mcslock: Replace GCC atomic builtins with C11 (#21)

Author: zondatw

mcslock/mcslock.c

@@ -1,3 +1,4 @@
+#include <stdatomic.h>
 #include <stddef.h>
 
 #include "mcslock.h"
@@ -14,34 +15,37 @@ enum { MCS_PROCEED = 0, MCS_WAIT = 1 };
 #define spin_wait() ((void) 0)
 #endif
 
-static inline void wait_until_equal_u8(uint8_t *loc, uint8_t val, int mm)
+static inline void wait_until_equal_u8(_Atomic uint8_t *loc,
+                                       uint8_t val,
+                                       memory_order mm)
 {
-    while (__atomic_load_n(loc, mm) != val)
+    while (atomic_load_explicit(loc, mm) != val)
         spin_wait();
 }
 
 void mcslock_init(mcslock_t *lock)
 {
-    *lock = NULL;
+    atomic_init(lock, NULL);
 }
 
 void mcslock_acquire(mcslock_t *lock, mcsnode_t *node)
 {
-    node->next = NULL;
+    atomic_init(&node->next, NULL);
     /* A0: Read and write lock, synchronized with A0/A1 */
-    mcsnode_t *prev = __atomic_exchange_n(lock, node, __ATOMIC_ACQ_REL);
+    mcsnode_t *prev =
+        atomic_exchange_explicit(lock, node, memory_order_acq_rel);
     if (LIKELY(!prev)) /* Lock uncontended, the lock is acquired */
         return;
     /* Otherwise, the lock is owned by another thread, waiting for its turn */
 
-    node->wait = MCS_WAIT;
+    atomic_store_explicit(&node->wait, MCS_WAIT, memory_order_release);
     /* B0: Write next, synchronized with B1/B2 */
-    __atomic_store_n(&prev->next, node, __ATOMIC_RELEASE);
+    atomic_store_explicit(&prev->next, node, memory_order_release);
 
     /* Waiting for the previous thread to signal using the assigned node
      * C0: Read wait, synchronized with C1
      */
-    wait_until_equal_u8(&node->wait, MCS_PROCEED, __ATOMIC_ACQUIRE);
+    wait_until_equal_u8(&node->wait, MCS_PROCEED, memory_order_acquire);
 }
 
 void mcslock_release(mcslock_t *lock, mcsnode_t *node)
@@ -50,26 +54,28 @@ void mcslock_release(mcslock_t *lock, mcsnode_t *node)
 
     /* Check if any waiting thread exists */
     /* B1: Read next, synchronized with B0 */
-    if ((next = __atomic_load_n(&node->next, __ATOMIC_ACQUIRE)) == NULL) {
+    if ((next = atomic_load_explicit(&node->next, memory_order_acquire)) ==
+        NULL) {
         /* No waiting threads detected, attempt lock release */
         /* Use temporary variable as it might be overwritten */
         mcsnode_t *tmp = node;
 
         /* A1: write lock, synchronize with A0 */
-        if (__atomic_compare_exchange_n(lock, &tmp, NULL, 0, __ATOMIC_RELEASE,
-                                        __ATOMIC_RELAXED)) {
+        if (atomic_compare_exchange_strong_explicit(
+                lock, &tmp, NULL, memory_order_release, memory_order_relaxed)) {
             /* No waiting threads yet, lock released successfully */
             return;
         }
         /* Otherwise, at least one waiting thread exists */
 
         /* Wait for the first waiting thread to link its node with ours */
         /* B2: Read next, synchronized with B0 */
-        while ((next = __atomic_load_n(&node->next, __ATOMIC_ACQUIRE)) == NULL)
+        while ((next = atomic_load_explicit(&node->next,
+                                            memory_order_acquire)) == NULL)
             spin_wait();
     }
 
     /* Signal the first waiting thread */
     /* C1: Write wait, synchronized with C0 */
-    __atomic_store_n(&next->wait, MCS_PROCEED, __ATOMIC_RELEASE);
+    atomic_store_explicit(&next->wait, MCS_PROCEED, memory_order_release);
 }

mcslock/mcslock.h

@@ -1,13 +1,14 @@
 #pragma once
 
+#include <stdatomic.h>
 #include <stdint.h>
 
 typedef struct mcsnode {
-    struct mcsnode *next;
-    uint8_t wait;
+    _Atomic(struct mcsnode *) next;
+    _Atomic(uint8_t) wait;
 } mcsnode_t;
 
-typedef mcsnode_t *mcslock_t;
+typedef _Atomic(mcsnode_t*) mcslock_t;
 
 /* Initialize an MCS lock */
 void mcslock_init(mcslock_t *lock);