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

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Date:   Mon, 13 Feb 2017 15:52:44 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Byungchul Park <byungchul.park@....com>
Cc:     "Huang\, Ying" <ying.huang@...el.com>, <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

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().

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.