Re: [PATCH v11 4/5] core: Add kernel message dumper to call on oopses and panics

Simon Kagstrom
2009-10-26T11:53:19Z

On Mon, 26 Oct 2009 12:36:33 +0200
"Shargorodsky Atal (EXT-Teleca/Helsinki)" <ext-atal.shargorodsky@nokia.com> wrote:

> > > I can think of a couple of way to figure it out in the module
> > > itself, but I could not think of any clean way to do it.
> >
> > This is correct, and the mtdoops driver has some provisions to handle
> > this. First, there is a parameter to the module to specify whether
> > oopses should be dumped at all - I added this for the particular case
> > that someone has panic_on_oops set.
>
> It takes care of most of the situations, but panic_on_oops
> can be changed any time, even after the module is loaded.

Yes, but this parameter is settable at runtime as well by writing
to /sys/module/mtdoops/parameters/dump_oops.

> > Second, it does not dump oopses directly anyway, but puts it in a work
> > queue. That way, if panic_on_oops is set, it will store the panic but
> > the oops (called from the workqueue) will not get written anyway.
> >
>
> AFAIK, mtdoops does not put oopses in a work queue. And if by any chance
> it does, then I think it's wrong and might lead to missed oopses, as
> the oops might be because of the work queues themselves, or it might
> look to the kernel like some non-fatal fault, but actually it's a
> sign of a much more catastrophic failure - IOMMU device garbaging
> memory, for instance.

I was referring to my patches to it, sorry. It's in the patch "[PATCH v7 5/5]:
mtdoops: refactor as a kmsg_dumper" (as well as the parameter to dump
oopses at all).

There are other situations which will make dumping problematic as well,
e.g., crashes in the mtd code, so there are certainly some cases which
will be difficult to catch. But in the panic_on_oops case or
oops-in-interrupt, the oops won't be missed and won't be outputted
twice for mtdoops.

Anyway, I understand your problem and agree that it would be good to
fix. Moving up crash_kexec() and the notifiers will at least fix your
second issue. For the double-output-of-oopses, I don't see a good way
to fix it unless relying on the module to correct it like above.

// Simon