Re: [PATCH v3 1/7] list: Add mutable iterator variants

[David Laight]

On Wed, 24 Jun 2026 15:23:47 +0200
Christian König <christian.koenig@xxxxxxx> wrote:

> On 6/24/26 15:14, Kaitao Cheng wrote:
> > 
> > 
> > 在 2026/6/22 16:42, David Laight 写道:  
> >> On Mon, 22 Jun 2026 12:05:31 +0800
> >> Kaitao Cheng <kaitao.cheng@xxxxxxxxx> wrote:
> >>  
> >>> From: Kaitao Cheng <chengkaitao@xxxxxxxxxx>
> >>>
> >>> The list_for_each*_safe() helpers are used when the loop body may
> >>> remove the current entry.  Their API exposes the temporary cursor at
> >>> every call site, even though most users only need it for the iterator
> >>> implementation and never reference it in the loop body.
> >>>
> >>> Add *_mutable() variants for list and hlist iteration.  The new helpers
> >>> support both forms: callers may keep passing an explicit temporary cursor
> >>> when they need to inspect or reset it, or omit it and let the helper use
> >>> a unique internal cursor.  
> >>
> >> I'm not really sure 'mutable' means anything either.
> >> It is possible to make it valid for the loop body (or even other threads)
> >> to delete arbitrary list items - but that needs significant extra overheads.
> >>
> >> It might be worth doing something that doesn't need the extra variable,
> >> but there is little point doing all the churn just to rename things.
> >>  
> >>>
> >>> This makes call sites that only mutate the list through the current entry
> >>> less noisy, while keeping the existing *_safe() helpers available for
> >>> compatibility.
> >>>
> >>> Signed-off-by: Kaitao Cheng <chengkaitao@xxxxxxxxxx>
> >>> ---
> >>>  include/linux/list.h | 269 +++++++++++++++++++++++++++++++++++++------
> >>>  1 file changed, 231 insertions(+), 38 deletions(-)
> >>>
> >>> diff --git a/include/linux/list.h b/include/linux/list.h
> >>> index 09d979976b3b..1081def7cea9 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>
> >>>  
> >>> @@ -763,28 +764,72 @@ static inline void list_splice_tail_init(struct list_head *list,
> >>>  #define list_for_each_prev(pos, head) \
> >>>  	for (pos = (head)->prev; !list_is_head(pos, (head)); pos = pos->prev)
> >>>  
> >>> -/**
> >>> - * list_for_each_safe - iterate over a list safe against removal of list entry
> >>> - * @pos:	the &struct list_head to use as a loop cursor.
> >>> - * @n:		another &struct list_head to use as temporary storage
> >>> - * @head:	the head for your list.
> >>> +/*
> >>> + * list_for_each_safe is an old interface, use list_for_each_mutable instead.
> >>>   */
> >>>  #define list_for_each_safe(pos, n, head) \
> >>>  	for (pos = (head)->next, n = pos->next; \
> >>>  	     !list_is_head(pos, (head)); \
> >>>  	     pos = n, n = pos->next)
> >>>  
> >>> +#define __list_for_each_mutable_internal(pos, tmp, head)		\
> >>> +	for (typeof(pos) tmp = (pos = (head)->next)->next;		\  
> >>
> >> Use auto
> >>  
> >>> +	     !list_is_head(pos, (head));				\
> >>> +	     pos = tmp, tmp = pos->next)
> >>> +
> >>> +#define __list_for_each_mutable1(pos, head)				\
> >>> +	__list_for_each_mutable_internal(pos, __UNIQUE_ID(next), head)
> >>> +
> >>> +#define __list_for_each_mutable2(pos, next, head)			\
> >>> +	list_for_each_safe(pos, next, head)
> >>> +
> >>>  /**
> >>> - * list_for_each_prev_safe - iterate over a list backwards safe against removal of list entry
> >>> + * list_for_each_mutable - iterate over a list safe against entry removal
> >>>   * @pos:	the &struct list_head to use as a loop cursor.
> >>> - * @n:		another &struct list_head to use as temporary storage
> >>> - * @head:	the head for your list.
> >>> + * @...:	either (head) or (next, head)
> >>> + *
> >>> + * next:	another &struct list_head to use as optional temporary storage.
> >>> + *		The temporary cursor is internal unless explicitly supplied by
> >>> + *		the caller.
> >>> + * head:	the head for your list.
> >>> + */
> >>> +#define list_for_each_mutable(pos, ...)					\
> >>> +	CONCATENATE(__list_for_each_mutable, COUNT_ARGS(__VA_ARGS__))	\
> >>> +		(pos, __VA_ARGS__)  
> >>
> >> The variable argument count logic really just slows down compilation.
> >> Maybe there aren't enough copies of this code to make that significant.
> >> But just because you can do it doesn't mean it is a gooD idea.
> >> I'm also not sure it really adds anything to the readability.
> >>
> >> And, it you are going to make the middle argument optional there is
> >> no need to change the macro name.  
> > 
> > Christian König and Jani Nikula also disagree with the variadic-argument
> > implementation approach. If we abandon that method, it means we will
> > inevitably need to add some new macros. If mutable is not a good name,
> > suggestions for better alternatives would be welcome; coming up with a
> > suitable name is indeed rather tricky.  
> 
> I don't think you need to add a new macro for the specific use case that people want to modify the next element of the iteration.
> 
> If I remember your numbers correctly that is a really corner case and keeping using the existing *_safe() macros for that sounds perfectly fine to me.

IIRC currently you have a choice of either:
	define               Item that can't be deleted
	list_for_each()	     The current item.
	list_for_each_safe() The next item.
There is also likely to be code that updates the variables to allow
for other scenarios.

Note that if increase a reference count and release a lock then list_for_each()
is likely safer than list_for_each_safe() :-)

list.h has 9 variants of the 'safe' loop.
The bloat of another 9 is getting excessive.

It has to be said that this is one of my least favourite type of list...

	David

> 
> Regards,
> Christian.