linux-kernel - Re: [PATCH v2 1/9] llist: Provide a safe version for llist_for

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Date:   Mon, 13 Feb 2017 16:58:05 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     "Huang, Ying" <ying.huang@...el.com>
Cc:     peterz@...radead.org, mingo@...nel.org, neilb@...e.de,
        nab@...ux-iscsi.org, viro@...iv.linux.org.uk, oleg@...hat.com,
        shli@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/9] llist: Provide a safe version for llist_for_each

On Mon, Feb 13, 2017 at 03:52:44PM +0800, Huang, Ying wrote:
> Byungchul Park <byungchul.park@....com> writes:
> 
> > On Mon, Feb 13, 2017 at 03:36:33PM +0800, Huang, Ying wrote:
> >> Byungchul Park <byungchul.park@....com> 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@....com>
> >> > ---
> >> >  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.

> 
> Best Regards,
> Huang, Ying
> 
> >> 
> >> Best Regards,
> >> Huang, Ying
> >> 
> >> > +/**
> >> >   * llist_for_each_entry - iterate over some deleted entries of lock-less list of given type
> >> >   * @pos:	the type * to use as a loop cursor.
> >> >   * @node:	the fist entry of deleted list entries.