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
