Fix race issues among panic, NMI and crash_kexec

When an HA clustering software or administrator detects unresponsiveness of a host, they issue an NMI to the host to completely stop current works and take a crash dump. If the kernel has already panicked or is capturing a crash dump at that time, further NMI can cause a crash dump failure.

Also, crash_kexec() called from oops context and panic() can cause race conditions.

To solve these issues, this patch set does following things:

- Don't call panic() on NMI if the kernel has already panicked
- Extend exclusion control currently done by panic_lock to crash_kexec
- Introduce "apic_extnmi=none" boot option which masks external NMI NMI at the boot time

Additionally, "apic_extnmi=all" is provieded. This option unmasks external NMI for all CPUs. This would help cause kernel panic even if CPU 0 can't handle an external NMI due to hang-up in NMI context or being handled by other NMI handlers.

This patch set can be applied to current -tip tree.

V6:
- Update comments and patch descriptions all over the patch series
- Add documentation for kernel.panic_on_io_nmi sysctl (PATCH 6/6)
- Separate PATCH 5/6 from PATCH 2/6 because the portion is actually needed for "apic_extnmi=all" case introduced by PATCH 4/6
- ...and various fixes (please see the change logs in each patch description for details)

V5: https://lkml.org/lkml/2015/11/20/228
- Use WRITE_ONCE() for crash_ipi_done to keep the instruction order (PATCH 2/4)
- Address concurrent unknown/external NMI case, too (PATCH 2/4)
- Fix build errors (PATCH 3/4)
- Rename "noextnmi" boot option to "apic_extnmi" and expand its feature (PATCH 4/4)

V4: https://lkml.org/lkml/2015/9/25/193
- Improve comments and descriptions (PATCH 1/4 to 3/4)
- Use new __crash_kexec(), no exclusion check version of crash_kexec(), instead of checking if panic_cpu is the current cpu or not (PATCH 3/4)

V3: https://lkml.org/lkml/2015/8/6/39
- Introduce nmi_panic() macro to reduce code duplication
- In the case of panic on NMI, don't return from NMI handlers if another cpu already panicked

V2: https://lkml.org/lkml/2015/7/27/31
- Use atomic_cmpxchg() instead of current spin_trylock() to exclude concurrent accesses to panic() and crash_kexec()
- Don't introduce no-lock version of panic() and crash_kexec()

V1: https://lkml.org/lkml/2015/7/22/81

---

Hidehiro Kawai (6):
panic/x86: Fix re-entrance problem due to panic on NMI
panic/x86: Allow CPUs to save registers even if they are looping in NMI context
kexec: Fix race between panic() and crash_kexec() called directly x86/apic: Introduce apic_extnmi boot option
x86/nmi: Fix to save registers for crash dump on external NMI broadcast
Documentation: Add documentation for kernel.panic_on_io_nmi sysctl

 Documentation/kernel-parameters.txt |    9 +++++++++
 Documentation/sysctl/kernel.txt     |   15 +++++++++++++++
 arch/x86/include/asm/apic.h         |    5 +++++
 arch/x86/include/asm/reboot.h       |    1 +
 arch/x86/kernel/apic/apic.c         |   35 +++++++++++++++++++++++++++++++++--
 arch/x86/kernel/nmi.c               |   27 ++++++++++++++++++++++-----
 arch/x86/kernel/reboot.c            |   28 ++++++++++++++++++++++++++++
 include/linux/kernel.h              |   29 +++++++++++++++++++++++++++++
 include/linux/kexec.h               |    2 ++
 kernel/kexec_core.c                 |   30 +++++++++++++++++++++++++++++-
 kernel/panic.c                      |   29 ++++++++++++++++++++++++-----
 kernel/watchdog.c                   |    2 +-
 12 files changed, 198 insertions(+), 14 deletions(-)

Hidehiro Kawai
Hitachi, Ltd. Research & Development Group