Re: [PATCH v2 1/9] llist: Provide a safe version for llist_for_each

[Byungchul Park]

On Mon, Feb 13, 2017 at 04:58:05PM +0900, Byungchul Park wrote:
> On Mon, Feb 13, 2017 at 03:52:44PM +0800, Huang, Ying wrote:
> > Byungchul Park <byungchul.park@xxxxxxx> writes:
> > 
> > > On Mon, Feb 13, 2017 at 03:36:33PM +0800, Huang, Ying wrote:
> > >> Byungchul Park <byungchul.park@xxxxxxx> writes:
> > >> 
> > >> > Sometimes we have to dereference next field of llist node before entering
> > >> > loop becasue the node might be deleted or the next field might be
> > >> > modified within the loop. So this adds the safe version of llist_for_each,
> > >> > that is, llist_for_each_safe.
> > >> >
> > >> > Signed-off-by: Byungchul Park <byungchul.park@xxxxxxx>
> > >> > ---
> > >> >  include/linux/llist.h | 19 +++++++++++++++++++
> > >> >  1 file changed, 19 insertions(+)
> > >> >
> > >> > diff --git a/include/linux/llist.h b/include/linux/llist.h
> > >> > index fd4ca0b..4c508a5 100644
> > >> > --- a/include/linux/llist.h
> > >> > +++ b/include/linux/llist.h
> > >> > @@ -105,6 +105,25 @@ static inline void init_llist_head(struct llist_head *list)
> > >> >  	for ((pos) = (node); pos; (pos) = (pos)->next)
> > >> >  
> > >> >  /**
> > >> > + * llist_for_each_safe - iterate over some deleted entries of a lock-less list
> > >> > + *			 safe against removal of list entry
> > >> > + * @pos:	the &struct llist_node to use as a loop cursor
> > >> > + * @n:		another type * to use as temporary storage
> > >> 
> > >> s/type */&struct llist_node/
> > >
> > > Yes.
> > >
> > >> 
> > >> > + * @node:	the first entry of deleted list entries
> > >> > + *
> > >> > + * In general, some entries of the lock-less list can be traversed
> > >> > + * safely only after being deleted from list, so start with an entry
> > >> > + * instead of list head.
> > >> > + *
> > >> > + * If being used on entries deleted from lock-less list directly, the
> > >> > + * traverse order is from the newest to the oldest added entry.  If
> > >> > + * you want to traverse from the oldest to the newest, you must
> > >> > + * reverse the order by yourself before traversing.
> > >> > + */
> > >> > +#define llist_for_each_safe(pos, n, node)			\
> > >> > +	for ((pos) = (node); (pos) && ((n) = (pos)->next, true); (pos) = (n))
> > >> > +
> > >> 
> > >> Following the style of other xxx_for_each_safe,
> > >> 
> > >> #define llist_for_each_safe(pos, n, node)			\
> > >> 	for (pos = (node), (pos && (n = pos->next)); pos; pos = n, n = pos->next)
> > >
> > > Do you think it should be modified? I think mine is simpler. No?
> > 
> > Personally I prefer the style of other xxx_for_each_safe().
> 
> Yes, I will modify it as you recommand.
> 
> Thank you very much.

I wanted to modify it as you recommanded but it has a bug. It should be
(to fix the bug):

   for (pos = (node), (pos && (n = pos->next)); pos; pos = n, (pos && \
   (n = pos->next)))

Don't you think this is too messy? Or do I miss something? I still think
the following is neater and simpler.

   for (pos = node; pos && (n = pos->next, true); pos = n)

Or could you recommand another preference?