linux-kernel - Re: [PATCH 1/2] rcu: introduce list_last_or_null

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Date:	Thu, 28 May 2015 14:05:46 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Dan Streetman <ddstreet@...e.org>
Cc:	josh@...htriplett.org, Andrew Morton <akpm@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] rcu: introduce list_last_or_null_rcu

On Thu, May 28, 2015 at 04:42:20PM -0400, Dan Streetman wrote:
> On Thu, May 28, 2015 at 4:39 PM,  <josh@...htriplett.org> wrote:
> > On Thu, May 28, 2015 at 04:35:27PM -0400, Dan Streetman wrote:
> >> Add list_last_or_null_rcu(), to simplify getting the last entry from a
> >> rcu-protected list.  The standard list_last_entry() can't be used as it
> >> is not rcu-protected; the list may be modified concurrently.  And the
> >> ->prev pointer can't be used, as only the ->next pointers are protected
> >> by rcu.
> >>
> >> This simply iterates forward through the entire list, to get to the last
> >> entry.  If the list is empty, it returns NULL.
> >>
> >> Signed-off-by: Dan Streetman <ddstreet@...e.org>
> >
> > The list iteration functions are macros because they introduce a loop
> > with attached loop block.  For this, is there any reason not to make it
> > an inline function instead of a macro?
> 
> true, there's no reason i can see not to make it inline, let me send
> an updated patch.

Hmmm...  If we can now do type-generic inline functions, it might make
sense to convert some of the others as well.

							Thanx, Paul

> >>  include/linux/rculist.h | 21 +++++++++++++++++++++
> >>  1 file changed, 21 insertions(+)
> >>
> >> diff --git a/include/linux/rculist.h b/include/linux/rculist.h
> >> index a18b16f..954fde5 100644
> >> --- a/include/linux/rculist.h
> >> +++ b/include/linux/rculist.h
> >> @@ -293,6 +293,27 @@ static inline void list_splice_init_rcu(struct list_head *list,
> >>  })
> >>
> >>  /**
> >> + * list_last_or_null_rcu - get the last element from a list
> >> + * @ptr:        the list head to take the element from.
> >> + * @type:       the type of the struct this is embedded in.
> >> + * @member:     the name of the list_head within the struct.
> >> + *
> >> + * Note that if the list is empty, it returns NULL.
> >> + *
> >> + * This primitive may safely run concurrently with the _rcu list-mutation
> >> + * primitives such as list_add_rcu() as long as it's guarded by rcu_read_lock().
> >> + */
> >> +#define list_last_or_null_rcu(ptr, type, member) \
> >> +({ \
> >> +     struct list_head *__ptr = (ptr); \
> >> +     struct list_head *__last = __ptr; \
> >> +     struct list_head *__entry = list_next_rcu(__ptr); \
> >> +     for (; __entry != __ptr; __entry = list_next_rcu(__entry)) \
> >> +             __last = __entry; \
> >> +     likely(__ptr != __last) ? list_entry_rcu(__last, type, member) : NULL; \
> >> +})
> >> +
> >> +/**
> >>   * list_for_each_entry_rcu   -       iterate over rcu list of given type
> >>   * @pos:     the type * to use as a loop cursor.
> >>   * @head:    the head for your list.
> >> --
> >> 2.1.0
> >>
> 

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