[lttng-dev] [PATCH v2 11/12] Add cmm_emit_legacy_smp_mb()
Olivier Dion
odion at efficios.com
Wed Jun 7 14:53:58 EDT 2023
Some public APIs stipulate implicit memory barriers on operations. These
were coherent with the memory model used at that time. However, with the
migration to a memory model closer to the C11 memory model, these memory
barriers are not strictly emitted by the atomic operations in the new
memory model.
Therefore, introducing the `--disable-legacy-mb' configuration
option. By default, liburcu is configured to emit these legacy memory
barriers, thus keeping backward compatibility at the expense of slower
performances. However, users can opt-out by disabling the legacy memory
barriers.
This options is publicly exported in the system configuration header
file and can be overrode manually on a compilation unit basis by
defining `CONFIG_RCU_EMIT_LEGACY_MB' before including any liburcu files.
The usage of this macro requires to re-write atomic operations in term
of the CMM memory model. This is done for the queue and stack APIs.
Change-Id: Ia5ce3b3d8cd1955556ce96fa4408a63aa098a1a6
Signed-off-by: Olivier Dion <odion at efficios.com>
---
configure.ac | 13 ++++++
include/urcu/arch.h | 6 +++
include/urcu/config.h.in | 3 ++
include/urcu/static/lfstack.h | 25 ++++++++----
include/urcu/static/rculfqueue.h | 14 ++++---
include/urcu/static/rculfstack.h | 8 +++-
include/urcu/static/wfcqueue.h | 68 +++++++++++++++++---------------
include/urcu/static/wfqueue.h | 9 +++--
include/urcu/static/wfstack.h | 24 +++++++----
9 files changed, 114 insertions(+), 56 deletions(-)
diff --git a/configure.ac b/configure.ac
index 4450a31..ca51b5b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -235,6 +235,11 @@ AE_FEATURE([cds-lfht-iter-debug], [Enable extra debugging checks for lock-free h
AE_FEATURE_DEFAULT_ENABLE
AE_FEATURE([atomic-builtins], [Disable the usage of toolchain atomic builtins.])
+# emit legacy memory barriers
+# Enable by default
+AE_FEATURE_DEFAULT_ENABLE
+AE_FEATURE([legacy-mb], [Disable legacy memory barriers.])
+
# When given, add -Werror to WARN_CFLAGS and WARN_CXXFLAGS.
# Disabled by default
AE_FEATURE_DEFAULT_DISABLE
@@ -282,6 +287,10 @@ AE_IF_FEATURE_ENABLED([atomic-builtins], [
[AE_FEATURE_DISABLE(atomic-builtins)])
])
+AE_IF_FEATURE_ENABLED([legacy-mb], [
+ AC_DEFINE([CONFIG_RCU_EMIT_LEGACY_MB], [1], [Emit legacy memory barriers that were documented in the APIs.])
+])
+
## ##
## Set automake variables for optional feature conditionnals in Makefile.am ##
## ##
@@ -387,6 +396,10 @@ PPRINT_PROP_BOOL([Lock-free HT iterator debugging], $value)
AE_IS_FEATURE_ENABLED([atomic-builtins]) && value=1 || value=0
PPRINT_PROP_BOOL([Use toolchain atomic builtins], $value)
+# legacy memory barriers
+AE_IS_FEATURE_ENABLED([legacy-mb]) && value=1 || value=0
+PPRINT_PROP_BOOL([Emit legacy memory barriers], $value)
+
PPRINT_PROP_BOOL([Multi-flavor support], 1)
report_bindir="`eval eval echo $bindir`"
diff --git a/include/urcu/arch.h b/include/urcu/arch.h
index d3914da..377a0ec 100644
--- a/include/urcu/arch.h
+++ b/include/urcu/arch.h
@@ -171,5 +171,11 @@
#error "Cannot build: unrecognized architecture, see <urcu/arch.h>."
#endif
+#ifdef CONFIG_RCU_EMIT_LEGACY_MB
+# define cmm_emit_legacy_smp_mb() cmm_smp_mb()
+#else
+# define cmm_emit_legacy_smp_mb() do { } while (0)
+#endif
+
#endif /* _URCU_ARCH_H */
diff --git a/include/urcu/config.h.in b/include/urcu/config.h.in
index 1daaa7e..d2f6c8c 100644
--- a/include/urcu/config.h.in
+++ b/include/urcu/config.h.in
@@ -22,6 +22,9 @@
/* Uatomic API uses atomic builtins? */
#undef CONFIG_RCU_USE_ATOMIC_BUILTINS
+/* Emit legacy memory barriers? */
+#undef CONFIG_RCU_EMIT_LEGACY_MB
+
/* Expose multi-flavor support */
#define CONFIG_RCU_HAVE_MULTIFLAVOR 1
diff --git a/include/urcu/static/lfstack.h b/include/urcu/static/lfstack.h
index a05acb4..07604db 100644
--- a/include/urcu/static/lfstack.h
+++ b/include/urcu/static/lfstack.h
@@ -114,7 +114,7 @@ bool ___cds_lfs_empty_head(struct cds_lfs_head *head)
static inline
bool _cds_lfs_empty(cds_lfs_stack_ptr_t s)
{
- return ___cds_lfs_empty_head(CMM_LOAD_SHARED(s._s->head));
+ return ___cds_lfs_empty_head(uatomic_load(&s._s->head, CMM_RELAXED));
}
/*
@@ -122,6 +122,8 @@ bool _cds_lfs_empty(cds_lfs_stack_ptr_t s)
*
* Does not require any synchronization with other push nor pop.
*
+ * Operations before push are consistent when observed after associated pop.
+ *
* Lock-free stack push is not subject to ABA problem, so no need to
* take the RCU read-side lock. Even if "head" changes between two
* uatomic_cmpxchg() invocations here (being popped, and then pushed
@@ -167,7 +169,9 @@ bool _cds_lfs_push(cds_lfs_stack_ptr_t u_s,
* uatomic_cmpxchg() implicit memory barrier orders earlier
* stores to node before publication.
*/
- head = uatomic_cmpxchg(&s->head, old_head, new_head);
+ cmm_emit_legacy_smp_mb();
+ head = uatomic_cmpxchg_mo(&s->head, old_head, new_head,
+ CMM_SEQ_CST, CMM_SEQ_CST);
if (old_head == head)
break;
}
@@ -179,6 +183,8 @@ bool _cds_lfs_push(cds_lfs_stack_ptr_t u_s,
*
* Returns NULL if stack is empty.
*
+ * Operations after pop are consistent when observed before associated push.
+ *
* __cds_lfs_pop needs to be synchronized using one of the following
* techniques:
*
@@ -203,7 +209,7 @@ struct cds_lfs_node *___cds_lfs_pop(cds_lfs_stack_ptr_t u_s)
struct cds_lfs_head *head, *next_head;
struct cds_lfs_node *next;
- head = _CMM_LOAD_SHARED(s->head);
+ head = uatomic_load(&s->head, CMM_CONSUME);
if (___cds_lfs_empty_head(head))
return NULL; /* Empty stack */
@@ -212,12 +218,14 @@ struct cds_lfs_node *___cds_lfs_pop(cds_lfs_stack_ptr_t u_s)
* memory barrier before uatomic_cmpxchg() in
* cds_lfs_push.
*/
- cmm_smp_read_barrier_depends();
- next = _CMM_LOAD_SHARED(head->node.next);
+ next = uatomic_load(&head->node.next, CMM_RELAXED);
next_head = caa_container_of(next,
struct cds_lfs_head, node);
- if (uatomic_cmpxchg(&s->head, head, next_head) == head)
+ if (uatomic_cmpxchg_mo(&s->head, head, next_head,
+ CMM_SEQ_CST, CMM_SEQ_CST) == head){
+ cmm_emit_legacy_smp_mb();
return &head->node;
+ }
/* busy-loop if head changed under us */
}
}
@@ -245,6 +253,7 @@ static inline
struct cds_lfs_head *___cds_lfs_pop_all(cds_lfs_stack_ptr_t u_s)
{
struct __cds_lfs_stack *s = u_s._s;
+ struct cds_lfs_head *head;
/*
* Implicit memory barrier after uatomic_xchg() matches implicit
@@ -256,7 +265,9 @@ struct cds_lfs_head *___cds_lfs_pop_all(cds_lfs_stack_ptr_t u_s)
* taking care to order writes to each node prior to the full
* memory barrier after this uatomic_xchg().
*/
- return uatomic_xchg(&s->head, NULL);
+ head = uatomic_xchg_mo(&s->head, NULL, CMM_SEQ_CST);
+ cmm_emit_legacy_smp_mb();
+ return head;
}
/*
diff --git a/include/urcu/static/rculfqueue.h b/include/urcu/static/rculfqueue.h
index ad73454..25a4ec8 100644
--- a/include/urcu/static/rculfqueue.h
+++ b/include/urcu/static/rculfqueue.h
@@ -148,26 +148,29 @@ void _cds_lfq_enqueue_rcu(struct cds_lfq_queue_rcu *q,
* uatomic_cmpxchg() implicit memory barrier orders earlier stores to
* node before publication.
*/
-
for (;;) {
struct cds_lfq_node_rcu *tail, *next;
tail = rcu_dereference(q->tail);
- next = uatomic_cmpxchg(&tail->next, NULL, node);
+ cmm_emit_legacy_smp_mb();
+ next = uatomic_cmpxchg_mo(&tail->next, NULL, node,
+ CMM_SEQ_CST, CMM_SEQ_CST);
if (next == NULL) {
/*
* Tail was at the end of queue, we successfully
* appended to it. Now move tail (another
* enqueue might beat us to it, that's fine).
*/
- (void) uatomic_cmpxchg(&q->tail, tail, node);
+ (void) uatomic_cmpxchg_mo(&q->tail, tail, node,
+ CMM_SEQ_CST, CMM_SEQ_CST);
return;
} else {
/*
* Failure to append to current tail.
* Help moving tail further and retry.
*/
- (void) uatomic_cmpxchg(&q->tail, tail, next);
+ (void) uatomic_cmpxchg_mo(&q->tail, tail, next,
+ CMM_SEQ_CST, CMM_SEQ_CST);
continue;
}
}
@@ -211,7 +214,8 @@ struct cds_lfq_node_rcu *_cds_lfq_dequeue_rcu(struct cds_lfq_queue_rcu *q)
enqueue_dummy(q);
next = rcu_dereference(head->next);
}
- if (uatomic_cmpxchg(&q->head, head, next) != head)
+ if (uatomic_cmpxchg_mo(&q->head, head, next,
+ CMM_SEQ_CST, CMM_SEQ_CST) != head)
continue; /* Concurrently pushed. */
if (head->dummy) {
/* Free dummy after grace period. */
diff --git a/include/urcu/static/rculfstack.h b/include/urcu/static/rculfstack.h
index 54ff377..2befb2a 100644
--- a/include/urcu/static/rculfstack.h
+++ b/include/urcu/static/rculfstack.h
@@ -83,7 +83,9 @@ int _cds_lfs_push_rcu(struct cds_lfs_stack_rcu *s,
* uatomic_cmpxchg() implicit memory barrier orders earlier
* stores to node before publication.
*/
- head = uatomic_cmpxchg(&s->head, old_head, node);
+ cmm_emit_legacy_smp_mb();
+ head = uatomic_cmpxchg_mo(&s->head, old_head, node,
+ CMM_SEQ_CST, CMM_SEQ_CST);
if (old_head == head)
break;
}
@@ -108,7 +110,9 @@ _cds_lfs_pop_rcu(struct cds_lfs_stack_rcu *s)
if (head) {
struct cds_lfs_node_rcu *next = rcu_dereference(head->next);
- if (uatomic_cmpxchg(&s->head, head, next) == head) {
+ if (uatomic_cmpxchg_mo(&s->head, head, next,
+ CMM_SEQ_CST, CMM_SEQ_CST) == head) {
+ cmm_emit_legacy_smp_mb();
return head;
} else {
/* Concurrent modification. Retry. */
diff --git a/include/urcu/static/wfcqueue.h b/include/urcu/static/wfcqueue.h
index 478e859..043b18a 100644
--- a/include/urcu/static/wfcqueue.h
+++ b/include/urcu/static/wfcqueue.h
@@ -91,6 +91,11 @@ static inline void _cds_wfcq_node_init(struct cds_wfcq_node *node)
node->next = NULL;
}
+static inline void _cds_wfcq_node_init_atomic(struct cds_wfcq_node *node)
+{
+ uatomic_store(&node->next, NULL, CMM_RELAXED);
+}
+
/*
* cds_wfcq_init: initialize wait-free queue (with lock). Pair with
* cds_wfcq_destroy().
@@ -153,8 +158,8 @@ static inline bool _cds_wfcq_empty(cds_wfcq_head_ptr_t u_head,
* common case to ensure that dequeuers do not frequently access
* enqueuer's tail->p cache line.
*/
- return CMM_LOAD_SHARED(head->node.next) == NULL
- && CMM_LOAD_SHARED(tail->p) == &head->node;
+ return uatomic_load(&head->node.next, CMM_CONSUME) == NULL
+ && uatomic_load(&tail->p, CMM_CONSUME) == &head->node;
}
static inline void _cds_wfcq_dequeue_lock(struct cds_wfcq_head *head,
@@ -188,7 +193,7 @@ static inline bool ___cds_wfcq_append(cds_wfcq_head_ptr_t u_head,
* stores to data structure containing node and setting
* node->next to NULL before publication.
*/
- old_tail = uatomic_xchg(&tail->p, new_tail);
+ old_tail = uatomic_xchg_mo(&tail->p, new_tail, CMM_SEQ_CST);
/*
* Implicit memory barrier after uatomic_xchg() orders store to
@@ -199,7 +204,8 @@ static inline bool ___cds_wfcq_append(cds_wfcq_head_ptr_t u_head,
* store will append "node" to the queue from a dequeuer
* perspective.
*/
- CMM_STORE_SHARED(old_tail->next, new_head);
+ uatomic_store(&old_tail->next, new_head, CMM_RELEASE);
+
/*
* Return false if queue was empty prior to adding the node,
* else return true.
@@ -210,8 +216,8 @@ static inline bool ___cds_wfcq_append(cds_wfcq_head_ptr_t u_head,
/*
* cds_wfcq_enqueue: enqueue a node into a wait-free queue.
*
- * Issues a full memory barrier before enqueue. No mutual exclusion is
- * required.
+ * Operations prior to enqueue are consistant with respect to dequeuing or
+ * splicing and iterating.
*
* Returns false if the queue was empty prior to adding the node.
* Returns true otherwise.
@@ -220,6 +226,8 @@ static inline bool _cds_wfcq_enqueue(cds_wfcq_head_ptr_t head,
struct cds_wfcq_tail *tail,
struct cds_wfcq_node *new_tail)
{
+ cmm_emit_legacy_smp_mb();
+
return ___cds_wfcq_append(head, tail, new_tail, new_tail);
}
@@ -270,8 +278,10 @@ ___cds_wfcq_node_sync_next(struct cds_wfcq_node *node, int blocking)
/*
* Adaptative busy-looping waiting for enqueuer to complete enqueue.
+ *
+ * Load node.next before loading node's content
*/
- while ((next = CMM_LOAD_SHARED(node->next)) == NULL) {
+ while ((next = uatomic_load(&node->next, CMM_CONSUME)) == NULL) {
if (___cds_wfcq_busy_wait(&attempt, blocking))
return CDS_WFCQ_WOULDBLOCK;
}
@@ -290,8 +300,7 @@ ___cds_wfcq_first(cds_wfcq_head_ptr_t u_head,
if (_cds_wfcq_empty(__cds_wfcq_head_cast(head), tail))
return NULL;
node = ___cds_wfcq_node_sync_next(&head->node, blocking);
- /* Load head->node.next before loading node's content */
- cmm_smp_read_barrier_depends();
+
return node;
}
@@ -343,16 +352,15 @@ ___cds_wfcq_next(cds_wfcq_head_ptr_t head __attribute__((unused)),
* out if we reached the end of the queue, we first check
* node->next as a common case to ensure that iteration on nodes
* do not frequently access enqueuer's tail->p cache line.
+ *
+ * Load node->next before loading next's content
*/
- if ((next = CMM_LOAD_SHARED(node->next)) == NULL) {
- /* Load node->next before tail->p */
- cmm_smp_rmb();
- if (CMM_LOAD_SHARED(tail->p) == node)
+ if ((next = uatomic_load(&node->next, CMM_CONSUME)) == NULL) {
+ if (uatomic_load(&tail->p, CMM_RELAXED) == node)
return NULL;
next = ___cds_wfcq_node_sync_next(node, blocking);
}
- /* Load node->next before loading next's content */
- cmm_smp_read_barrier_depends();
+
return next;
}
@@ -414,7 +422,7 @@ ___cds_wfcq_dequeue_with_state(cds_wfcq_head_ptr_t u_head,
return CDS_WFCQ_WOULDBLOCK;
}
- if ((next = CMM_LOAD_SHARED(node->next)) == NULL) {
+ if ((next = uatomic_load(&node->next, CMM_CONSUME)) == NULL) {
/*
* @node is probably the only node in the queue.
* Try to move the tail to &q->head.
@@ -422,17 +430,13 @@ ___cds_wfcq_dequeue_with_state(cds_wfcq_head_ptr_t u_head,
* NULL if the cmpxchg succeeds. Should the
* cmpxchg fail due to a concurrent enqueue, the
* q->head.next will be set to the next node.
- * The implicit memory barrier before
- * uatomic_cmpxchg() orders load node->next
- * before loading q->tail.
- * The implicit memory barrier before uatomic_cmpxchg
- * orders load q->head.next before loading node's
- * content.
*/
- _cds_wfcq_node_init(&head->node);
- if (uatomic_cmpxchg(&tail->p, node, &head->node) == node) {
+ _cds_wfcq_node_init_atomic(&head->node);
+ if (uatomic_cmpxchg_mo(&tail->p, node, &head->node,
+ CMM_SEQ_CST, CMM_SEQ_CST) == node) {
if (state)
*state |= CDS_WFCQ_STATE_LAST;
+ cmm_emit_legacy_smp_mb();
return node;
}
next = ___cds_wfcq_node_sync_next(node, blocking);
@@ -442,7 +446,7 @@ ___cds_wfcq_dequeue_with_state(cds_wfcq_head_ptr_t u_head,
* (currently NULL) back to its original value.
*/
if (!blocking && next == CDS_WFCQ_WOULDBLOCK) {
- head->node.next = node;
+ uatomic_store(&head->node.next, node, CMM_RELAXED);
return CDS_WFCQ_WOULDBLOCK;
}
}
@@ -450,10 +454,9 @@ ___cds_wfcq_dequeue_with_state(cds_wfcq_head_ptr_t u_head,
/*
* Move queue head forward.
*/
- head->node.next = next;
+ uatomic_store(&head->node.next, next, CMM_RELAXED);
+ cmm_emit_legacy_smp_mb();
- /* Load q->head.next before loading node's content */
- cmm_smp_read_barrier_depends();
return node;
}
@@ -515,6 +518,8 @@ ___cds_wfcq_dequeue_nonblocking(cds_wfcq_head_ptr_t head,
/*
* __cds_wfcq_splice: enqueue all src_q nodes at the end of dest_q.
*
+ * Operations after splice are consistant with respect to enqueue.
+ *
* Dequeue all nodes from src_q.
* dest_q must be already initialized.
* Mutual exclusion for src_q should be ensured by the caller as
@@ -548,10 +553,10 @@ ___cds_wfcq_splice(
* uatomic_xchg, as well as tail pointer vs head node
* address.
*/
- head = uatomic_xchg(&src_q_head->node.next, NULL);
+ head = uatomic_xchg_mo(&src_q_head->node.next, NULL, CMM_SEQ_CST);
if (head)
break; /* non-empty */
- if (CMM_LOAD_SHARED(src_q_tail->p) == &src_q_head->node)
+ if (uatomic_load(&src_q_tail->p, CMM_CONSUME) == &src_q_head->node)
return CDS_WFCQ_RET_SRC_EMPTY;
if (___cds_wfcq_busy_wait(&attempt, blocking))
return CDS_WFCQ_RET_WOULDBLOCK;
@@ -563,7 +568,8 @@ ___cds_wfcq_splice(
* concurrent enqueue on src_q, which exchanges the tail before
* updating the previous tail's next pointer.
*/
- tail = uatomic_xchg(&src_q_tail->p, &src_q_head->node);
+ cmm_emit_legacy_smp_mb();
+ tail = uatomic_xchg_mo(&src_q_tail->p, &src_q_head->node, CMM_SEQ_CST);
/*
* Append the spliced content of src_q into dest_q. Does not
diff --git a/include/urcu/static/wfqueue.h b/include/urcu/static/wfqueue.h
index d04f66f..290fe0a 100644
--- a/include/urcu/static/wfqueue.h
+++ b/include/urcu/static/wfqueue.h
@@ -81,13 +81,14 @@ static inline void _cds_wfq_enqueue(struct cds_wfq_queue *q,
* structure containing node and setting node->next to NULL before
* publication.
*/
- old_tail = uatomic_xchg(&q->tail, &node->next);
+ cmm_emit_legacy_smp_mb();
+ old_tail = uatomic_xchg_mo(&q->tail, &node->next, CMM_SEQ_CST);
/*
* At this point, dequeuers see a NULL old_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);
+ uatomic_store(old_tail, node, CMM_RELEASE);
}
/*
@@ -102,7 +103,7 @@ ___cds_wfq_node_sync_next(struct cds_wfq_node *node)
/*
* Adaptative busy-looping waiting for enqueuer to complete enqueue.
*/
- while ((next = CMM_LOAD_SHARED(node->next)) == NULL) {
+ while ((next = uatomic_load(&node->next, CMM_CONSUME)) == NULL) {
if (++attempt >= WFQ_ADAPT_ATTEMPTS) {
(void) poll(NULL, 0, WFQ_WAIT); /* Wait for 10ms */
attempt = 0;
@@ -129,7 +130,7 @@ ___cds_wfq_dequeue_blocking(struct cds_wfq_queue *q)
/*
* Queue is empty if it only contains the dummy node.
*/
- if (q->head == &q->dummy && CMM_LOAD_SHARED(q->tail) == &q->dummy.next)
+ if (q->head == &q->dummy && uatomic_load(&q->tail, CMM_CONSUME) == &q->dummy.next)
return NULL;
node = q->head;
diff --git a/include/urcu/static/wfstack.h b/include/urcu/static/wfstack.h
index 088e6e3..cfaf675 100644
--- a/include/urcu/static/wfstack.h
+++ b/include/urcu/static/wfstack.h
@@ -124,7 +124,7 @@ static inline bool _cds_wfs_empty(cds_wfs_stack_ptr_t u_stack)
{
struct __cds_wfs_stack *s = u_stack._s;
- return ___cds_wfs_end(CMM_LOAD_SHARED(s->head));
+ return ___cds_wfs_end(uatomic_load(&s->head, CMM_RELAXED));
}
/*
@@ -133,6 +133,8 @@ static inline bool _cds_wfs_empty(cds_wfs_stack_ptr_t u_stack)
* Issues a full memory barrier before push. No mutual exclusion is
* required.
*
+ * Operations before push are consistent when observed after associated pop.
+ *
* Returns 0 if the stack was empty prior to adding the node.
* Returns non-zero otherwise.
*/
@@ -148,12 +150,13 @@ int _cds_wfs_push(cds_wfs_stack_ptr_t u_stack, struct cds_wfs_node *node)
* uatomic_xchg() implicit memory barrier orders earlier stores
* to node (setting it to NULL) before publication.
*/
- old_head = uatomic_xchg(&s->head, new_head);
+ cmm_emit_legacy_smp_mb();
+ old_head = uatomic_xchg_mo(&s->head, new_head, CMM_SEQ_CST);
/*
* At this point, dequeuers see a NULL node->next, they should
* busy-wait until node->next is set to old_head.
*/
- CMM_STORE_SHARED(node->next, &old_head->node);
+ uatomic_store(&node->next, &old_head->node, CMM_RELEASE);
return !___cds_wfs_end(old_head);
}
@@ -169,7 +172,7 @@ ___cds_wfs_node_sync_next(struct cds_wfs_node *node, int blocking)
/*
* Adaptative busy-looping waiting for push to complete.
*/
- while ((next = CMM_LOAD_SHARED(node->next)) == NULL) {
+ while ((next = uatomic_load(&node->next, CMM_CONSUME)) == NULL) {
if (!blocking)
return CDS_WFS_WOULDBLOCK;
if (++attempt >= CDS_WFS_ADAPT_ATTEMPTS) {
@@ -194,7 +197,7 @@ ___cds_wfs_pop(cds_wfs_stack_ptr_t u_stack, int *state, int blocking)
if (state)
*state = 0;
for (;;) {
- head = CMM_LOAD_SHARED(s->head);
+ head = uatomic_load(&s->head, CMM_CONSUME);
if (___cds_wfs_end(head)) {
return NULL;
}
@@ -203,9 +206,11 @@ ___cds_wfs_pop(cds_wfs_stack_ptr_t u_stack, int *state, int blocking)
return CDS_WFS_WOULDBLOCK;
}
new_head = caa_container_of(next, struct cds_wfs_head, node);
- if (uatomic_cmpxchg(&s->head, head, new_head) == head) {
+ if (uatomic_cmpxchg_mo(&s->head, head, new_head,
+ CMM_SEQ_CST, CMM_SEQ_CST) == head) {
if (state && ___cds_wfs_end(new_head))
*state |= CDS_WFS_STATE_LAST;
+ cmm_emit_legacy_smp_mb();
return &head->node;
}
if (!blocking) {
@@ -220,6 +225,8 @@ ___cds_wfs_pop(cds_wfs_stack_ptr_t u_stack, int *state, int blocking)
*
* Returns NULL if stack is empty.
*
+ * Operations after pop push are consistent when observed before associated push.
+ *
* __cds_wfs_pop_blocking needs to be synchronized using one of the
* following techniques:
*
@@ -278,6 +285,8 @@ ___cds_wfs_pop_nonblocking(cds_wfs_stack_ptr_t u_stack)
/*
* __cds_wfs_pop_all: pop all nodes from a stack.
*
+ * Operations after pop push are consistent when observed before associated push.
+ *
* __cds_wfs_pop_all does not require any synchronization with other
* push, nor with other __cds_wfs_pop_all, but requires synchronization
* matching the technique used to synchronize __cds_wfs_pop_blocking:
@@ -309,7 +318,8 @@ ___cds_wfs_pop_all(cds_wfs_stack_ptr_t u_stack)
* taking care to order writes to each node prior to the full
* memory barrier after this uatomic_xchg().
*/
- head = uatomic_xchg(&s->head, CDS_WFS_END);
+ head = uatomic_xchg_mo(&s->head, CDS_WFS_END, CMM_SEQ_CST);
+ cmm_emit_legacy_smp_mb();
if (___cds_wfs_end(head))
return NULL;
return head;
--
2.40.1
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