On Tue, Sep 11, 2012 at 03:32:47PM +0100, David Gould wrote:
> Subject: Re: [PATCH] clear_child_for_cleanup must correctly manage
> children_to_clean
Thanks for the patch. Overall it looks good, but let me nit-pick your
commit message a little (not because it is that horrible, but because
you are so close to perfect that I want to fix the minor things and then
encourage you to submit more patches :) ).
Your subject is a bit vague, and it is not clear if it is not correct
now, and this is a bugfix, or if it is a feature enhancement. I would
have said something like:
Subject: fix broken list iteration in clear_child_for_cleanup
which is _also_ vague about what exactly the breakage is, but is clear
that this is a bugfix. So then you can go on to describe the actual
problem:
We iterate through the list of children to cleanup, but do not keep
our "last" pointer up to date. As a result, we may end up cutting off
part of the list instead of removing a single element.
And then describe your fix:
> Iterate through children_to_clean using 'next' fields but with an
> extra level of indirection. This allows us to update the chain when
> we remove a child and saves us managing several variables around
> the loop mechanism.
which I think is good.
> - last = &children_to_clean;
> - for (p = children_to_clean; p; p = p->next) {
> - if (p->pid == pid) {
> - *last = p->next;
> - free(p);
> + for (pp = &children_to_clean; *pp; pp = &(*pp)->next) {
> + if ((*pp)->pid == pid) {
> + struct child_to_clean *clean_me = *pp;
> + *pp = clean_me->next;
> + free(clean_me);
> return;
> }
I think using the indirect pointer is a nice compromise; it makes it
clear from just the for loop that this is not an ordinary for-each
traversal. You could hoist the extra pointer out of the conditional and
save one set of parentheses in the "if" statement, but I don't think it
is a big deal either way.
Acked-by: Jeff King <peff@xxxxxxxx>
Thanks for the bug report and the patch.
-Peff
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