Messages in this thread | | | Date | Fri, 7 Jun 2013 12:36:58 -0400 | From | Jörn Engel <> | Subject | Re: [PATCH 1/2] list: add list_for_each_entry_del |
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On Thu, 6 June 2013 22:49:22 +0300, Andy Shevchenko wrote: > On Thu, Jun 6, 2013 at 9:12 PM, Jörn Engel <joern@logfs.org> wrote: > > On Thu, 6 June 2013 22:32:55 +0300, Andy Shevchenko wrote: > >> On Mon, Jun 3, 2013 at 8:28 PM, Joern Engel <joern@logfs.org> wrote: > >> > I have seen a lot of boilerplate code that either follows the pattern of > >> > while (!list_empty(head)) { > >> > pos = list_entry(head->next, struct foo, list); > >> > list_del(pos->list); > >> > ... > >> > } > >> > or some variant thereof. > >> > >> What the problem to use list_for_each_safe()? > > > > The loop may terminate with elements left on the list. There is more, > > but I would consider this the main problem. > > I didn't quite get what you mean.
Take two threads, one doing a list_for_each_entry_safe loop and dropping the lock after list_del, the other doing list_add. Result is that you finish the list_for_each_entry_safe loop with something remaining on the list.
spin_lock list_for_each_entry_safe list_del spin_unlock spin_lock list_add spin_unlock
If you search for this pattern in the kernel, you won't find too many examples. Quite likely that is because a) people realized this and used a while (!list_empty()) loop to begin with or b) they started out wrong and fixed the bug later. Not sure how many examples of b) there are.
At any rate, this is a purely janitorial patch. It is almost by definition of moderate utility and if there is significant opposition or the patch actually causes harm in some way, we should drop it.
Jörn
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