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[RFC V2 PATCH 0/1] kexec: crash_kexec_post_notifiers boot option related fixes

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  [RFC V2 PATCH 0/1] kexec: crash_kexec_post_notifiers boot option  related fixes Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com> - 2015-07-24 03:30 +0200
    Re: [RFC V2 PATCH 0/1] kexec: crash_kexec_post_notifiers boot  option related fixes Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com> - 2015-08-03 13:10 +0200
      Re: [RFC V2 PATCH 0/1] kexec: crash_kexec_post_notifiers boot option related fixes ebiederm@xmission.com (Eric W. Biederman) - 2015-08-03 18:40 +0200
        RE: [RFC V2 PATCH 0/1] kexec: crash_kexec_post_notifiers boot  option related fixes 河合英宏 / KAWAI，HIDEHIRO   <hidehiro.kawai.ez@hitachi.com> - 2015-08-04 13:50 +0200

#1191421 — [RFC V2 PATCH 0/1] kexec: crash_kexec_post_notifiers boot option related fixes

This is a bugfix patch for crash_kexec_post_notifiers boot option
which allows users to call panic notifiers and kmsg dumpers before
kdump.

This fixes one of the problems reported by Daniel Walker
(https://lkml.org/lkml/2015/6/24/44). 

 Problem 1:
 If crash_kexec_post_notifiers boot option is specified, some
 shutting down process which assume other cpus are still alive
 don't work properly.

 Problem 2 (addressed by this patch):
 If crash_kexec_post_notifiers boot option is specified, register
 information of other cpus are not saved to crash dumps.

Following Vivek's opinion, this patch replaces smp_send_stop()
in panic() with suitable version for crash_kexec which saves
cpu states and other things like cleaning up VMX/SVM.  Since this
needs architecture specific implementation and it's not so trivial,
this version only support for x86.  So the problem 1, known to
happen on MIPS/OCTEON, is not addressed now.

To keep the modification impact low, this patch doesn't change
the logic basically if crash_kexec_post_notifiers is not specified.

Please note that crash_kexec() can be called directly without
entering panic().  Stopping other cpus functionality is still
needed in crash_kexec().

Changes in V2:
- Replace smp_send_stop() call with crash_kexec version which
  saves cpu states and does cleanups instead of changing execution
  flow
- Drop a fix for Problem 1
- Drop other patches because they aren't needed anymore

V1: https://lkml.org/lkml/2015/7/10/316

---

Hidehiro Kawai (1):
      panic/x86: Replace smp_send_stop() with crash_kexec version


 arch/x86/kernel/crash.c |   16 +++++++++++-----
 kernel/panic.c          |   29 +++++++++++++++++++++++------
 2 files changed, 34 insertions(+), 11 deletions(-)


-- 
Hidehiro Kawai
Hitachi, Ltd. Research & Development Group


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#1198744 — Re: [RFC V2 PATCH 0/1] kexec: crash_kexec_post_notifiers boot option related fixes

Hello Eric and Vivek,

Do you have any comments?

(2015/07/24 10:16), Hidehiro Kawai wrote:
> This is a bugfix patch for crash_kexec_post_notifiers boot option
> which allows users to call panic notifiers and kmsg dumpers before
> kdump.
> 
> This fixes one of the problems reported by Daniel Walker
> (https://lkml.org/lkml/2015/6/24/44). 
> 
>  Problem 1:
>  If crash_kexec_post_notifiers boot option is specified, some
>  shutting down process which assume other cpus are still alive
>  don't work properly.
> 
>  Problem 2 (addressed by this patch):
>  If crash_kexec_post_notifiers boot option is specified, register
>  information of other cpus are not saved to crash dumps.
> 
> Following Vivek's opinion, this patch replaces smp_send_stop()
> in panic() with suitable version for crash_kexec which saves
> cpu states and other things like cleaning up VMX/SVM.  Since this
> needs architecture specific implementation and it's not so trivial,
> this version only support for x86.  So the problem 1, known to
> happen on MIPS/OCTEON, is not addressed now.
> 
> To keep the modification impact low, this patch doesn't change
> the logic basically if crash_kexec_post_notifiers is not specified.
> 
> Please note that crash_kexec() can be called directly without
> entering panic().  Stopping other cpus functionality is still
> needed in crash_kexec().
> 
> Changes in V2:
> - Replace smp_send_stop() call with crash_kexec version which
>   saves cpu states and does cleanups instead of changing execution
>   flow
> - Drop a fix for Problem 1
> - Drop other patches because they aren't needed anymore
> 
> V1: https://lkml.org/lkml/2015/7/10/316
> 
> ---
> 
> Hidehiro Kawai (1):
>       panic/x86: Replace smp_send_stop() with crash_kexec version
> 
> 
>  arch/x86/kernel/crash.c |   16 +++++++++++-----
>  kernel/panic.c          |   29 +++++++++++++++++++++++------
>  2 files changed, 34 insertions(+), 11 deletions(-)
> 
> 


-- 
Hidehiro Kawai
Hitachi, Ltd. Research & Development Group


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#1199030 — Re: [RFC V2 PATCH 0/1] kexec: crash_kexec_post_notifiers boot option related fixes

Hidehiro Kawai <hidehiro.kawai.ez@hitachi.com> writes:

> Hello Eric and Vivek,
>
> Do you have any comments?

crash_kexec_post_notifiers is a debugging hack to allow people to test
if the kmsg_dump works better than kexec.  crash_kexec_post_notifiers is
not, nor has it ever been a solution for general operation (which is
what I perceive this work trying to push).

I will not support any work that expands crash_kexec_post_notifiers to
be more than it currently is, because people want ``panic hooks'' to
run before kexec.  That appropach was extensively tested before kexec on
panic was implemented in the kernel and every implementation failed.
The practical symptom was that everything would work ok in testing but
on failures in the real world there would be enough going on in the
dying kernel that no crash dump would be taken.  kexec on panic on the
other hand works a reasonable fraction of the time.

I deeply and fundamentally can not support a general purpose hook being
called before kexec.  In 15 years of practice I have never heard of a
case where using a general purpose hook does anything but make kexec on
panic undebuggable in practice.

A specific hook for a very specific purpose when there is no other way
we can consider.

If you don't have something that generalises well into a general purpose
operation that it makes sense for everyone to call you can always use
the world's largest aka you can run code before the new kernel starts
that is loaded with kexec_load.

If you absolutely must run code in the dying code because you need lots
of the kernel infrastructure to work, and it is too hard to code a small
little bit of stand-alone assembly, I am sorry for you.  Experience shows
that will never work when the kernel fails in interesting ways.

So no.  I don't think there is any point to putting any more effort into
the crash_kexec_post_notifiers path because experience has shown over
the years that in practice it won't work for anyone, and if the code
doesn't work in practice there is no point in developing or implementing
it.

Eric
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#1199700 — RE: [RFC V2 PATCH 0/1] kexec: crash_kexec_post_notifiers boot option related fixes

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