Re: [5/6] 2.6.21-rc2: known regressions

Randy Dunlap
2007-03-16T16:19:47Z

On Tue, 6 Mar 2007 08:44:43 -0800 (PST) Linus Torvalds wrote:

>
> This is just a coding style thing, but I thought I should really point it
> out, because these kinds of things quite often result in nasty bugs simply
> because the source code is so hard to read properly:
>
> On Tue, 6 Mar 2007, Ingo Molnar wrote:
> >
> > -static void hrtimer_switch_to_hres(void)
> > +static int hrtimer_switch_to_hres(void)
>
> Ok, so here's the quiz: does this function return "true on success, false
> on failure", or does it return "zero on success, negative on failure"?
>
> > if (base->hres_active)
> > - return;
> > + return 1;
>
> Ahh, it must be "true on success", right?
>
> > local_irq_save(flags);
> >
> > if (tick_init_highres()) {
> > local_irq_restore(flags);
> > - return;
> > + return 0;
>
> Ohh-oh! This is clearly a failure schenario! And indeed,
> "tick_init_highres()" will do the "negative on failure, zero on success"
> thing.
>
> BUT! That means that you're testing the return value WRONG!
>
> A function that returns a negative error value should be tested with
>
> if (tick_init_highres() < 0) {
> local_irq_restore(flags);
> return 0;
> }
>
> because now you *see* that it's a failure.
>
> So here's the coding style:
>
> - "true on success, false on failure" should be tested by just doing the
> implicit test against zero (because that's how C booleans work!)
>
> Example:
>
> if (everything_is_done())
> return;
>
> Or:
>
> if (!something_worked_ok()) {
> printk("Aiee! Bug!\n");
> return;
> }
>
> - "negative error values" should preferably always be tested as such
>
> if (tick_init_highres() < 0) {
> printk("Aieee! Couldn't init!\n");
> return 0;
> }
>
> or, much better, actually use a temporary variable called "err" or
> "error" or something, at which point "!error" is suddenly readable
> again:
>
[1b]
> err = tick_init_highres();
> if (!err)
> return;

So this one above and [2] below lose the obvious "negative error"
information, and also prevent such functions from returning a
positive value (> 0), e.g., to indicate a successful amount of
work done (like bytes read or written). The second version above
[1b] also does not quite agree with your statement:
- "negative error values" should preferably always be tested as such

so in the interest of CodingStyle, can you be even clearer?

> I know this sounds stupid, but we've long since come to the point where
> source code readability on a *local* scale is damn important, simply
> because that's how people look at code: they may not always remember
> whether "zero is success" or "zero is false".
>
> In general, I would suggest:
>
> - ALWAYS use "negative means error". If you had done that in this case,
> then hrtimer_switch_to_hres() would have been a lot more readable,
> *and* it could actually have returned the error code that it got to the
> caller. In general, it's just more information when you see
>
[2]
> error = some_function();
> if (error)
> return error;
>
> because even if it may generate basically *exactly* same code as the
> reversed "positive" version:
>
> if (!some_version_is_true())
> return 0;
>
> it simply has more semantic information for *humans*.
>
> And when you do this, *test it as such*. Either use an explicit "< 0"
> so that you *see* that you're testing an error value, or use that
> "retval/error = xyzzy()" pattern that is already showing "it's more
> than just true/false"
>
> - use "true/false" only for things where it's *really* obvious that the
> answer is never an error, and always a "was it true"?
>
> Yeah, even so, the true/false kind of thing may be more common (especially
> with small helper functions that are literally *designed* to be used just
> as a conditional), but I think in this case, you really should have done
> it as a "returns error" function. Partly because now it was throwing away
> an error code, partly simply because in this case, it really wasn't about
> true/false as much as about "did something error out and keep it from
> succeeding?".
>
> Maybe I'm just getting anal in my old age. I at one time tried to make
> sparse check for these things, but there was no really sane thing I could
> come up with (way way WAAY too much manual annotation).
>
> I might have to break down and suggest people use
>
> bool somefunction(..)
> {
> if (... < 0)
> return false;
> ...
> return true;
> }
>
> just to (a) eventually have sparse check for these things but more
> importantly (b) have people see more at a glance whether a function is
> supposed to return "negative or success" or "true or false".
>
> I've not generally been a huge fan of "boolean", especially in the
> traditional C kind of sense (capital screaming letters, and really just an
> "int" with lipstick). But with modern C, and "bool" defined as really
> holding just 0/1 (in practice - "unsigned char"), we could actually check
> these things (and verify with sparse that you never assign any integer
> except for 0/1 to a boolean, and otherwise always have to use a real
> boolean construct).
>
> Thus endeth my overly long coding style rant.
>
> Linus

---
~Randy
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