Re: [PATCH] panic.c: export panic_on_oops

Ingo Molnar
2009-10-12T14:30:38Z

* Simon Kagstrom <simon.kagstrom@netinsight.net> wrote:

> OK, I don't think we understand each other. Sorry if I'm being slow
> here, please tell me if I'm misunderstanding something fundamental
> below.

[ it could easily be me being confused - i dont know the mtdoops code
that well - i just raised an eyebrow at the export request, which
yelled 'layering violation' at me ;-) ]

> On Mon, 12 Oct 2009 15:15:29 +0200
> Ingo Molnar <mingo@elte.hu> wrote:
>
> > > I'm afraid I don't really see this issue. The workqueue is used to
> > > write the buffer to the mtd device if we are not in a panic or
> > > interrupt context - in which case we do it directly.
> > >
> > > So it's only used when an oops is ongoing.
> >
> > This fixation on 'panic' is so wrong!
> >
> > 90% of the bugs users care about dont involve any panic. And even if
> > there is a panic down the line, most of the interesting messages are in
> > the stream leading up to the panic - now tucked away in that async
> > workqueue mechanism and not visible.
>
> Well, this is what my patch [1] aims to fix. What it does is to put
> all messages in a circular buffer, and when an oops or panic occurs it
> writes them out. The current version only collects messages _during_
> an oops. I'll rework it with using kfifo as per Alans suggestion
> though.
>
> Neither the current code nor the new patch has them stored in the work
> queue during a panic though. If this happens, they will call
> panic_write (if it's available) to write it out directly.
>
> > There's two clean solutions i think:
> >
> > 1) add some new "ok, there's trouble!" callback to struct console and
> > the console driver could via that mechanism send out the _last_ 2KB
> > (or more) of kernel log messages. Basically we can go back in time by
> > looking at the dmesg buffer. The low level console driver does not
> > need to 'follow' the high level console state - it only wants to
> > print in case of trouble anyway.
> >
> > 2) or add buffered (flash-friendly) writes for all printk output - panic
> > and non-panic alike. This would be useful to debug suspend/resume
> > bugs for example. This would also optimize the packets of netconsole
> > output. (last i checked we sent a packet per line.)
>
> Well, suspend/resume hangs is one of the cases which mtdoops won't
> catch. [...]

( Sidenote: i see no reason why that wouldnt be possible if it's
implemented properly. )

> [...] But at least on NAND flash, I'd be a bit weary about logging all
> printk output for fear of wearing out the flash.

Clearly should be optional - like the s2ram debug hack to RTC registers
is optional on x86.

> > The workqueue looks wrong in both variants. If we are panic-ing (or
> > hanging, or ...) then we are halting the machine - the workqueue has
> > no chance to actually execute.
>
> but then we are using mtd->panic_write to write it out directly, not
> via the work queue.

... i might be confused, but in which case _is_ the workqueue used?

It clearly shows up in the codepaths i've read, but maybe i've
misinterpreted what it does.

Ingo