linux-kernel - Re: [PATCH 1/2] list: add list_for_each_entry

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Message-ID: <CAHp75VfEUWYNXuo57x6JkOQfRM=03iGFaBKUeqz77SEeg_7=mA@mail.gmail.com>
Date:	Sat, 8 Jun 2013 03:03:24 +0300
From:	Andy Shevchenko <andy.shevchenko@...il.com>
To:	Jörn Engel <joern@...fs.org>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Chris Mason <chris.mason@...ionio.com>,
	linux-btrfs@...r.kernel.org
Subject: Re: [PATCH 1/2] list: add list_for_each_entry_del

On Fri, Jun 7, 2013 at 9:48 PM, Jörn Engel <joern@...fs.org> wrote:
> On Fri, 7 June 2013 21:30:16 +0300, Andy Shevchenko wrote:
>> >
>> > spin_lock
>> > list_for_each_entry_safe
>> >         list_del
>> >         spin_unlock
>>
>> Who is doing such thing?
>
> Replace list_for_each_entry_safe with 'while (!list_empty(...))' and
> just grep.  My patch is about 'while (!list_empty(...))', not about
> list_for_each_entry_safe.

I saw your patch against btrfs.
I didn't see locking there.

Any excerpt like

 while (!list_empty(&prefs)) {
 ref = list_first_entry(&prefs, struct __prelim_ref, list);
 list_del(&ref->list);

could be transformed to
struct __prelim_ref *_ref;
list_for_each_entry_safe(ref, _ref, &prefs, list) {
 list_del(&ref->list);
 ...
}

but is it worth to do? (Same question to your approach)

I see two potential issues with while_list_drain_entry() or whatever
name you like:
 - you delete list as a first operation - you limit user to think in
that way, if you choose deletion as last operation, who will go to
free resources (call kfree() for example)?
- you always use the same ordering (list_first_entry() call), someone
may not be satisfied by that

So, the approach with while (!list_empty()) { e = list_entry();
list_del(&e->list); ... }
actually not bad. If there any bugs in code, better to fix those bugs
explicitly.

What do I not understand?

--
With Best Regards,
Andy Shevchenko
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