Re: [PATCH] [BUGFIX] crash/ioapic: Prevent crash_kexec() from deadlocking of ioapic_lock

[Eric W. Biederman]

Don Zickus <dzickus@xxxxxxxxxx> writes:

> On Tue, Aug 27, 2013 at 12:41:51PM +0900, Yoshihiro YUNOMAE wrote:
>> Hi Don,
>> 
>> Sorry for the late reply.
>> 
>> (2013/08/22 22:11), Don Zickus wrote:
>> >On Thu, Aug 22, 2013 at 05:38:07PM +0900, Yoshihiro YUNOMAE wrote:
>> >>>So, I agree with Eric, let's remove the disable_IO_APIC() stuff and keep
>> >>>the code simpler.
>> >>
>> >>Thank you for commenting about my patch.
>> >>I didn't know you already have submitted the patches for this deadlock
>> >>problem.
>> >>
>> >>I can't answer definitively right now that no problems are induced by
>> >>removing disable_IO_APIC(). However, my patch should be work well (and
>> >>has already been merged to -tip tree). So how about taking my patch at
>> >>first, and then discussing the removal of disabled_IO_APIC()?
>> >
>> >It doesn't matter to me.  My orignal patch last year was similar to yours
>> >until it was suggested that we were working around a problem which was we
>> >shouldn't touch the IO_APIC code on panic.  Then I wrote the removal of
>> >disable_IO_APIC patch and did lots of testing on it.  I don't think I have
>> >seen any issues with it (just the removal of disabling the lapic stuff).
>> 
>> Yes, you really did a lot of testing about this problem according to
>> your patch(https://lkml.org/lkml/2012/1/31/391). Although you
>> said jiffies calibration code does not need the PIT in
>> http://lists.infradead.org/pipermail/kexec/2012-February/006017.html,
>> I don't understand yet why we can remove disable_IO_APIC.
>> Would you please explain about the calibration codes?
>
> I forgot a lot of this, Eric B. might remember more (as he was the one that
> pointed this out initially).  I believe initially the io_apic had to be in
> a pre-configured state in order to do some early calibration of the timing
> code.  Later on, it was my understanding, that the calibration of various
> time keeping stuff did not need the io_apic in a correct state.  The code
> might have switched to tsc instead of PIT, I forget.

Yes.  Alan Coxe's initial SMP port had a few cases where it still
exepected the system to be in PIT mode during boot and it took us a
decade or so before those assumptions were finally expunged.

> Then again looking at the output of the latest dmesg, it seems the IO APIC
> is initialized way before the tsc is calibrated.  So I am not sure what
> needed to get done or what interrupts are needed before the IO APIC gets
> initialized.

The practical issue is that jiffies was calibrated off of the PIT timer
if I recall.  But that is all old news.

>> By the way, can we remove disable_IO_APIC even if an old dump capture
>> kernel is used?
>
> Good question.  I did a bunch of testing with RHEL-6 too, which is 2.6.32
> based.  But I think we added some IRR fixes (commit 1e75b31d638), which
> may or may not have helped in this case.  So I don't know when a kernel
> started worked correctly during init (with the right changes).  I believe
> 2.6.32 had everything.

A sufficient old and buggy dump capture kernel will fail because of bugs
in it's startup path, but I don't think anyone cares.

The kernel startup path has been fixed for years, and disable_IO_APIC in
crash_kexec has always been a bug work-around for deficiencies in the
kernel's start up path (not part of the guaranteed interface).
Furthermore every real system configuration I have encountered used the
same kernel version for the crashdump kernel and the production kernel.
So we should be good.

> However, at the same time, the memory layout of current kernels has
> changed and I am not sure if older kernels can read them correctly (or if
> you just need the latest makedumpfile tool).  In other words, an old
> kernel like 2.6.32 might not work as a kdump kernel for a 3.10 kernel.  I
> don't know.

Memory layout should not be an issue at all.  The details are passed
from one kernel to another in a set of ELF headers.  So if the crash
dump kernel can run in the memory reserved for it, all should work well.

Eric
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