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

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

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).

> 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