linux-kernel - Re: [PATCH] doc: add note on usleep

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Date:   Tue, 13 Dec 2016 12:24:11 +0000
From:   Nicholas Mc Guire <der.herr@...r.at>
To:     Julia Lawall <julia.lawall@...6.fr>
Cc:     Jani Nikula <jani.nikula@...ux.intel.com>,
        Nicholas Mc Guire <hofrat@...dl.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jonathan Corbet <corbet@....net>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, Dan Carpenter <dan.carpenter@...cle.com>
Subject: Re: [PATCH] doc: add note on usleep_range range

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@...dl.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