[lttng-dev] [rp] [URCU PATCH 0/3] wait-free concurrent queues (wfcqueue)

[Lai Jiangshan]

On 10/08/2012 11:07 PM, Mathieu Desnoyers wrote:
> * Lai Jiangshan (laijs at cn.fujitsu.com) wrote:
>> On 10/04/2012 05:04 AM, Mathieu Desnoyers wrote:
>>> * Paul E. McKenney (paulmck at linux.vnet.ibm.com) wrote:
>>>> On Tue, Oct 02, 2012 at 10:13:07AM -0400, Mathieu Desnoyers wrote:
>>>>> Implement wait-free concurrent queues, with a new API different from
>>>>> wfqueue.h, which is already provided by Userspace RCU. The advantage of
>>>>> splitting the head and tail objects of the queue into different
>>>>> arguments is to allow these to sit on different cache-lines, thus
>>>>> eliminating false-sharing, leading to a 2.3x speed increase.
>>>>>
>>>>> This API also introduces a "splice" operation, which moves all nodes
>>>>> from one queue into another, and postpones the synchronization to either
>>>>> dequeue or iteration on the list. The splice operation does not need to
>>>>> touch every single node of the queue it moves them from. Moreover, the
>>>>> splice operation only needs to ensure mutual exclusion with other
>>>>> dequeuers, iterations, and splice operations from the list it splices
>>>>> from, but acts as a simple enqueuer on the list it splices into (no
>>>>> mutual exclusion needed for that list).
>>>>>
>>>>> Feedback is welcome,
>>>>
>>>> These look sane to me, though I must confess that the tail pointer
>>>> referencing the node rather than the node's next pointer did throw
>>>> me for a bit.  ;-)
>>>
>>> Yes, this was originally introduced with Lai's original patch to
>>> wfqueue, which I think is a nice simplification: it's pretty much the
>>> same thing to use the last node address as tail rather than the address
>>> of its first member (its next pointer address (_not_ value)). It ends up
>>> being the same address in this case, but more interestingly, we don't
>>> have to use a struct cds_wfcq_node ** type: a simple struct
>>> cds_wfcq_node *  suffice.
>>>
>>> Thanks Paul, I will therefore merge these 3 patches with your Acked-by.
>>>
>>> Lai, you are welcome to provide improvements to this code against the
>>> master branch. I will gladly consider any change you propose.
>>>
>>
>> I did not remember that there is any improvement idea not included.
>> The patchset is OK for me.
> 
> Great! Would you be OK if I commit the following patch ? Let me know if
> you want me to put your signed-off-by on this (I can even put your email
> as From if you like):
> 
> 
> wfcqueue: update credits in patch documentation
> 
> Give credits to those responsible for the design and implementation of
> commit 8ad4ce587f001ae026d5560ac509c2e48986130b, "wfcqueue: implement
> concurrency-efficient queue", which happened through rounds of email and
> patch exchanges.
> 
> Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
> ---
> diff --git a/urcu/static/wfcqueue.h b/urcu/static/wfcqueue.h
> index a989984..153143d 100644
> --- a/urcu/static/wfcqueue.h
> +++ b/urcu/static/wfcqueue.h
> @@ -41,8 +41,10 @@ extern "C" {
>  /*
>   * Concurrent queue with wait-free enqueue/blocking dequeue.
>   *
> - * Inspired from half-wait-free/half-blocking queue implementation done by
> - * Paul E. McKenney.
> + * This queue has been designed and implemented collaboratively by
> + * Mathieu Desnoyers and Lai Jiangshan. Inspired from
> + * half-wait-free/half-blocking queue implementation done by Paul E.
> + * McKenney.
>   *
>   * Mutual exclusion of __cds_wfcq_* API
>   *
> diff --git a/urcu/wfcqueue.h b/urcu/wfcqueue.h
> index 5576cbf..940dc7d 100644
> --- a/urcu/wfcqueue.h
> +++ b/urcu/wfcqueue.h
> @@ -37,8 +37,10 @@ extern "C" {
>  /*
>   * Concurrent queue with wait-free enqueue/blocking dequeue.
>   *
> - * Inspired from half-wait-free/half-blocking queue implementation done by
> - * Paul E. McKenney.
> + * This queue has been designed and implemented collaboratively by
> + * Mathieu Desnoyers and Lai Jiangshan. Inspired from
> + * half-wait-free/half-blocking queue implementation done by Paul E.
> + * McKenney.
>   */
>  
>  struct cds_wfcq_node {
> 
> 
>> I think you can reimplement wfqueue via wfcqueue without cacheline opt.
> 
> Hrm, semantically this can indeed be done, but I fear that we might not
> be strictly ABI-compatible with the old wfqueue. So I would be tempted
> to leave the old wfqueue implementation as-is, and maybe deprecate it at
> some point. Thoughts ?
> 

All APIs is not changed, they are forwarded to wfcqueue APIs in implementation,
so ABI of APIs is compatible. The only thing is struct cds_wfq_queue:

struct cds_wfq_queue {
	struct cds_wfq_node *head, **tail;
	struct cds_wfq_node dummy;	/* Dummy node */
	pthread_mutex_t lock;
};


We can redefine it as:

#define cds_wfq_node cds_wfcq_node

struct cds_wfq_queue {
	union {
		struct cds_wfq_node *head; /* make bug-user who wrongly directly access to ->head happy */
		struct cds_wfcq_node *__pad; /* not used in new implement */
	};
	union {
		struct cds_wfq_node **tail;  /* make bug-user who wrongly directly access to ->tail happy */
		struct cds_wfcq_tail real_tail;
	};
	union {
		struct {
			struct cds_wfq_node dummy;	/* Dummy node */
			pthread_mutex_t lock;
		}
		struct cds_wfcq_head real_head;
	};
}

static inline void _cds_wfq_init(struct cds_wfq_queue *q)
{
	q->head = &q->dummy; /* make bug-user who wrongly directly access to ->head happy */
	_cds_wfcq_init(&q->real_head, &q->real_tail);
}

after this change, struct cds_wfq_queue is not changed.
Even bug-user wrongly directly access to struct cds_wfq_queue by old-view,
the queue is compatible:
	head->dummy node->real node->real node....
	tail->real tail node(or dummy node)

the only different is that: dummy node is always the first node by old-view.


And if we deprecate struct cds_wfq_queue, I think we should provide a new default wfqueue to users.

Thanks,
Lai