* Lai Jiangshan ([email protected]) 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 <[email protected]> > --- > 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 <[email protected]> > + * Copyright 2011-2012 - Lai Jiangshan <[email protected]> > * > * 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 <[email protected]> > + * Copyright 2011-2012 - Lai Jiangshan <[email protected]> > * > * 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 <[email protected]> > + * Copyright 2011-2012 - Lai Jiangshan <[email protected]> > * > * 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 > > > _______________________________________________ > lttng-dev mailing list > [email protected] > http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev -- Mathieu Desnoyers Operating System Efficiency R&D Consultant EfficiOS Inc. http://www.efficios.com _______________________________________________ lttng-dev mailing list [email protected] http://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
