ip6_tables iptable_nat nf_conntrack_ipv4 nf_defrag_ipv4 nf_nat_ipv4 nf_nat nf_conntrack iptable_mangle iptable_security iptable_raw iptable_filter snd_hda_intel snd_hda_codec snd_hwdep snd_seq snd_seq_device snd_pcm snd_page_alloc snd_timer snd serio_raw pcspkr i2c_piix4 virtio_balloon soundcore nfsd auth_rpcgss nfs_acl lockd sunrpc ip_tables xfs libcrc32c ata_generic pata_acpi ata_piix cirrus syscopyarea sysfillrect sysimgblt drm_kms_helper ttm virtio_net virtio_blk drm libata virtio_pci virtio_ring virtio i2c_core floppy dm_mirror dm_region_hash dm_log dm_mod
[  112.090263] Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2007
[  112.090263] task: ffff880077275a90 ti: ffff880076494000 task.ti: ffff880076494000
[  112.090263] RIP: 0010:[<ffffffff8134abb6>]  [<ffffffff8134abb6>] sysrq_handle_crash+0x16/0x20
[  112.090263] RSP: 0018:ffff880076495e80  EFLAGS: 00010096
[  112.090263] RAX: 000000000000000f RBX: ffffffff8190dbc0 RCX: ffff88007fd0eec0
[  112.090263] RDX: 0000000000000000 RSI: ffff88007fd0d368 RDI: 0000000000000063
[  112.090263] RBP: ffff880076495e80 R08: ffffffff81b9069c R09: 0000000000000245
[  112.090263] R10: 0000000000000244 R11: 0000000000000003 R12: 0000000000000063
[  112.090263] R13: 0000000000000246 R14: 0000000000000007 R15: 0000000000000000
[  112.090263] FS:  00007f53aec0c740(0000) GS:ffff88007fd00000(0000) knlGS:0000000000000000
[  112.090263] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  112.090263] CR2: 0000000000000000 CR3: 00000000764bd000 CR4: 00000000000006e0
[  112.090263] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
[  112.104034] DR3: 0000000000000000 DR6: 00000000ffff0ff0 DR7: 0000000000000400
[  112.104034] Stack:
[  112.104034]  ffff880076495eb8 ffffffff8134b312 0000000000000002 00007f53aec0a000
[  112.104034]  ffff880076495f50 0000000000000002 0000000000000000 ffff880076495ed8
[  112.104034]  ffffffff8134b80a 00007f53aec0a000 ffff88007661c000 ffff880076495ef8
[  112.104034] Call Trace:
[  112.104034]  [<ffffffff8134b312>] __handle_sysrq+0xa2/0x170
[  112.104034]  [<ffffffff8134b80a>] write_sysrq_trigger+0x4a/0x50
[  112.104034]  [<ffffffff811fb61d>] proc_reg_write+0x3d/0x80
[  112.104034]  [<ffffffff811993bd>] vfs_write+0xbd/0x1e0
[  112.104034]  [<ffffffff81199d89>] SyS_write+0x49/0xa0
[  112.104034]  [<ffffffff815a27c7>] tracesys+0xdd/0xe2
[  112.104034] Code: eb 9b 45 01 f4 45 39 65 34 75 e5 4c 89 ef e8 02 f8 ff ff eb db 0f 1f 44 00 00 55 c7 05 20 17 54 00 01 00 00 00 48 89 e5 0f ae f8 <c6> 04 25 00 00 00 00 01 5d c3 0f 1f 44 00 00 55 31 c0 c7 05 7e
[  112.104034] RIP  [<ffffffff8134abb6>] sysrq_handle_crash+0x16/0x20
[  112.104034]  RSP <ffff880076495e80>
[  112.104034] CR2: 0000000000000000
[  112.104034] ---[ end trace 4484e44d0167e21c ]---
[  112.104034] Kernel panic - not syncing: Fatal exception
PANIC: Fatal exception

Entering kdb (current=0xffff880077275a90, pid 2352) on processor 1 due to Keyboard Entry

[Note:] kdb command "go" is used to continue kernel execution. In above case
it will not take you back to bash shell. :)

Few questions for you;

[Q:1] What exactly you are trying to achieve ?
[Q:2] What is the exact issue ?

Regards,
BKS

[[1]] http://people.redhat.com/anderson/crash_whitepaper/#INVOCATION
[[2]] https://www.kernel.org/pub/linux/kernel/people/jwessel/kdb/