[lttng-dev] [PATCH v2] wfstack: implement mutex-free wfstack with transparent union

[Eric Wong]

This allows users more freedom to use alternative synchronization
mechanisms.

Changes since v1:
- Fix typos in cds_wfs_stack_ptr_t documentation.
  Thanks to Mathieu for spotting.

Signed-off-by: Eric Wong <normalperson at yhbt.net>
Cc: Paul E. McKenney <paulmck at linux.vnet.ibm.com>
Cc: Lai Jiangshan <laijs at cn.fujitsu.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
---
  > > + * The transparent union allows calling functions that work on both
  > > + * struct cds_wfcq_head and struct __cds_wfcq_head on any of those two
  > 
  > should be:  struct cds_wfs_stack and struct __cds_wfs_stack
  > 
  > Other than that, looks good! Can you send an updated version ?

 urcu/static/wfstack.h | 18 +++++++++++++++---
 urcu/wfstack.h        | 23 +++++++++++++++++++++--
 wfstack.c             |  9 +++++++--
 3 files changed, 43 insertions(+), 7 deletions(-)

diff --git a/urcu/static/wfstack.h b/urcu/static/wfstack.h
index db0d5b8..eb12f37 100644
--- a/urcu/static/wfstack.h
+++ b/urcu/static/wfstack.h
@@ -77,6 +77,14 @@ void _cds_wfs_node_init(struct cds_wfs_node *node)
 }
 
 /*
+ * __cds_wfs_init: initialize wait-free stack.
+ */
+static inline void ___cds_wfs_init(struct __cds_wfs_stack *s)
+{
+	s->head = CDS_WFS_END;
+}
+
+/*
  * cds_wfs_init: initialize wait-free stack.
  */
 static inline
@@ -99,8 +107,10 @@ static inline bool ___cds_wfs_end(void *node)
  *
  * No memory barrier is issued. No mutual exclusion is required.
  */
-static inline bool _cds_wfs_empty(struct cds_wfs_stack *s)
+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));
 }
 
@@ -114,8 +124,9 @@ static inline bool _cds_wfs_empty(struct cds_wfs_stack *s)
  * Returns non-zero otherwise.
  */
 static inline
-int _cds_wfs_push(struct cds_wfs_stack *s, struct cds_wfs_node *node)
+int _cds_wfs_push(cds_wfs_stack_ptr_t u_stack, struct cds_wfs_node *node)
 {
+	struct __cds_wfs_stack *s = u_stack._s;
 	struct cds_wfs_head *old_head, *new_head;
 
 	assert(node->next == NULL);
@@ -269,8 +280,9 @@ ___cds_wfs_pop_nonblocking(struct cds_wfs_stack *s)
  */
 static inline
 struct cds_wfs_head *
-___cds_wfs_pop_all(struct cds_wfs_stack *s)
+___cds_wfs_pop_all(cds_wfs_stack_ptr_t u_stack)
 {
+	struct __cds_wfs_stack *s = u_stack._s;
 	struct cds_wfs_head *head;
 
 	/*
diff --git a/urcu/wfstack.h b/urcu/wfstack.h
index fc0b44b..b914f41 100644
--- a/urcu/wfstack.h
+++ b/urcu/wfstack.h
@@ -83,11 +83,25 @@ struct cds_wfs_head {
 	struct cds_wfs_node node;
 };
 
+struct __cds_wfs_stack {
+	struct cds_wfs_head *head;
+};
+
 struct cds_wfs_stack {
 	struct cds_wfs_head *head;
 	pthread_mutex_t lock;
 };
 
+/*
+ * The transparent union allows calling functions that work on both
+ * struct cds_wfs_stack and struct __cds_wfs_stack on any of those two
+ * types.
+ */
+typedef union __attribute__((__transparent_union__)) {
+	struct __cds_wfs_stack *_s;
+	struct cds_wfs_stack *s;
+} cds_wfs_stack_ptr_t;
+
 #ifdef _LGPL_SOURCE
 
 #include <urcu/static/wfstack.h>
@@ -136,11 +150,16 @@ extern void cds_wfs_node_init(struct cds_wfs_node *node);
 extern void cds_wfs_init(struct cds_wfs_stack *s);
 
 /*
+ * __cds_wfs_init: initialize wait-free stack.
+ */
+extern void __cds_wfs_init(struct __cds_wfs_stack *s);
+
+/*
  * cds_wfs_empty: return whether wait-free stack is empty.
  *
  * No memory barrier is issued. No mutual exclusion is required.
  */
-extern bool cds_wfs_empty(struct cds_wfs_stack *s);
+extern bool cds_wfs_empty(cds_wfs_stack_ptr_t u_stack);
 
 /*
  * cds_wfs_push: push a node into the stack.
@@ -151,7 +170,7 @@ extern bool cds_wfs_empty(struct cds_wfs_stack *s);
  * Returns 0 if the stack was empty prior to adding the node.
  * Returns non-zero otherwise.
  */
-extern int cds_wfs_push(struct cds_wfs_stack *s, struct cds_wfs_node *node);
+extern int cds_wfs_push(cds_wfs_stack_ptr_t u_stack, struct cds_wfs_node *node);
 
 /*
  * cds_wfs_pop_blocking: pop a node from the stack.
diff --git a/wfstack.c b/wfstack.c
index c8bd7e6..fec9e47 100644
--- a/wfstack.c
+++ b/wfstack.c
@@ -38,6 +38,11 @@ void cds_wfs_init(struct cds_wfs_stack *s)
 	_cds_wfs_init(s);
 }
 
+void __cds_wfs_init(struct __cds_wfs_stack *s)
+{
+	___cds_wfs_init(s);
+}
+
 bool cds_wfs_empty(struct cds_wfs_stack *s)
 {
 	return _cds_wfs_empty(s);
@@ -112,7 +117,7 @@ struct cds_wfs_node *
 	return ___cds_wfs_pop_with_state_nonblocking(s, state);
 }
 
-struct cds_wfs_head *__cds_wfs_pop_all(struct cds_wfs_stack *s)
+struct cds_wfs_head *__cds_wfs_pop_all(cds_wfs_stack_ptr_t u_stack)
 {
-	return ___cds_wfs_pop_all(s);
+	return ___cds_wfs_pop_all(u_stack);
 }
-- 
EW