update

.gitignore

@@ -1 +1,2 @@
+test_queue
 test_queue.*

lock_free_queue.c

@@ -3,6 +3,23 @@
 #include <stdatomic.h>
 #include "lock_free_queue.h"
 
+#include <stdio.h>
+#include <stdarg.h>
+static int
+kind_of_atomic_fprintf(FILE *stream, const char *fmt, ...)
+{
+    char msg[256];
+
+    va_list args;
+    va_start(args, fmt);
+    int len = vsnprintf(msg, sizeof(msg), fmt, args);
+    va_end(args);
+
+    if (len < 0) return 0;
+
+    return fwrite(msg, 1, len, stream);
+}
+
 void
 lock_free_queue_init(struct lock_free_queue *q, 
                      void *arrptr, int arrlen,
@@ -12,7 +29,8 @@ lock_free_queue_init(struct lock_free_queue *q,
 
     q->head = 0;
     q->tail = 0;
-    q->temp = 0;
+    q->temp_head = 0;
+    q->temp_tail = 0;
     q->size = arrlen / cell;
     q->base = arrptr;
     q->cell = cell;
@@ -21,24 +39,35 @@ lock_free_queue_init(struct lock_free_queue *q,
 static void*
 item_addr(struct lock_free_queue *q, int index)
 {
-    assert(index >= 0 && index < q->size);
+    return q->base + q->cell * (index % q->size);
-    return q->base + q->cell * index;
 }
 
-static int
+static uint64_t
 acquire_push_location(struct lock_free_queue *q)
 {
-    int old_temp;
+    uint64_t old_temp_tail;
-    int new_temp;
+    uint64_t new_temp_tail;
+
     do {
-        old_temp = q->temp;
+
-        new_temp = (old_temp + 1) % q->size;
+        uint64_t old_temp_head;
-    } while (!atomic_compare_exchange_strong(&q->temp, &old_temp, new_temp));
+
-    return old_temp;
+        old_temp_tail = q->temp_tail;
+        old_temp_head = q->temp_head;
+
+        if (old_temp_head + q->size == old_temp_tail)
+            return UINT64_MAX; // Queue is full
+
+        assert(old_temp_head + q->size > old_temp_tail);
+
+        new_temp_tail = old_temp_tail + 1;
+    } while (!atomic_compare_exchange_weak(&q->temp_tail, &old_temp_tail, new_temp_tail));
+
+    return old_temp_tail;
 }
 
-static void 
+static void
-release_push_location(struct lock_free_queue *q, int index)
+release_push_location(struct lock_free_queue *q, uint64_t index)
 {
     // The current thread already inserted an
     // item at position "index", which comes
@@ -51,34 +80,83 @@ release_push_location(struct lock_free_queue *q, int index)
     // Before being able to move the tail over
     // this element, we need to wait for other
     // threads to do it.
+
     while (q->tail != index);
 
-    // At this point only this thread is allowed
+    q->tail++;
-    // to push the tail forward
+}
 
-    q->tail = (q->tail + 1) % q->size;
+bool
+lock_free_queue_try_push(struct lock_free_queue *q, void *src)
+{
+    uint64_t index = acquire_push_location(q);
+    if (index == UINT64_MAX) return false;
+
+    void *dst = item_addr(q, index);
+    memcpy(dst, src, q->cell);
+
+    //kind_of_atomic_fprintf(stdout, "push at %d\n", index % q->size);
+
+    release_push_location(q, index);
+    return true;
 }
 
 void
 lock_free_queue_push(struct lock_free_queue *q, void *src)
 {
-    int index = acquire_push_location(q);
+    while (!lock_free_queue_try_push(q, src));
+}
 
-    void *dst = item_addr(q, index);
+static uint64_t
+acquire_pop_location(struct lock_free_queue *q)
+{
+    uint64_t old_head;
+    uint64_t new_head;
+    do {
+        uint64_t old_tail;
+
+        // It's important to get head before tail
+        // or head may be incremented over tail before
+        // querying it.
+        old_head = q->head;
+        old_tail = q->tail;
+
+        if (old_head == old_tail)
+            return UINT64_MAX;
+
+        assert(old_tail > old_head);
+
+        new_head = old_head + 1;
+    } while (!atomic_compare_exchange_weak(&q->head, &old_head, new_head));
+    return old_head;
+}
+
+static void
+release_pop_location(struct lock_free_queue *q, uint64_t index)
+{
+    while (q->temp_head != index);
+
+    q->temp_head++;
+}
+
+bool
+lock_free_queue_try_pop(struct lock_free_queue *q, void *dst)
+{
+    uint64_t index = acquire_pop_location(q);
+    if (index == UINT64_MAX)
+        return false;
+
+    void *src = item_addr(q, index);
     memcpy(dst, src, q->cell);
 
-    release_push_location(q, index);
+    //kind_of_atomic_fprintf(stdout, "pop at %d\n", index % q->size);
+
+    release_pop_location(q, index);
+    return true;
 }
 
 void
 lock_free_queue_pop(struct lock_free_queue *q, void *dst)
 {
-    int old_head;
+    while (!lock_free_queue_try_pop(q, dst));
-    int new_head;
-    do {
-        old_head = q->head;
-        new_head = (old_head + 1) % q->size;
-        void *src = item_addr(q, old_head);
-        memcpy(dst, src, q->cell);
-    } while (!atomic_compare_exchange_strong(&q->head, &old_head, new_head));
 }

lock_free_queue.h

@@ -1,19 +1,33 @@
+#include <stdint.h>
+#include <stdbool.h>
+
+/*
+ * +--------------+-----------------+----------------+-------------------+-------------+
+ * | unused       | Pop in progress |     Items      | Push in progress  | Unused      |
+ * +--------------+-----------------+----------------+-------------------+-------------+
+ *                ^                 ^                ^                   ^
+ *                temp_head         head             tail                temp_tail
+ */
 
 struct lock_free_queue {
 
-    _Atomic(int) head;
+    _Atomic(uint64_t) head;
-    _Atomic(int) tail;
+    _Atomic(uint64_t) tail;
-    _Atomic(int) temp;
+    _Atomic(uint64_t) temp_head;
+    _Atomic(uint64_t) temp_tail;
 
     // The following aren't written to by
     // push and pop operations, therefore
     // don't need to be atomic.
-    int   size; // Capacity of the queue (item count)
+    uint64_t size; // Capacity of the queue (item count)
-    int   cell; // Size of a single item (in bytes)
+    uint64_t cell; // Size of a single item (in bytes)
     char *base; // Pointer to the first item
 };
+
 void lock_free_queue_init(struct lock_free_queue *q, void *arrptr, int arrlen, int item_size);
 void lock_free_queue_push(struct lock_free_queue *q, void *src);
 void lock_free_queue_pop(struct lock_free_queue *q, void *dst);
+bool lock_free_queue_try_push(struct lock_free_queue *q, void *src);
+bool lock_free_queue_try_pop(struct lock_free_queue *q, void *dst);
 
 #define lock_free_queue_INIT(q, arr) lock_free_queue_init(q, arr, sizeof(arr), sizeof((arr)[0]))

lock_free_queue_test.c

@@ -2,17 +2,15 @@
 #include <stdlib.h>
 #include <stdarg.h>
 #include "tinycthread.h"
-#include "tinycsem.h"
 #include "lock_free_queue.h"
 
 #define NUM_THREADS 30
 
+#define OPERATIONS_PER_WORKER 100
+
 struct lock_free_queue queue;
 
-struct tinycsem consumed;
+static int
-struct tinycsem produced;
-
-static int 
 kind_of_atomic_fprintf(FILE *stream, const char *fmt, ...)
 {
     char msg[256];
@@ -27,8 +25,6 @@ kind_of_atomic_fprintf(FILE *stream, const char *fmt, ...)
     return fwrite(msg, 1, len, stream);
 }
 
-#define OPERATIONS_PER_WORKER 10000
-
 int consumer(void *arg)
 {
     (void) arg;
@@ -37,19 +33,12 @@ int consumer(void *arg)
 
     for (int i = 0; i < OPERATIONS_PER_WORKER; i++) {
 
-        tinycsem_wait(&produced);
-
         int value;
         lock_free_queue_pop(&queue, &value);
 
         sum += value;
-
-        kind_of_atomic_fprintf(stderr, "Thread consumed %d (i=%d)\n", value, i);
-
-        tinycsem_signal(&consumed);
     }
 
-    kind_of_atomic_fprintf(stderr, "Consumer stopped (sum=%d)\n", sum);
     return sum;
 }
 
@@ -62,33 +51,24 @@ int producer(void *arg)
     int sum = 0;
     
     for (int i = 0; i < OPERATIONS_PER_WORKER; i++) {
-        tinycsem_wait(&consumed);
 
         int value = rand() % 10000;
 
         lock_free_queue_push(&queue, &value);
 
-        kind_of_atomic_fprintf(stderr, "Thread produced %d (i=%d)\n", value, i);
-
         sum += value;
-
-        tinycsem_signal(&produced);
     }
 
-    kind_of_atomic_fprintf(stderr, "Producer stopped (sum=%d)\n", sum);
     return sum;
 }
 
-#define MAX_ITEMS 100
+#define MAX_ITEMS 10
 
 int main()
 {
     int items[MAX_ITEMS];
     lock_free_queue_INIT(&queue, items);
 
-    tinycsem_init(&consumed, MAX_ITEMS, MAX_ITEMS);
-    tinycsem_init(&produced, 0,         MAX_ITEMS);
-
     thrd_t threads[NUM_THREADS];
     int created = 0;
     for (int i = 0; i < NUM_THREADS; i++) {
@@ -122,7 +102,5 @@ int main()
         produced_sum, consumed_sum,
         (produced_sum == consumed_sum) ? "OK" : "Don't match!!");
 
-    tinycsem_free(&consumed);
-    tinycsem_free(&produced);
     return 0;
 }

makefile

@@ -1,3 +1,3 @@
 
 all:
-	gcc lock_free_queue_test.c lock_free_queue.c tinycsem.c tinycthread.c -o test_queue -Wall -Wextra
+	gcc lock_free_queue_test.c lock_free_queue.c tinycthread.c -o test_queue -ggdb -Wall -Wextra #-fsanitize=thread

tinycsem.c

@@ -1,45 +0,0 @@
-#include "tinycsem.h"
-
-bool tinycsem_init(struct tinycsem *sem, int count, int max)
-{
-    #ifdef _WIN32
-    SECURITY_ATTRIBUTES *attr = NULL; // Default
-    const char          *name = NULL; // No name
-    HANDLE handle = CreateSemaphore(attr, count, max, name);
-    if (handle == NULL)
-        return false;
-    sem->data = handle;
-    return true;
-    #else
-    (void) max; // POSIX doesn't use this
-    return sem_init(&sem->data, 0, count) == 0;
-    #endif
-}
-
-bool tinycsem_free(struct tinycsem *sem)
-{
-    #ifdef _WIN32
-    CloseHandle(sem->data);
-    return true;
-    #else
-    return sem_destroy(&sem->data) == 0;
-    #endif
-}
-
-bool tinycsem_wait(struct tinycsem *sem)
-{
-    #ifdef _WIN32
-    return WaitForSingleObject(sem->data, INFINITE) == WAIT_OBJECT_0;
-    #else
-    return sem_wait(&sem->data) == 0;
-    #endif
-}
-
-bool tinycsem_signal(struct tinycsem *sem)
-{
-    #ifdef _WIN32
-    return ReleaseSemaphore(sem->data, 1, NULL);
-    #else
-    return sem_post(&sem->data) == 0;
-    #endif
-}

tinycsem.h

@@ -1,47 +0,0 @@
-
-#include <stddef.h>
-#include <stdbool.h>
-
-#ifdef _WIN32
-#include <windows.h>
-#else
-#include <semaphore.h>
-#endif
-
-/*
- * Semaphore structure
- */
-struct tinycsem {
-#ifdef _WIN32
-    HANDLE data;
-#else
-    sem_t  data;
-#endif
-};
-
-/*
- * Initialize a semaphore with a value of "count".
- *
- * The count of the semaphore must always be lower
- * than "max", else behaviour is undefined.
- */
-bool tinycsem_init(struct tinycsem *sem, int count, int max);
-
-/*
- * Releases the resources held by a semaphore previously
- * initialized with "tinycsem_init". 
- * 
- * After this function is used on a semaphore, you can't
- * reuse it unless you initialize it again.
- */
-bool tinycsem_free(struct tinycsem *sem);
-
-/*
- * The usual semaphore wait operation.
- */
-bool tinycsem_wait(struct tinycsem *sem);
-
-/*
- * The usual semaphore signal operation.
- */
-bool tinycsem_signal(struct tinycsem *sem);