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

[Andy Shevchenko]

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?

> >> 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.

-- 
With Best Regards,
Andy Shevchenko