[lttng-dev] [PATCH 2/2] urcu: new wfqueue implementation
Mathieu Desnoyers
mathieu.desnoyers at efficios.com
Sat Aug 11 09:14:17 EDT 2012
* Lai Jiangshan (laijs at cn.fujitsu.com) wrote:
> Some guys would be surprised by this fact:
> There are already TWO implementations of wfqueue in urcu.
>
> The first one is in urcu/static/wfqueue.h:
> 1) enqueue: exchange the tail and then update previous->next
> 2) dequeue: wait for first node's next pointer and them shift, a dummy node
> is introduced to avoid the queue->tail become NULL when shift.
>
> The second one shares some code with the first one, and the left code
> are spreading in urcu-call-rcu-impl.h:
> 1) enqueue: share with the first one
> 2) no dequeue operation: and no shift, so it don't need dummy node,
> Although the dummy node is queued when initialization, but it is removed
> after the first dequeue_all operation in call_rcu_thread().
> call_rcu_data_free() forgets to handle the dummy node if it is not removed.
> 3)dequeue_all: record the old head and tail, and queue->head become the special
> tail node.(atomic record the tail and change the tail).
>
> The second implementation's code are spreading, bad for review, and it is not
> tested by tests/test_urcu_wfq.
>
> So we need a better implementation avoid the dummy node dancing and can service
> both generic wfqueue APIs and dequeue_all API for call rcu.
>
> The new implementation:
> 1) enqueue: share with the first one/original implementation.
> 2) dequeue: shift when node count >= 2, cmpxchg when node count = 1.
> no dummy node, save memory.
> 3) dequeue_all: simply set queue->head.next to NULL, xchg the tail
> and return the old head.next.
>
> More implementation details are in the code.
> tests/test_urcu_wfq will be update in future for testing new APIs.
Hi Lai,
Some other style-related questions below,
>
>
> Signed-off-by: Lai Jiangshan <laijs at cn.fujitsu.com>
> ---
> urcu-call-rcu-impl.h | 50 ++++++++++--------------
> urcu/static/wfqueue.h | 104 ++++++++++++++++++++++++++++++++++++------------
> urcu/wfqueue.h | 25 ++++++++++--
> wfqueue.c | 29 ++++++++++++++
> 4 files changed, 149 insertions(+), 59 deletions(-)
>
> diff --git a/urcu-call-rcu-impl.h b/urcu-call-rcu-impl.h
> index 13b24ff..dbfb410 100644
> --- a/urcu-call-rcu-impl.h
> +++ b/urcu-call-rcu-impl.h
> @@ -221,7 +221,7 @@ static void *call_rcu_thread(void *arg)
> {
> unsigned long cbcount;
> struct cds_wfq_node *cbs;
> - struct cds_wfq_node **cbs_tail;
> + struct cds_wfq_node *cbs_tail;
> struct call_rcu_data *crdp = (struct call_rcu_data *)arg;
> struct rcu_head *rhp;
> int rt = !!(uatomic_read(&crdp->flags) & URCU_CALL_RCU_RT);
> @@ -243,24 +243,18 @@ static void *call_rcu_thread(void *arg)
> cmm_smp_mb();
> }
> for (;;) {
> - if (&crdp->cbs.head != _CMM_LOAD_SHARED(crdp->cbs.tail)) {
> - while ((cbs = _CMM_LOAD_SHARED(crdp->cbs.head)) == NULL)
> - poll(NULL, 0, 1);
> - _CMM_STORE_SHARED(crdp->cbs.head, NULL);
> - cbs_tail = (struct cds_wfq_node **)
> - uatomic_xchg(&crdp->cbs.tail, &crdp->cbs.head);
> + cbs = __cds_wfq_dequeue_all_blocking(&crdp->cbs, &cbs_tail);
> + if (cbs) {
> synchronize_rcu();
> cbcount = 0;
> do {
> - while (cbs->next == NULL &&
> - &cbs->next != cbs_tail)
> - poll(NULL, 0, 1);
> - if (cbs == &crdp->cbs.dummy) {
> - cbs = cbs->next;
> - continue;
> - }
> rhp = (struct rcu_head *)cbs;
> - cbs = cbs->next;
> +
> + if (cbs != cbs_tail)
> + cbs = __cds_wfq_node_sync_next(cbs);
> + else
> + cbs = NULL;
> +
> rhp->func(rhp);
> cbcount++;
> } while (cbs != NULL);
> @@ -270,8 +264,7 @@ static void *call_rcu_thread(void *arg)
> break;
> rcu_thread_offline();
> if (!rt) {
> - if (&crdp->cbs.head
> - == _CMM_LOAD_SHARED(crdp->cbs.tail)) {
> + if (cds_wfq_empty(&crdp->cbs)) {
> call_rcu_wait(crdp);
> poll(NULL, 0, 10);
> uatomic_dec(&crdp->futex);
> @@ -625,32 +618,31 @@ void call_rcu(struct rcu_head *head,
> */
> void call_rcu_data_free(struct call_rcu_data *crdp)
> {
> - struct cds_wfq_node *cbs;
> - struct cds_wfq_node **cbs_tail;
> - struct cds_wfq_node **cbs_endprev;
> + struct cds_wfq_node *head, *tail;
>
> if (crdp == NULL || crdp == default_call_rcu_data) {
> return;
> }
> +
> if ((uatomic_read(&crdp->flags) & URCU_CALL_RCU_STOPPED) == 0) {
> uatomic_or(&crdp->flags, URCU_CALL_RCU_STOP);
> wake_call_rcu_thread(crdp);
> while ((uatomic_read(&crdp->flags) & URCU_CALL_RCU_STOPPED) == 0)
> poll(NULL, 0, 1);
> }
> - if (&crdp->cbs.head != _CMM_LOAD_SHARED(crdp->cbs.tail)) {
> - while ((cbs = _CMM_LOAD_SHARED(crdp->cbs.head)) == NULL)
> - poll(NULL, 0, 1);
> - _CMM_STORE_SHARED(crdp->cbs.head, NULL);
> - cbs_tail = (struct cds_wfq_node **)
> - uatomic_xchg(&crdp->cbs.tail, &crdp->cbs.head);
> +
> + if (!cds_wfq_empty(&crdp->cbs)) {
> + head = __cds_wfq_dequeue_all_blocking(&crdp->cbs, &tail);
> + assert(head);
> +
> /* Create default call rcu data if need be */
> (void) get_default_call_rcu_data();
> - cbs_endprev = (struct cds_wfq_node **)
> - uatomic_xchg(&default_call_rcu_data, cbs_tail);
> - *cbs_endprev = cbs;
> +
> + __cds_wfq_append_list(&default_call_rcu_data->cbs, head, tail);
> +
> uatomic_add(&default_call_rcu_data->qlen,
> uatomic_read(&crdp->qlen));
> +
> wake_call_rcu_thread(default_call_rcu_data);
> }
>
> diff --git a/urcu/static/wfqueue.h b/urcu/static/wfqueue.h
> index 636e1af..15ea9fc 100644
> --- a/urcu/static/wfqueue.h
> +++ b/urcu/static/wfqueue.h
> @@ -10,6 +10,7 @@
> * dynamically with the userspace rcu library.
> *
> * Copyright 2010 - Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
> + * Copyright 2011-2012 - Lai Jiangshan <laijs at cn.fujitsu.com>
> *
> * This library is free software; you can redistribute it and/or
> * modify it under the terms of the GNU Lesser General Public
> @@ -29,6 +30,7 @@
> #include <pthread.h>
> #include <assert.h>
> #include <poll.h>
> +#include <stdbool.h>
> #include <urcu/compiler.h>
> #include <urcu/uatomic.h>
>
> @@ -38,8 +40,6 @@ extern "C" {
>
> /*
> * Queue with wait-free enqueue/blocking dequeue.
> - * This implementation adds a dummy head node when the queue is empty to ensure
> - * we can always update the queue locklessly.
> *
> * Inspired from half-wait-free/half-blocking queue implementation done by
> * Paul E. McKenney.
> @@ -57,31 +57,43 @@ static inline void _cds_wfq_init(struct cds_wfq_queue *q)
> {
> int ret;
>
> - _cds_wfq_node_init(&q->dummy);
> /* Set queue head and tail */
> - q->head = &q->dummy;
> - q->tail = &q->dummy.next;
> + _cds_wfq_node_init(&q->head);
> + q->tail = &q->head;
> ret = pthread_mutex_init(&q->lock, NULL);
> assert(!ret);
> }
>
> -static inline void _cds_wfq_enqueue(struct cds_wfq_queue *q,
> - struct cds_wfq_node *node)
> +static inline bool _cds_wfq_empty(struct cds_wfq_queue *q)
> {
> - struct cds_wfq_node **old_tail;
> + /*
> + * Queue is empty if no node is pointed by q->head.next nor q->tail.
> + */
> + return q->head.next == NULL && CMM_LOAD_SHARED(q->tail) == &q->head;
> +}
>
> +static inline void ___cds_wfq_append_list(struct cds_wfq_queue *q,
> + struct cds_wfq_node *head, struct cds_wfq_node *tail)
> +{
> /*
> * uatomic_xchg() implicit memory barrier orders earlier stores to data
> * structure containing node and setting node->next to NULL before
> * publication.
> */
> - old_tail = uatomic_xchg(&q->tail, &node->next);
> + tail = uatomic_xchg(&q->tail, tail);
I'd prefer to keep "old_tail" here, because it becomes clearer to anyone
reviewing that uatomic_xchg() returns the old tail (and this extra
clarity comes without any overhead).
> +
> /*
> - * At this point, dequeuers see a NULL old_tail->next, which indicates
> + * At this point, dequeuers see a NULL tail->next, which indicates
> * that the queue is being appended to. The following store will append
> * "node" to the queue from a dequeuer perspective.
> */
> - CMM_STORE_SHARED(*old_tail, node);
> + CMM_STORE_SHARED(tail->next, head);
> +}
> +
> +static inline void _cds_wfq_enqueue(struct cds_wfq_queue *q,
> + struct cds_wfq_node *node)
> +{
> + ___cds_wfq_append_list(q, node, node);
> }
Why not keep ___cds_wfq_append_list() merged into _cds_wfq_enqueue() ?
This would keep the number of symbols exported minimal.
So if I get it right, one "_" prefix is the "normally used" functions
(exposed through the LGPL symbol API).
The "__" prefix are somewhat more internal, but can also be used
externally.
Finally, the "___" prefix seem to be quite similar to the
double-underscores.
We might need more consistency, I'm not sure the triple-underscores are
needed. Also, I'm not sure should export the double-underscore functions
outside of LGPL use (in other words, maybe we should not expose them to
!LGPL_SOURCE code). So we would emit the static inlines, but no symbols
for those. This covers ___cds_wfq_node_sync_next(), and
___cds_wfq_dequeue_all_blocking (which requires the caller to use
sync_next). Currently, all code that needs to fine-grained integration
is within the userspace RCU tree, which defines LGPL_SOURCE.
>
> /*
> @@ -120,27 +132,46 @@ ___cds_wfq_dequeue_blocking(struct cds_wfq_queue *q)
> {
> struct cds_wfq_node *node, *next;
>
> - /*
> - * Queue is empty if it only contains the dummy node.
> - */
> - if (q->head == &q->dummy && CMM_LOAD_SHARED(q->tail) == &q->dummy.next)
> + if (_cds_wfq_empty(q))
> return NULL;
> - node = q->head;
>
> - next = ___cds_wfq_node_sync_next(node);
> + node = ___cds_wfq_node_sync_next(&q->head);
> +
> + if ((next = CMM_LOAD_SHARED(node->next)) == NULL) {
> + if (CMM_LOAD_SHARED(q->tail) == node) {
> + /*
> + * @node is the only node in the queue.
> + * Try to move the tail to &q->head
> + */
> + _cds_wfq_node_init(&q->head);
> + if (uatomic_cmpxchg(&q->tail, node, &q->head) == node)
> + return node;
> + }
> + next = ___cds_wfq_node_sync_next(node);
> + }
>
> /*
> * Move queue head forward.
> */
> - q->head = next;
> - /*
> - * Requeue dummy node if we just dequeued it.
> - */
> - if (node == &q->dummy) {
> - _cds_wfq_node_init(node);
> - _cds_wfq_enqueue(q, node);
> - return ___cds_wfq_dequeue_blocking(q);
> - }
> + q->head.next = next;
> +
> + return node;
> +}
> +
> +/* dequeue all nodes, the nodes are not synchronized for the next pointer */
> +static inline struct cds_wfq_node *
> +___cds_wfq_dequeue_all_blocking(struct cds_wfq_queue *q,
> + struct cds_wfq_node **tail)
> +{
> + struct cds_wfq_node *node;
> +
> + if (_cds_wfq_empty(q))
> + return NULL;
> +
> + node = ___cds_wfq_node_sync_next(&q->head);
> + _cds_wfq_node_init(&q->head);
> + *tail = uatomic_xchg(&q->tail, &q->head);
> +
> return node;
> }
>
> @@ -158,6 +189,27 @@ _cds_wfq_dequeue_blocking(struct cds_wfq_queue *q)
> return retnode;
> }
>
> +static inline struct cds_wfq_node *
> +_cds_wfq_dequeue_all_blocking(struct cds_wfq_queue *q,
> + struct cds_wfq_node **tail)
> +{
> + struct cds_wfq_node *node, *next;
> + int ret;
> +
> + ret = pthread_mutex_lock(&q->lock);
> + assert(!ret);
> + node = ___cds_wfq_dequeue_all_blocking(q, tail);
> + ret = pthread_mutex_unlock(&q->lock);
> + assert(!ret);
So we take the queue lock on dequeue_all, but not on dequeue. It might
be good to have a consistent behavior: either we lock dequeue and
dequeue_all, or leave the lock entirely to the caller (and document it).
Thoughts ?
Thanks!
Mathieu
> +
> + /* synchronize all nodes' next pointer */
> + next = node;
> + while (next != *tail)
> + next = ___cds_wfq_node_sync_next(next);
> +
> + return node;
> +}
> +
> #ifdef __cplusplus
> }
> #endif
> diff --git a/urcu/wfqueue.h b/urcu/wfqueue.h
> index 03a73f1..985f540 100644
> --- a/urcu/wfqueue.h
> +++ b/urcu/wfqueue.h
> @@ -7,6 +7,7 @@
> * Userspace RCU library - Queue with Wait-Free Enqueue/Blocking Dequeue
> *
> * Copyright 2010 - Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
> + * Copyright 2011-2012 - Lai Jiangshan <laijs at cn.fujitsu.com>
> *
> * This library is free software; you can redistribute it and/or
> * modify it under the terms of the GNU Lesser General Public
> @@ -25,6 +26,7 @@
>
> #include <pthread.h>
> #include <assert.h>
> +#include <stdbool.h>
> #include <urcu/compiler.h>
>
> #ifdef __cplusplus
> @@ -33,8 +35,6 @@ extern "C" {
>
> /*
> * Queue with wait-free enqueue/blocking dequeue.
> - * This implementation adds a dummy head node when the queue is empty to ensure
> - * we can always update the queue locklessly.
> *
> * Inspired from half-wait-free/half-blocking queue implementation done by
> * Paul E. McKenney.
> @@ -45,8 +45,7 @@ struct cds_wfq_node {
> };
>
> struct cds_wfq_queue {
> - struct cds_wfq_node *head, **tail;
> - struct cds_wfq_node dummy; /* Dummy node */
> + struct cds_wfq_node head, *tail;
> pthread_mutex_t lock;
> };
>
> @@ -56,18 +55,36 @@ struct cds_wfq_queue {
>
> #define cds_wfq_node_init _cds_wfq_node_init
> #define cds_wfq_init _cds_wfq_init
> +#define cds_wfq_empty _cds_wfq_empty
> +#define __cds_wfq_append_list ___cds_wfq_append_list
> #define cds_wfq_enqueue _cds_wfq_enqueue
> #define __cds_wfq_dequeue_blocking ___cds_wfq_dequeue_blocking
> #define cds_wfq_dequeue_blocking _cds_wfq_dequeue_blocking
> +#define __cds_wfq_node_sync_next ___cds_wfq_node_sync_next
> +#define __cds_wfq_dequeue_all_blocking ___cds_wfq_dequeue_all_blocking
> +#define cds_wfq_dequeue_all_blocking _cds_wfq_dequeue_all_blocking
>
> #else /* !_LGPL_SOURCE */
>
> extern void cds_wfq_node_init(struct cds_wfq_node *node);
> extern void cds_wfq_init(struct cds_wfq_queue *q);
> +extern bool cds_wfq_empty(struct cds_wfq_queue *q);
> +/* __cds_wfq_append_list: caller ensures mutual exclusion between dequeues */
> +extern void __cds_wfq_append_list(struct cds_wfq_queue *q,
> + struct cds_wfq_node *head, struct cds_wfq_node *tail);
> extern void cds_wfq_enqueue(struct cds_wfq_queue *q, struct cds_wfq_node *node);
> /* __cds_wfq_dequeue_blocking: caller ensures mutual exclusion between dequeues */
> extern struct cds_wfq_node *__cds_wfq_dequeue_blocking(struct cds_wfq_queue *q);
> extern struct cds_wfq_node *cds_wfq_dequeue_blocking(struct cds_wfq_queue *q);
> +extern struct cds_wfq_node *__cds_wfq_node_sync_next(struct cds_wfq_node *node);
> +/*
> + * __cds_wfq_dequeue_all_blocking: caller ensures mutual exclusion between
> + * dequeues, and need synchronize next pointer berfore use it.
> + */
> +extern struct cds_wfq_node *__cds_wfq_dequeue_all_blocking(
> + struct cds_wfq_queue *q, struct cds_wfq_node **tail);
> +extern struct cds_wfq_node *cds_wfq_dequeue_all_blocking(
> + struct cds_wfq_queue *q, struct cds_wfq_node **tail);
>
> #endif /* !_LGPL_SOURCE */
>
> diff --git a/wfqueue.c b/wfqueue.c
> index 3337171..28a7b58 100644
> --- a/wfqueue.c
> +++ b/wfqueue.c
> @@ -4,6 +4,7 @@
> * Userspace RCU library - Queue with Wait-Free Enqueue/Blocking Dequeue
> *
> * Copyright 2010 - Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
> + * Copyright 2011-2012 - Lai Jiangshan <laijs at cn.fujitsu.com>
> *
> * This library is free software; you can redistribute it and/or
> * modify it under the terms of the GNU Lesser General Public
> @@ -38,6 +39,17 @@ void cds_wfq_init(struct cds_wfq_queue *q)
> _cds_wfq_init(q);
> }
>
> +bool cds_wfq_empty(struct cds_wfq_queue *q)
> +{
> + return _cds_wfq_empty(q);
> +}
> +
> +void __cds_wfq_append_list(struct cds_wfq_queue *q,
> + struct cds_wfq_node *head, struct cds_wfq_node *tail)
> +{
> + return ___cds_wfq_append_list(q, head, tail);
> +}
> +
> void cds_wfq_enqueue(struct cds_wfq_queue *q, struct cds_wfq_node *node)
> {
> _cds_wfq_enqueue(q, node);
> @@ -52,3 +64,20 @@ struct cds_wfq_node *cds_wfq_dequeue_blocking(struct cds_wfq_queue *q)
> {
> return _cds_wfq_dequeue_blocking(q);
> }
> +
> +struct cds_wfq_node *__cds_wfq_node_sync_next(struct cds_wfq_node *node)
> +{
> + return ___cds_wfq_node_sync_next(node);
> +}
> +
> +struct cds_wfq_node *__cds_wfq_dequeue_all_blocking(
> + struct cds_wfq_queue *q, struct cds_wfq_node **tail)
> +{
> + return ___cds_wfq_dequeue_all_blocking(q, tail);
> +}
> +
> +struct cds_wfq_node *cds_wfq_dequeue_all_blocking(
> + struct cds_wfq_queue *q, struct cds_wfq_node **tail)
> +{
> + return _cds_wfq_dequeue_all_blocking(q, tail);
> +}
> --
> 1.7.7
>
>
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--
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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