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

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Message-ID: <CALZtONA6yy+nS0tBd2ndKhrZdNWXrkMM_LL-nmwR3oOWzfUGbA@mail.gmail.com>
Date:	Thu, 28 May 2015 16:44:59 -0400
From:	Dan Streetman <ddstreet@...e.org>
To:	Josh Triplett <josh@...htriplett.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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 4:42 PM, Dan Streetman <ddstreet@...e.org> 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.

ha, as soon as i sent that email, i realized it can't be an inline
function, because the return value is (type *), not a predefined
value.  Of course it could return void*, but unless there's a benefit
of making it an inline function, it seems to me like it would be
better as a #define.

>
>>
>>>  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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