On 24.08.22 23:59, John Hubbard wrote: > On 8/24/22 09:30, David Hildenbrand wrote: >> diff --git a/Documentation/process/coding-style.rst b/Documentation/process/coding-style.rst >> index 03eb53fd029a..a6d81ff578fe 100644 >> --- a/Documentation/process/coding-style.rst >> +++ b/Documentation/process/coding-style.rst >> @@ -1186,6 +1186,33 @@ expression used. For instance: >> #endif /* CONFIG_SOMETHING */ >> > > I like the idea of adding this documentation, and this is the right > place. Naturally, if one likes something, one must immediately change > it. :) Therefore, here is an alternative writeup that I think captures > what you and the email threads were saying. > > How's this sound? Much better, thanks! :) > > diff --git a/Documentation/process/coding-style.rst b/Documentation/process/coding-style.rst > index 03eb53fd029a..32df0d503388 100644 > --- a/Documentation/process/coding-style.rst > +++ b/Documentation/process/coding-style.rst > @@ -1185,6 +1185,53 @@ expression used. For instance: > ... > #endif /* CONFIG_SOMETHING */ > > +22) Do not crash the kernel > +--------------------------- > + > +Use WARN() rather than BUG() > +**************************** > + > +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'll tend to keep in this section: "Unavoidable data corruption / security issues might be a very rare exception to this rule and need good justification." Because there are rare exceptions, and I'd much rather document the clear exception to this rule. > + > +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. (For example, once per file, or once per struct page.) This can fill up I'll drop the "For example" part. I feel like this doesn't really need an example -- most probably we've all been there already when the kernel log was flooded :) > +and wrap the kernel log, and can even slow the system enough that the excessive > +logging turns into its own, additional problem. > + > +Do not WARN lightly > +******************* > + > +WARN*() is intended for unexpected, this-should-never-happen situations. WARN*() > +macros are not to be used for anything that is expected to happen during normal > +operation. These are not pre- or post-condition asserts, for example. Again: > +WARN*() must not be used for a condition that is expected to trigger easily, for > +example, by user space actions. pr_warn_once() is a possible alternative, if you > +need to notify the user of a problem. > + > +Do not worry about panic_on_warn users > +************************************** > + > +A few more words about panic_on_warn: Remember that ``panic_on_warn`` is an > +available kernel option, and that many users set this option. This is why there > +is a "Do not WARN lightly" writeup, above. However, the existence of > +panic_on_warn users is not a valid reason to avoid the judicious use WARN*(). > +That is because, whoever enables panic_on_warn has explicitly asked the kernel > +to crash if a WARN*() fires, and such users must be prepared to deal with the > +consequences of a system that is somewhat more likely to crash. Side note: especially with kdump() I feel like we might see much more widespread use of panic_on_warn to be able to actually extract debug information in a controlled manner -- for example on enterprise distros. ... which would then make these systems more likely to crash, because there is no way to distinguish a rather harmless warning from a severe warning :/ . But let's see if some kdump() folks will share their opinion as reply to the cover letter. -- Thanks, David / dhildenb
