add seqlock

[model-checker-benchmarks.git] / seqlock / seqlock.c

diff --git a/seqlock/seqlock.c b/seqlock/seqlock.c
new file mode 100644 (file)
index 0000000..3762374
--- /dev/null
+++ b/seqlock/seqlock.c
@@ -0,0 +1,75 @@
+#include <stdatomic.h>
+#include <threads.h>
+
+typedef struct seqlock {
+       // Sequence for reader consistency check
+       atomic_int _seq;
+       // It needs to be atomic to avoid data races
+       atomic_int _data;
+
+       seqlock() {
+               atomic_init(&_seq, 0);
+               atomic_init(&_data, 0);
+       }
+
+       int read() {
+               while (true) {
+                       int old_seq = _seq.load(memory_order_acquire); // acquire
+                       if (old_seq % 2 == 1) continue;
+
+                       int res = _data.load(memory_order_acquire); // acquire
+                       if (_seq.load(memory_order_relaxed) == old_seq) { // relaxed
+                               return res;
+                       }
+               }
+       }
+       
+       void write(int new_data) {
+               while (true) {
+                       // This might be a relaxed too
+                       int old_seq = _seq.load(memory_order_acquire); // acquire
+                       if (old_seq % 2 == 1)
+                               continue; // Retry
+
+                       // Should be relaxed!!! 
+                       if (_seq.compare_exchange_strong(old_seq, old_seq + 1,
+                               memory_order_relaxed, memory_order_relaxed)) // relaxed 
+                               break;
+               }
+
+               // Update the data
+               _data.store(new_data, memory_order_release); // release
+
+               _seq.fetch_add(1, memory_order_release); // release
+       }
+
+} seqlock_t;
+
+
+seqlock_t *lock;
+
+static void a(void *obj) {
+       lock->write(3);
+}
+
+static void b(void *obj) {
+       lock->write(2);
+}
+
+static void c(void *obj) {
+       int r1 = lock->read();
+}
+
+int user_main(int argc, char **argv) {
+       thrd_t t1, t2, t3, t4;
+       lock = new seqlock_t();
+
+       thrd_create(&t1, (thrd_start_t)&a, NULL);
+       thrd_create(&t2, (thrd_start_t)&b, NULL);
+       thrd_create(&t3, (thrd_start_t)&c, NULL);
+
+       thrd_join(t1);
+       thrd_join(t2);
+       thrd_join(t3);
+       return 0;
+}