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

Linus Torvalds
2007-03-06T16:49:50Z

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:

err = tick_init_highres();
if (!err)
return;

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

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