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?
diff --git a/include/linux/list.h b/include/linux/list.h
index 9df84a56a789..306554ab1841 100644
--- a/include/linux/list.h
+++ b/include/linux/list.h
@@ -7,6 +7,7 @@
#include <linux/stddef.h>
#include <linux/poison.h>
#include <linux/const.h>
+#include <linux/args.h>
#include <asm/barrier.h>
@@ -911,20 +912,34 @@ static inline size_t list_count_nodes(struct list_head
*head)
for (; !list_entry_is_head(pos, head, member); \
pos = list_prev_entry(pos, member))
+#define __list_for_each_entry_safe_internal(pos, next, head, member) \
+ for (typeof(pos) next = list_next_entry(pos = \
+ list_first_entry(head, typeof(*pos), member), member); \
+ !list_entry_is_head(pos, head, member); \
+ pos = next, next = list_next_entry(next, member))
+
+#define __list_for_each_entry_safe2(pos, head, member) \
+ __list_for_each_entry_safe_internal(pos, __UNIQUE_ID(next), head,
member)
+
+#define __list_for_each_entry_safe3(pos, next, head, member) \
+ for (pos = list_first_entry(head, typeof(*pos), member), \
+ next = list_next_entry(pos, member); \
+ !list_entry_is_head(pos, head, member); \
+ pos = next, next = list_next_entry(next, member))
+
/**
* list_for_each_entry_safe - iterate over list of given type safe against
removal of list entry
* @pos: the type * to use as a loop cursor.
- * @n: another type * to use as temporary storage
- * @head: the head for your list.
- * @member: the name of the list_head within the struct.
+ * @...: either (head, member) or (next, head, member)
+ * @next: another type * to use as optional temporary storage. The
temporary
+ * cursor is internal unless explicitly supplied by the caller.
+ * @head: the head for your list.
+ * @member:the name of the list_head within the struct.
*
*/
-#define list_for_each_entry_safe(pos, n, head, member) \
- for (pos = list_first_entry(head, typeof(*pos), member), \
- n = list_next_entry(pos, member); \
- !list_entry_is_head(pos, head, member); \
- pos = n, n = list_next_entry(n, member))
+#define list_for_each_entry_safe(pos, ...) \
+ CONCATENATE(__list_for_each_entry_safe, COUNT_ARGS(__VA_ARGS__))\
+ (pos, __VA_ARGS__)
/**
* list_for_each_entry_safe_continue - continue list iteration safe against
removal
>> 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
