Re: [PATCH] printk/kdb: Redirect printk messages into kdb in any context

[Doug Anderson]

Hi,

On Fri, May 15, 2020 at 9:36 AM Sergey Senozhatsky
<sergey.senozhatsky@xxxxxxxxx> wrote:
>
> On (20/05/15 17:32), Sumit Garg wrote:
> > > Can I please have some context what problem does this solve?
> >
> > You can find the problem description here [1] which leads to this fix.
>
> [..]
>
> > [1] https://lkml.org/lkml/2020/5/12/213
>
> Thanks for the link. I'm slightly surprised it took so many years
> to notice the addition of printk_nmi/printk_safe :)

I haven't looked at all the details, but IIUC we don't normally enter
kgdb on the primary CPU through a NMI context, but the secondary ones
(on x86) always do.  Most things are run on the primary CPU and I
think it's relatively unlikely for people to change the primary CPU
(though it is possible).

Probably things got worse when I changed the way "btc" worked to make
it common between all architectures.  See commit 9ef50a686b53
("UPSTREAM: kdb: Fix stack crawling on 'running' CPUs that aren't the
master").  Though theoretically someone could have changed masters and
reproduced the problem with a simple "bt" before my patch, now a
relatively normal command "btc" would tickle the problem.  I didn't
notice it because I work almost totally on arm/arm64 machines and they
don't have NMI (yet).

In general I've always wondered about why (historically) kgdb bugs
have sometimes gone unnoticed for a period of time.  That does seem to
be changing, though, and I've seen a few longstanding bugs getting
fixed recently.  :-)


-Doug