Re: [PATCH RFC 2/2] checkpatch: warn on usage of VM_BUG_ON() and friends

David Hildenbrand
2022-08-25T11:51:33Z

On 25.08.22 13:43, Jani Nikula wrote:
> On Thu, 25 Aug 2022, David Hildenbrand <david@redhat.com> wrote:
>> On 24.08.22 18:52, Joe Perches wrote:
>>> On Wed, 2022-08-24 at 18:31 +0200, David Hildenbrand wrote:
>>>> checkpatch does not point out that VM_BUG_ON() and friends should be
>>>> avoided, however, Linus notes:
>>>>
>>>> VM_BUG_ON() has the exact same semantics as BUG_ON. It is literally
>>>> no different, the only difference is "we can make the code smaller
>>>> because these are less important". [1]
>>>>
>>>> So let's warn on VM_BUG_ON() and friends as well. While at it, make it
>>>> clearer that the kernel really shouldn't be crashed.
>>>>
>>>> Note that there are some other *_BUG_ON flavors, but they are not all
>>>> bad: for example, KVM_BUG_ON() only triggers a WARN_ON_ONCE and then
>>>> flags KVM as being buggy, so we'll not care about them for now here.
>>> []
>>>> diff --git a/scripts/checkpatch.pl b/scripts/checkpatch.pl
>>> []
>>>> @@ -4695,12 +4695,12 @@ sub process {
>>>> }
>>>> }
>>>>
>>>> -# avoid BUG() or BUG_ON()
>>>> - if ($line =~ /\b(?:BUG|BUG_ON)\b/) {
>>>> +# do not use BUG(), BUG_ON(), VM_BUG_ON() and friends.
>>>> + if ($line =~ /\b(?:BUG|BUG_ON|VM_BUG_ON|VM_BUG_ON_[A-Z]+)\b/) {
>>>
>>> Perhaps better as something like the below to pick up more variants
>>>
>>
>> Trying to find more possible variants and exceptions
>
>> CI_BUG_ON(
>> -> Bad with CONFIG_DRM_I915_DEBUG
>> GEM_BUG_ON(
>> -> Bad with CONFIG_DRM_I915_DEBUG_GEM_ONCE
>
> These are hidden behind debug knobs that we use in our CI to
> specifically catch "should not happen" cases fast and loud. Should not
> be a problem for regular users.
>

I tend to agree but I don't think this is worth an exception.
VM_BUG_ON also requires CONFIG_DEBUG_VM and absolutely shouldn't
be used as I learned.

Quoting Linus:

Really. BUG_ON() IS NOT FOR DEBUGGING. [1]

This kind of "I don't think this can happen" is _never_ an excuse for it. [2]

For CI work, it might be sufficient to use WARN_ON_ONCE() combined with panic_on_warn.

[1] https://lore.kernel.org/all/CAHk-=wiEAH+ojSpAgx_Ep=NKPWHU8AdO3V56BXcCsU97oYJ1EA@mail.gmail.com/
[2] https://lore.kernel.org/all/CAHk-=wg40EAZofO16Eviaj7mfqDhZ2gVEbvfsMf6gYzspRjYvw@mail.gmail.com/

--
Thanks,

David / dhildenb