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

Kalle Valo
2022-09-21T04:40:35Z

David Hildenbrand <david@redhat.com> writes:

> 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>

[...]

> +Use WARN_ON_ONCE() rather than WARN() or WARN_ON()
> +**************************************************
> +
> +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.

FWIW I have had cases where WARN() messages caused a reboot, maybe
mention that here? In my case the logging was so excessive that the
watchdog wasn't updated and in the end the device was forcefully
rebooted.

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