linux-kernel - Re: [PATCH RFC 2/2] checkpatch: warn on usage of VM_BUG

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Message-ID: <7d146520-52e2-bd8d-3c53-a444e93e4006@redhat.com>
Date:   Thu, 25 Aug 2022 13:51:16 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Jani Nikula <jani.nikula@...ux.intel.com>,
        Joe Perches <joe@...ches.com>, linux-kernel@...r.kernel.org
Cc:     linux-mm@...ck.org, linux-doc@...r.kernel.org,
        kexec@...ts.infradead.org,
        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>,
        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: Re: [PATCH RFC 2/2] checkpatch: warn on usage of VM_BUG_ON() and
 friends

On 25.08.22 13:43, Jani Nikula wrote:
> On Thu, 25 Aug 2022, David Hildenbrand <david@...hat.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