Re: [PATCH v1 1/3] coding-style.rst: document BUG() and WARN() rules ("do not crash the kernel")

Akira Yokosawa
2022-09-22T13:44:00Z

Hi,

Minor nits on section title adornments.
See inline comments below.

On Tue, 20 Sep 2022 14:23:00 +0200, David Hildenbrand wrote:
> Linus notes [1] that the introduction of new code that uses VM_BUG_ON()
> is just as bad as BUG_ON(), because it will crash the kernel on
> distributions that enable CONFIG_DEBUG_VM (like Fedora):
>
> 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". [2]
>
> This resulted in a more generic discussion about usage of BUG() and
> friends. While there might be corner cases that still deserve a BUG_ON(),
> most BUG_ON() cases should simply use WARN_ON_ONCE() and implement a
> recovery path if reasonable:
>
> The only possible case where BUG_ON can validly be used is "I have
> some fundamental data corruption and cannot possibly return an
> error". [2]
>
> As a very good approximation is the general rule:
>
> "absolutely no new BUG_ON() calls _ever_" [2]
>
> ... not even if something really shouldn't ever happen and is merely for
> documenting that an invariant always has to hold. However, there are sill
> exceptions where BUG_ON() may be used:
>
> If you have a "this is major internal corruption, there's no way we can
> continue", then BUG_ON() is appropriate. [3]
>
> There is only one good BUG_ON():
>
> Now, that said, there is one very valid sub-form of BUG_ON():
> BUILD_BUG_ON() is absolutely 100% fine. [2]
>
> While WARN will also crash the machine with panic_on_warn set, that's
> exactly to be expected:
>
> So we have two very different cases: the "virtual machine with good
> logging where a dead machine is fine" - use 'panic_on_warn'. And
> the actual real hardware with real drivers, running real loads by
> users. [4]
>
> The basic idea is that warnings will similarly get reported by users
> and be found during testing. However, in contrast to a BUG(), there is a
> way to actually influence the expected behavior (e.g., panic_on_warn)
> and to eventually keep the machine alive to extract some debug info.
>
> Ingo notes that not all WARN_ON_ONCE cases need recovery. If we don't ever
> expect this code to trigger in any case, recovery code is not really
> helpful.
>
> I'd prefer to keep all these warnings 'simple' - i.e. no attempted
> recovery & control flow, unless we ever expect these to trigger.
> [5]
>
> There have been different rules floating around that were never properly
> documented. Let's try to clarify.
>
> [1] https://lkml.kernel.org/r/CAHk-=wiEAH+ojSpAgx_Ep=NKPWHU8AdO3V56BXcCsU97oYJ1EA@mail.gmail.com
> [2] https://lore.kernel.org/r/CAHk-=wg40EAZofO16Eviaj7mfqDhZ2gVEbvfsMf6gYzspRjYvw@mail.gmail.com
> [2] https://lkml.kernel.org/r/CAHk-=wit-DmhMfQErY29JSPjFgebx_Ld+pnerc4J2Ag990WwAA@mail.gmail.com
> [4] https://lore.kernel.org/r/CAHk-=wgF7K2gSSpy=m_=K3Nov4zaceUX9puQf1TjkTJLA2XC_g@mail.gmail.com
> [5] https://lore.kernel.org/r/YwIW+mVeZoTOxn%2F4@gmail.com
>
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
> Documentation/process/coding-style.rst | 61 ++++++++++++++++++++++++++
> 1 file changed, 61 insertions(+)
>
> diff --git a/Documentation/process/coding-style.rst b/Documentation/process/coding-style.rst
> index 03eb53fd029a..e05899cbfd49 100644
> --- a/Documentation/process/coding-style.rst
> +++ b/Documentation/process/coding-style.rst
> @@ -1186,6 +1186,67 @@ expression used. For instance:
> #endif /* CONFIG_SOMETHING */
>
>
> +22) Do not crash the kernel
> +---------------------------
> +
> +In general, it is not the kernel developer's decision to crash the kernel.
> +
> +Avoid panic()
> +=============
This looks to me like a subsection-level title. The adornment symbol
needs to be:

*************

> +
> +panic() should be used with care and primarily only during system boot.
> +panic() is, for example, acceptable when running out of memory during boot and
> +not being able to continue.
> +
> +Use WARN() rather than BUG()
> +============================
Ditto.

> +
> +Do not add new code that uses any of the BUG() variants, such as BUG(),
> +BUG_ON(), or VM_BUG_ON(). Instead, use a WARN*() variant, preferably
> +WARN_ON_ONCE(), and possibly with recovery code. Recovery code is not
> +required if there is no reasonable way to at least partially recover.
> +
> +"I'm too lazy to do error handling" is not an excuse for using BUG(). Major
> +internal corruptions with no way of continuing may still use BUG(), but need
> +good justification.
> +
> +Use WARN_ON_ONCE() rather than WARN() or WARN_ON()
> +**************************************************
These wrong adornment symbol confuse ReST parser of Sphinx and results in
the build error from "make htmldocs" at this title (long message folded):

Sphinx parallel build error:

docutils.utils.SystemMessage: /xxx/Documentation/process/coding-style.rst:1213:
(SEVERE/4) Title level inconsistent:

Please fix in v2.

Thanks, Akira

> +
> +WARN_ON_ONCE() is generally preferred over WARN() or WARN_ON(), because it
> +is common for a given warning condition, if it occurs at all, to occur
> +multiple times. This can fill up and wrap the kernel log, and can even slow
> +the system enough that the excessive logging turns into its own, additional
> +problem.
> + …