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

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Message-ID: <CAHp75VdBb3Gt+sYwDbJo8BXRSsxwN0K6qw_UeJve6ycQwz2WBQ@mail.gmail.com>
Date:	Fri, 7 Jun 2013 21:30:16 +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 7:36 PM, Jörn Engel <joern@...fs.org> wrote:
> On Thu, 6 June 2013 22:49:22 +0300, Andy Shevchenko wrote:
>> On Thu, Jun 6, 2013 at 9:12 PM, Jörn Engel <joern@...fs.org> wrote:
>> > On Thu, 6 June 2013 22:32:55 +0300, Andy Shevchenko wrote:
>> >> 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

Who is doing such thing?

Usually if you unlock, you exit from function, or you already done
iteration through the list.

Like
lock()
list_for_each_safe() {
  if (condition) {
  list_del()
  unlock()
  return;
 }
}
unlock()
return;

In case you have to do unlock()/lock() routine inside iteration you always can
do an additional check at the end

list_for_each_safe() {
 unlock();
 lock();
}
if (!list_empty()) {
do_smth()
}
unlock();

Thus, I don't see how list*del will help.

> 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.

--
With Best Regards,
Andy Shevchenko
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