Re: [PATCH RFC 1/2] coding-style.rst: document BUG() and WARN() rules ("do not crash the kernel")

Dave Young
2022-08-29T01:56:14Z

Hi David,

On Sat, 27 Aug 2022 at 01:02, David Hildenbrand <david@redhat.com> wrote:
>
> On 26.08.22 03:43, Dave Young wrote:
> > Hi David,
> >
> > [Added more people in cc]
> >
>
> Hi Dave,
>
> thanks for your input!

You are welcome :)

>
> [...]
>
> >> Side note: especially with kdump() I feel like we might see much more
> >> widespread use of panic_on_warn to be able to actually extract debug
> >> information in a controlled manner -- for example on enterprise distros.
> >> ... which would then make these systems more likely to crash, because
> >> there is no way to distinguish a rather harmless warning from a severe
> >> warning :/ . But let's see if some kdump() folks will share their
> >> opinion as reply to the cover letter.
> >
> > I can understand the intention of this patch, and I totally agree that
> > BUG() should be used carefully, this is a good proposal if we can
> > clearly define the standard about when to use BUG(). But I do have
>
> Essentially, the general rule from Linus is "absolutely no new BUG_ON()
> calls ever" -- but I think the consensus in that thread was that there
> are corner cases when it comes to unavoidable data corruption/security
> issues. And these are rare cases, not the usual case where we'd have
> used BUG_ON()/VM_BUG_ON().

Yes, probably.. (say probably because those cases are hidden and not
clear sometimes)

>
> > some worries, I think this standard is different for different sub
> > components, it is not clear to me at least, so this may introduce an
> > unstable running kernel and cause troubles (eg. data corruption) with
> > a WARN instead of a BUG. Probably it would be better to say "Do not
> > WARN lightly, and do not hesitate to use BUG if it is really needed"?
>
>
> Well, I don't make the rules, I document them and share them for general
> awareness/comments :) Documenting this is valuable, because there seem
> to be quite some different opinions floating around in the community --
> and I've been learning different rules from different people over the years.

Understand.

>
> >
> > About "patch_on_warn", it will depend on the admin/end user to set it,
> > it is not a good idea for distribution to set it. It seems we are
> > leaving it to end users to take the risk of a kernel panic even with
> > all kernel WARN even if it is sometimes not necessary.
>
> My question would be what we could add/improve to keep systems with
> kdump armed running as expected for end users, that is most probably:
>
> 1) don't crash on harmless WARN() that can just be reported and the
> machine will continue running mostly fine without real issues.
> 2) crash on severe issues (previously BUG) such that we can properly
> capture a system dump via kdump. The restart the machine.
>
> Of course, once one would run into 2), one could try reproducing with
> "panic_on_warn" to get a reasonable system dump. But I guess that's not
> what enterprise customers expect.
>

Sometimes the bug can not be easily reproduced again. So there seems
no easy and good way to use..

>
> One wild idea (in the cover letter) was to add something new that can be
> configured by user space and that expresses that something is more
> severe than just some warning that can be recovered easily. But it can
> eventually be recovered to keep the system running to some degree. But
> still, it's configurable if we want to trigger a panic or let the system
> run.
>
> John mentioned PANIC_ON().
>

I would vote for PANIC_ON(), it sounds like a good idea, because
BUG_ON() is not obvious and, PANIC_ON() can alert the code author that
this will cause a kernel panic and one will be more careful before
using it.

>
> What would be your expectation for kdump users under which conditions we
> want to trigger kdump and when not?
>
> Regarding panic_on_warn, how often do e.g., RHEL users observe warnings
> that we're not able to catch during testing, such that "panic_on_warn"
> would be a real no-go?

Well, I'm not sure how to answer the questions, when to panic should
be decided by kernel developers instead of kdump users, but I think
the panic behaviour does impact the supporting team. I added Stephen
who is from the RH supporting team, maybe he can have some inputs.

BTW, I vaguely remember Prarit introduced the panic_on_warn, see if he
has any comments here.

Thanks
Dave

>
> --
> Thanks,
>
> David / dhildenb
>