Hi Dan,
On Wednesday 30 Nov 2016 15:33:26 Dan Carpenter wrote:
> On Mon, Nov 28, 2016 at 04:49:36PM +0200, Laurent Pinchart wrote:
> > On Monday 28 Nov 2016 14:54:58 Julia Lawall wrote:
> >> On Mon, 28 Nov 2016, Dan Carpenter wrote:
> >>> I understand the comparison, but I just think it's better if people
> >>> always keep track of what has been allocated and what has not. I
> >>> tried so hard to get Markus to stop sending those hundreds of patches
> >>> where he's like "this function has a sanity check so we can pass
> >>> pointers that weren't allocated"... It's garbage code.
> >>>
> >>> But I understand that other people don't agree.
> >>
> >> In my opinion, it is good for code understanding to only do what is
> >> useful to do. It's not a hard and fast rule, but I think it is
> >> something to take into account.
> >
> > On the other hand it complicates the error handling code by increasing the
> > number of goto labels, and it then becomes pretty easy when reworking code
> > to goto the wrong label. This is even more of an issue when the rework
> > doesn't touch the error handling code, in which case the reviewers can
> > easily miss the issue if they don't open the original source file to
> > check the goto labels.
>
> It's really not. I've looked at a lot of error handling in the past
> five years and sent hundreds of fixes for error paths, more than any
> other kernel developer during that time. Although it seems obvious in
> retrospect, it took me years to realize this but the canonical way of
> doing error handling is the least error prone.
>
> Counting the labels is the wrong way to measure complexity. That's like
> counting the number of functions. Code with lots of functions is not
> necessarily more complicated than if it's just one big function.
>
> Part of the key to unwinding correctly is using good label names. It
> should say what the label does. Some people use come-from labels names
> like "goto kmalloc_failed". Those are totally useless. It's like
> naming your functions "called_from_foo()". If there is only one goto
> then come-from label names are useless and if there are more than one
> goto which go to the same label then it's useless *and* misleading.
Yes, label naming is (or at least should be) largely agreed upon, they should
name the unwinding action, not the cause of the failure.
> Functions should be written so you can read them from top to bottom
> without scrolling back and forth.
>
> a = alloc();
> if (!a)
> return -ENOMEM;
>
> b = alloc();
> if (!b) {
> ret = -ENOMEM;
> goto free_a;
> }
But then you get the following patch (which, apart from being totally made up,
probably shows what I've watched yesterday evening).
@@ ... @@
return -ENOMEM;
}
+ ret = check_time_vortex();
+ if (ret < 0)
+ goto power_off_tardis;
+
matt_smith = alloc_regeneration();
if (math_smith->status != OK) {
ret = -E_NEEDS_FISH_FINGERS_AND_CUSTARD;
>From that code only you can't tell whether the jump label is the right one. If
a single jump label is used with an unwinding code block that supports non-
allocated resources, you don't have to ask yourself any question.
> That code tells a complete story without any scrolling. It's future
> proof too. You can tell the goto is correct just from the name. But
> when it's:
>
> a = alloc();
> if (!a)
> goto out;
> b = alloc();
> goto out;
>
> That code doesn't have enough information to be understandable on it's
> own. It's way more bug prone than the first sample.
--
Regards,
Laurent Pinchart
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