Re: [PATCH] drivers/block/ub.c: use list_for_each_entry()

Roland Dreier
2007-05-30T23:42:51Z

> > > The negative is the sheer number of helper functions in list.h. Personally,
> > > I find it difficult to retain a working knowledge of them. Iterators are
> > > particularly nasty that way. I'm thinking about dropping all of these
> > > list_for_each_with_murky_argument_requirements_and_odd_side_effects()
> > > and use plain for(;;), as a courtesy to someone who has to read the
> > > code years down the road.
> >
> > I think I disagree with this reasoning. If I'm reading your code and
> > I see, say, list_for_each_entry_safe(), I can be pretty confident that
> > your loop works correctly. If you write your own for loop, then I
> > have to check that you actually got the linked list walking right.
>
> You have to check that I used list_for_each_entry_safe correctly too,
> which is harder. Are you aware that we had (and probably still have)
> dozens of cases where the use of list_for_each_entry_safe was buggy?
> Most of them involved IHV programmers being lured into false sense
> of security by the _safe suffix and getting their locking wrong.
>
> You could not find a better way to blow up your own argument
> than to mention list_for_each_entry_safe(), which is anything but.
> Matthias' use of list_for_each_entry() actually IS safe, which is
> why I am not NAKing it. Andrew has accepted it already. I just
> think we aren't winning squat here.

Well, actually, I chose list_for_each_entry_safe() quite conscious of
the locking issues. If I see list_XXX_safe() then I know that I need
to be suspicious when reviewing code -- the same way seeing "atomic_t"
makes me think "is there any reason to use atomic_t??"

If I just see

for (pos = list_entry((head)->next, typeof(*pos), member),
n = list_entry(pos->member.next, typeof(*pos), member);
&pos->member != (head);
pos = n, n = list_entry(n->member.next, typeof(*n), member))

then what am I to think?

- R.