Messages in this thread | | | Date | Tue, 6 Mar 2007 08:44:43 -0800 (PST) | From | Linus Torvalds <> | Subject | Re: [5/6] 2.6.21-rc2: known regressions |
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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.
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