#define _LGPL_SOURCE
#include <stdlib.h>
+#include <errno.h>
+#include <assert.h>
+#include <stdio.h>
+
#include <urcu.h>
+#include <urcu-defer.h>
#include <arch.h>
#include <arch_atomic.h>
-#include <assert.h>
#include <compiler.h>
-#include <urcu-defer.h>
-#include <errno.h>
-#include <urcu-ht.h>
#include <urcu/jhash.h>
#include <stdio.h>
#include <pthread.h>
+#include <urcu-ht.h>
+
+/* node flags */
+#define NODE_STOLEN (1 << 0)
struct rcu_ht_node;
struct rcu_ht_node *next;
void *key;
void *data;
+ unsigned int flags;
};
struct rcu_ht {
new_head = calloc(1, sizeof(struct rcu_ht_node));
new_head->key = key;
new_head->data = data;
+ new_head->flags = 0;
/* here comes the fun and tricky part.
* Add at the beginning with a cmpxchg.
* Hold a read lock between the moment the first element is read
/*
* Restart until we successfully remove the entry, or no entry is left
* ((void *)(unsigned long)-ENOENT).
- * Deal with concurrent stealers by verifying that there are no element
- * in the list still pointing to the element stolen. (del_node)
+ * Deal with concurrent stealers by doing an extra verification pass to check
+ * that no element in the list are still pointing to the element stolen.
+ * This could happen if two concurrent steal for consecutive objects are
+ * executed. A pointer to an object being stolen could be saved by the
+ * concurrent stealer for the previous object.
+ * Also, given that in this precise scenario, another stealer can also want to
+ * delete the doubly-referenced object; use a "stolen" flag to let only one
+ * stealer delete the object.
*/
void *ht_steal(struct rcu_ht *ht, void *key)
{
if (del_node) {
goto end;
} else {
- data = (void *)(unsigned long)-ENOENT;
goto error;
}
}
prev = &node->next;
node = rcu_dereference(*prev);
}
+
+ if (!del_node) {
+ /*
+ * Another concurrent thread stole it ? If so, let it deal with
+ * this. Assume NODE_STOLEN is the only flag. If this changes,
+ * read flags before cmpxchg.
+ */
+ if (cmpxchg(&node->flags, 0, NODE_STOLEN) != 0)
+ goto error;
+ }
+
/* Found it ! pointer to object is in "prev" */
- if (rcu_cmpxchg_pointer(prev, node, node->next) != node)
+ if (rcu_cmpxchg_pointer(prev, node, node->next) == node)
del_node = node;
goto restart;
*/
rcu_read_unlock();
- data = node->data;
- call_rcu(free, node);
+ data = del_node->data;
+ call_rcu(free, del_node);
return data;
error:
+ data = (void *)(unsigned long)-ENOENT;
rcu_read_unlock();
return data;