Messages in this thread | | | Date | Thu, 22 Sep 2022 16:12:05 +0200 | Subject | Re: [PATCH v1 1/3] coding-style.rst: document BUG() and WARN() rules ("do not crash the kernel") | From | David Hildenbrand <> |
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On 21.09.22 06:40, Kalle Valo wrote: > 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. >
That should be covered by the last part, no? What would be your suggestion?
-- Thanks,
David / dhildenb
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