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

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Date:	Thu, 28 May 2015 16:42:20 -0400
From:	Dan Streetman <ddstreet@...e.org>
To:	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 4:39 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.

>
>>  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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