Re: [PATCH v2 00/14] list: Prepare entry iterators to cache cursor state

[Kaitao Cheng]

在 2026/6/11 14:54, Andy Shevchenko 写道:
> On Thu, Jun 11, 2026 at 12:42:02PM +0800, Kaitao Cheng wrote:
>> 在 2026/6/10 22:43, Andy Shevchenko 写道:
>>> On Wed, Jun 10, 2026 at 02:14:06PM +0800, Kaitao Cheng wrote:
>>>> 在 2026/6/9 18:33, Christian König 写道:
>>>>> On 6/9/26 08:13, Kaitao Cheng wrote:
> 
>>>>>> 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.
>>>
>>> Ah, I think the distinct macro families is that what we want.
>>> But the hiding of the parameter can be done inside list_for_each_*_safe().
>>> You can do a treewide change with coccinelle.
>>>
>>> Sorry if I didn't get the whole idea from your previous contributions.
>>>
>>> Note, even cases that would need a temporary cursor may be switched to
>>> new list_for_each_*_safe(), see how PCI macros for iterating over resources
>>> are implemented (include/linux/pci.h).
>>
>> Thanks for your suggestions. I've written a demo based on your feedback.
>> Could you please review it and share your thoughts on this approach?
> 
> Have you checked how many users actually need the temporary storage?

In Muchun's reply, he mentioned the following:

There are 9,925 list_for_each_entry() call sites in total. Among them,
9,919 do not require any adaptation, and only 6 need to be refactored:

As for list_for_each_entry_safe(), there are 4,572 callers. 4,550 of them
can be directly replaced by the new list_for_each_entry(), while 22 cannot
be replaced

https://lore.kernel.org/all/2B3BFA1E-08B8-42AB-87D6-A28BF15E5C58@xxxxxxxxx/


I only used Coccinelle to scan for list_for_each_entry() call sites, and
found the 13 call sites shown in the current patch series, which cover
the 6 cases mentioned in Muchun's email. I have not yet run the Coccinelle
scan for list_for_each_entry_safe().

If we need to handle all 9,925 list_for_each_entry() call sites or all 4,572
list_for_each_entry_safe() call sites in one go, would such a change be too
large? I expect it would affect almost every kernel subsystem.

I wonder whether it would be better to first provide the necessary
compatibility APIs, and then let each subsystem owner update their code as
appropriate. That would make the impact more controlled, similar to how
the current folio replacement of page is being handled.

>>>> 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.
>>>>
>>>>>> The final patch changes include/linux/list.h to keep a private cursor in
>>>>>> the common entry iterators while preserving the public macro interface.
>>>>>> The safe variants remain available when callers need the temporary
>>>>>> cursor explicitly or have stronger mutation requirements.
> 

-- 
Thanks
Kaitao Cheng