linux-kernel - [PATCH RFC 0/2] coding-style.rst: document BUG() and WARN() rules

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Message-Id: <20220824163100.224449-1-david@redhat.com>
Date:   Wed, 24 Aug 2022 18:30:58 +0200
From:   David Hildenbrand <david@...hat.com>
To:     linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, linux-doc@...r.kernel.org,
        kexec@...ts.infradead.org, David Hildenbrand <david@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        David Laight <David.Laight@...LAB.COM>,
        Jonathan Corbet <corbet@....net>,
        Andy Whitcroft <apw@...onical.com>,
        Joe Perches <joe@...ches.com>,
        Dwaipayan Ray <dwaipayanray1@...il.com>,
        Lukas Bulwahn <lukas.bulwahn@...il.com>,
        Baoquan He <bhe@...hat.com>, Vivek Goyal <vgoyal@...hat.com>,
        Dave Young <dyoung@...hat.com>
Subject: [PATCH RFC 0/2] coding-style.rst: document BUG() and WARN() rules

As it seems to be rather unclear if/when to use BUG(), BUG_ON(),
VM_BUG_ON(), WARN_ON_ONCE(), ... let's try to document the result of a
recent discussion.

Details can be found in patch #1.

--------------------------------------------------------------------------

Here is some braindump after thinking about BUG_ON(), WARN_ON(), ... and
how it interacts with kdump.

I was wondering what the expectation on a system with armed kdump are,
for example, after we removed most BUG_ON() instances and replaced them
by WARN_ON_ONCE(). I would assume that we actually want to panic in some
cases to capture a proper system dump instead of continuing and eventually
ending up with a completely broken system where it's hard to extract any
useful debug information. We'd have to enable panic_on_warn. But we'd only
want to do that in case kdump is actually armed after boot.

So one idea would be to have some kind of "panic_on_warn_with_kdump" mode.
But then, we'd actually crash+kdump even on the most harmless WARN_ON()
conditions, because they all look alike. To compensate, we would need
some kind of "severity" levels of a warning -- at least some kind of
"this is harmless and we can easily recover, but please tell the
developers" vs. "this is real bad and unexpected, capture a dump
immediately instead of trying to recover and eventually failing miserably".

But then, maybe we really want something like BUG_ON() -- let's call it
CBUG_ON() for simplicity -- but be able to make it be usable in
conditionals (to implement recovery code if easily possible) and make the
runtime behavior configurable.

if (CBUG_ON(whatever))
	try_to_recover()

Whereby, for example, "panic_on_cbug" and "panic_on_cbug_with_kdump"
could control the runtime behavior.

But this is just a braindump and I assume people reading along have other,
better ideas. Especially, a better name for CBUG.

Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: David Laight <David.Laight@...LAB.COM>
Cc: Jonathan Corbet <corbet@....net>
Cc: Andy Whitcroft <apw@...onical.com>
Cc: Joe Perches <joe@...ches.com>
Cc: Dwaipayan Ray <dwaipayanray1@...il.com>
Cc: Lukas Bulwahn <lukas.bulwahn@...il.com>
Cc: Baoquan He <bhe@...hat.com>
Cc: Vivek Goyal <vgoyal@...hat.com>
Cc: Dave Young <dyoung@...hat.com>

David Hildenbrand (2):
  coding-style.rst: document BUG() and WARN() rules ("do not crash the
    kernel")
  checkpatch: warn on usage of VM_BUG_ON() and friends

 Documentation/process/coding-style.rst | 27 ++++++++++++++++++++++++++
 scripts/checkpatch.pl                  |  6 +++---
 2 files changed, 30 insertions(+), 3 deletions(-)

-- 
2.37.1