在 2026/6/10 16:07, Christian König 写道:
> On 6/10/26 08:14, Kaitao Cheng wrote:
>> 在 2026/6/9 18:33, Christian König 写道:
>>> On 6/9/26 08:13, Kaitao Cheng wrote:
>>>> From: Kaito Cheng <[email protected]>
>>>>
>>>> This series prepares for, and then updates, the list_for_each_entry()
>>>> family so the common entry iterators cache their next or previous cursor
>>>> before the loop body runs.
>>>
>>> Why in the world would we want to do that?
>>>
>>> The safe and non-safe variants have very distinct use cases and that is
>>> completely intentional.
>>>
>>> What we could improve maybe is the documentation, from my experience an
>>> astonishing large amount of people have misconceptions about the safe
>>> variants.
>>>
>>>> The first 13 patches open-code loops that intentionally depend on the
>>>> old "derive the next entry from the current cursor at the end of the
>>>> iteration" behaviour. These loops append work to the list being walked,
>>>> restart traversal after dropping a lock, skip an entry consumed by the
>>>> current iteration, or otherwise adjust the cursor in the loop body.
>>>
>>> Well I have to clearly reject the changes for subsystems/components I'm
>>> maintaining, that just looks horrible to me and I clearly don't see a good
>>> reason for that.
>>
>> Hi Christian and Andy Shevchenko,
>>
>> Thanks for taking a look. I would like to clarify the point you raised.
>>
>> The reason I started looking at this is the original motivation behind
>> the _safe() variants. They exist because some users need to remove, move
>> or otherwise consume the current entry while walking the list. In that
>> case the next cursor has to be preserved before the loop body can modify
>> the current entry.
>>
>> The unfortunate part is that this could not be expressed with the
>> existing list_for_each_entry() interface without changing its calling
>> convention. The _safe() variants had to grow an extra argument for the
>> temporary cursor, and that is why we ended up with a separate family of
>> macros.
>>
>> But conceptually, the distinction does not have to be exposed as two
>> different iterator families forever. The difference is an implementation
>> detail: whether the iterator keeps the next/previous cursor before the
>> body runs. This series makes the common list_for_each_entry() iterators
>> do that internally, so the safe and non-safe forms can effectively be
>> folded together, or at least the need for a separate public _safe()
>> interface becomes much weaker.
>>
>> There is also a usability issue with the current _safe() interface. The
>> caller is forced to define a temporary cursor outside the macro and pass
>> it in, even though almost all users never use that cursor directly. It is
>> just boilerplate required by the macro implementation. I find that
>> redundant and awkward: the temporary cursor is an internal detail of the
>> iteration, but every caller has to spell it out.
>>
>> With the updated list_for_each_entry() implementation, that extra cursor
>> can be kept inside the iterator itself. Callers that only want to walk
>> the list, including callers that delete or consume the current entry, no
>> longer need to carry an otherwise-unused temporary variable just to make
>> the macro work.
>
> Well the distinction between list_for_each_entry() and
> list_for_each_entry_safe() is *not* there because you need an extra variable
> to hold the next pointer, but because just 'iterating the list' and
> 'iterating the list while you modify it' are two distinct use cases.
>
> Apart from the technical implications this also has some documentation value
> for the code using it.
>
> What we could consider with C99 at hand is to have _safe() variants who uses
> a local hidden variable to hold the next element.
>
> Or maybe come up with a better name instead of _safe() because people seem to
> misunderstand that quite often.
>
> But mangling the two use cases together just because it is now technical
> possible is among the worst ideas I've ever heard.
>
Should we revert to v1, or keep list_for_each_entry() and
list_for_each_entry_safe() as they are, close this thread, and make no
changes?
Link to v1:
https://lore.kernel.org/all/[email protected]/
Or do you have any better suggestions?
--
Thanks
Kaitao Cheng
