[Discuss] Kernel interpretation of pressing the reset button?

[Alan W. Irwin]

On 2018-06-09 12:19-0700 Cy Schubert wrote:

> In message <4E6AE5F528744E479C4F2AC326FFF840024809C57E at EX10MBX01.intra.c
> amosun.
> bc.ca>, "Deryk Barker (Honorary Colleague)" writes:
>> Alan, unless things have changed recently, the reset button simply asserts a
>> reset line/signal on the bus (not that PCIe is really a bus any more).
>>
>> If the OS is totally locked up and not even sampling the bus lines...
>
> There is no sampling of the bus. The reset pin on the CPU is also
> asserted when the bus is.

@Cy:

The details are important of course, but so is the overview.  Can I
conclude from what Deryk and you have said that the Linux kernel has
complete control of how pressing the reset button is interpreted?

If so, that would explain how badly knackered kernel ==> no result
from pressing the reset button.  Beyond that general conclusion, if
the kernel is working sufficiently well so the reset button works, why
is the result always a dirty shutdown that requires a journal replay
on the next boot that is typically accompanied by removal of inodes?
Is such a dirty shutdown considered safer than the alternative of
doing a clean shutdown when you have lost control of your computer
sufficiently so the reset button has to be hit to regain control?  And
is that inode removal (which I assume means removal of all files that
were memory resident but still needed to be written out to disk at the
time the reset button is hit) anything to worry about or does it mean,
for example, that effectively only a few lines of some log file that
was being actively written are missing on the new system rather than
the entire log file going missing.

I have never thought about these questions before because I have very
rarely (until the last several weeks) ever had to hit the system reset
button, and I only have a general knowledge of exactly what an inode
means, and I don't know, for example, whether a dirty file in memory
that corresponds to a log file that has had some lines added in memory
has a different inode than the old version of the log file.  Anyhow, I
would appreciate replies (from the overview perspective) to my
questions.

Alan
__________________________
Alan W. Irwin

Astronomical research affiliation with Department of Physics and Astronomy,
University of Victoria (astrowww.phys.uvic.ca).

Programming affiliations with the FreeEOS equation-of-state
implementation for stellar interiors (freeeos.sf.net); the Time
Ephemerides project (timeephem.sf.net); PLplot scientific plotting
software package (plplot.sf.net); the libLASi project
(unifont.org/lasi); the Loads of Linux Links project (loll.sf.net);
and the Linux Brochure Project (lbproject.sf.net).
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Linux-powered Science
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