Re: [PATCH] doc: add note on usleep_range range

Nicholas Mc Guire
2016-12-13T09:28:24Z

On Tue, Dec 13, 2016 at 11:10:50AM +0200, Jani Nikula wrote:
> On Tue, 13 Dec 2016, Nicholas Mc Guire <hofrat@osadl.org> wrote:
> > useleep_range() with a delta of 0 makes no sense and only prevents the
> > timer subsystem from optimizing interrupts. As any user of usleep_range()
> > is in non-atomic context the timer jitter is in the range of 10s of
> > microseconds anyway.
> >
> > This adds a note making it clear that a range of 0 is a bad idea.
>
> So I don't really have anything to do with the timer subsystem, I'm just
> their "consumer", so take this with a grain of salt.
>
> Documentation is good, but I don't think this will be enough.
>
> I think the only thing that will work is to detect and complain about
> things like this automatically. Some ideas:
>
> * WARN_ON(min == max) or WARN_ON_ONCE(min == max) in usleep_range()
> might be drastic, but it would get the job done eventually.
>
> * If you want to avoid the runtime overhead (and complaints about the
> backtraces), you could wrap usleep_range() in a macro that does
> BUILD_BUG_ON(min == max) if the parameters are build time constants
> (they usually are). But you'd have to fix all the problem cases first.
>
> * You could try (to persuade Julia or Dan) to come up with a
> cocci/smatch check for usleep_range() calls where min == max, so we
> could get bug reports for this. This probably works on expressions, so
> this would catch also cases where the parameters aren't built time
> constants.
>

I fully agree - without automation it is almost usless
the coccinelle spatch is a seperate patch and it is tested butnot yet
submitted.

the spatch for this iss actually trivial

@nulldelta@
constant C;
position p;
@@

* usleep_range@p(C,C)

thx!
hofrat