coding-style.rst: document BUG() and WARN() rules ("do not crash the kernel")

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
[3] 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

Reviewed-by: John Hubbard <jhubbard@nvidia.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20220923113426.52871-2-david@redhat.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

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