Message-ID: <1481675252.29291.39.camel@perches.com>
Subject: Re: [PATCH] doc: add note on usleep_range range
From: Joe Perches <joe@perches.com>
To: Nicholas Mc Guire <der.herr@hofr.at>,
        Jani Nikula <jani.nikula@linux.intel.com>
Cc: Nicholas Mc Guire <hofrat@osadl.org>,
        Thomas Gleixner <tglx@linutronix.de>, Jonathan Corbet <corbet@lwn.net>,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
        Dan Carpenter <dan.carpenter@oracle.com>,
        Julia Lawall <julia.lawall@lip6.fr>
Date: Tue, 13 Dec 2016 16:27:32 -0800
In-Reply-To: <20161213091912.GA6347@osadl.at>
References: <1481601523-14004-1-git-send-email-hofrat@osadl.org>
         <87r35ctcrp.fsf@intel.com> <20161213091912.GA6347@osadl.at>
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a, On Tue, 2016-12-13 at 09:19 +0000, 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 timea,
> >   constants.

You could also add a macro for usleep_range like

#define usleep_range(a, b) \
({ \
	if (__builtin_constant_p(a) && __builtin_constant_p(b)) { \
		if (a == b) \
			__compiletime_warning("Better to use usleep_range with different values"); \
		else if (a > b) \
			__compiletime_error("usleep_range uses smaller value first"); \
	} \
	usleep_range(a, b); \
})

and add parentheses around the actual function
definition for usleep_range in kernel/time/timer.c
so the macro works and these messages get emitted
at compile-time.