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

Nicholas Mc Guire
2016-12-13T12:24:22Z

On Tue, Dec 13, 2016 at 01:05:12PM +0100, Julia Lawall wrote:
>
>
> On Tue, 13 Dec 2016, Nicholas Mc Guire wrote:
>
> > 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)
>
> People never use more complex expressions?
>
well yes
@nulldelta@
expression E;
position p;
@@

* usleep_range@p(E,E)

but that seems to be it.
and the vast majority is simply constants

thx!
hofrat