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/*
* File : lock_free_queue.c
* Author : Jérémy Zurcher <jeremy@asynk.ch>
* Date : 2009/11/01
* License : stolen from http://www.cs.rochester.edu/~scott/papers/1996_PODC_queues.pdf
*/
#include "stdlib.h"
#include "lock_free_queue.h"
#ifdef DEBUG
#include "stdio.h"
#endif
/* CMPXCHG8B m64 Compare EDX:EAX with m64. If equal, set ZF and load ECX:EBX into m64. Else, clear ZF and load m64 into EDX:EAX. */
static inline unsigned int compare_and_swap(volatile unsigned long long *mem,
volatile unsigned long long old,
volatile unsigned long long new)
{
char result;
__asm__ __volatile__("lock; cmpxchg8b %0; setz %1;"
: "=m"(*mem), "=q"(result)
: "m"(*mem), "d" ((unsigned long)(old>>32)), "a" ((unsigned long)old)
, "c" ((unsigned long)(new>>32)), "b" ((unsigned long)new)
: "memory");
return (int)result;
}
void init(lfq_t *q)
{
node_t *node = (node_t*)malloc(sizeof(node_t));
node->next.split.ptr = NULL;
node->next.split.count = 0;
q->head.split.ptr = q->tail.split.ptr = node;
q->head.split.count = q->tail.split.count = 0;
}
void enqueue(lfq_t *q, void *data)
{
node_t *node;
pointer_t tail;
pointer_t next;
pointer_t tmp;
node = (node_t*)malloc(sizeof(node_t));
node->data = data;
node->next.split.ptr = NULL;
/* node->next.split.count = 0; */
for(;;)
{
tail.concat = q->tail.concat; /* copy tail pointer */
next.concat = tail.split.ptr->next.concat; /* copy next pointer */
if (tail.concat == q->tail.concat)
{ /* tail is still consistent */
if (next.split.ptr == NULL)
{ /* next is still the last node */
/* if tail->next is the same as next, link node at the end of the list */
tmp.split.ptr = node;
tmp.split.count = next.split.count+1;
if ( compare_and_swap( &tail.split.ptr->next.concat, next.concat, tmp.concat ) ) break;
}
else
{
/* try to swing tail to the next node, if q-> tail is still tail => next is ok */
tmp.split.ptr = next.split.ptr;
tmp.split.count = tail.split.count+1;
compare_and_swap( &q->tail.concat, tail.concat, tmp.concat );
}
}
}
/* try to swing tail to the next node, may have been done by a concurrent push */
tmp.split.ptr = node;
tmp.split.count = tail.split.count+1;
compare_and_swap( &q->tail.concat, tail.concat, tmp.concat );
}
void* dequeue(lfq_t *q)
{
void *data;
pointer_t head;
pointer_t tail;
node_t *next;
pointer_t tmp;
for(;;)
{
head.concat = q->head.concat;
tail.concat = q->tail.concat;
next = (node_t *)head.split.ptr;
if (head.concat == q->head.concat)
{
if (head.split.ptr == tail.split.ptr)
{ /* still consistent */
if (next->next.split.ptr == NULL)
{ /* queue empty */
return NULL;
}
/* new node has been linked, but tail is behind, should advance it */
tmp.split.ptr = next->next.split.ptr;
tmp.split.count = tail.split.count+1;
compare_and_swap( &q->tail.concat, tail.concat, tmp.concat );
}
else
{
data = next->data;
/* try to swing head to the next node */
tmp.split.ptr = next->next.split.ptr;
tmp.split.count = head.split.count+1;
if( compare_and_swap( &q->head.concat, head.concat, tmp.concat ) ) break;
}
}
}
free((void*)head.split.ptr);
return data;
}
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