Add the Seq module, a simple seqlock implementation.

This allows fast reader-writer concurrency in cases where writers are rare.  The
immediate use case is for the hooking implementaiton.

test/unit/seq.c

@@ -0,0 +1,95 @@
+#include "test/jemalloc_test.h"
+
+#include "jemalloc/internal/seq.h"
+
+typedef struct data_s data_t;
+struct data_s {
+	int arr[10];
+};
+
+static void
+set_data(data_t *data, int num) {
+	for (int i = 0; i < 10; i++) {
+		data->arr[i] = num;
+	}
+}
+
+static void
+assert_data(data_t *data) {
+	int num = data->arr[0];
+	for (int i = 0; i < 10; i++) {
+		assert_d_eq(num, data->arr[i], "Data consistency error");
+	}
+}
+
+seq_define(data_t, data)
+
+typedef struct thd_data_s thd_data_t;
+struct thd_data_s {
+	seq_data_t data;
+};
+
+static void *
+seq_reader_thd(void *arg) {
+	thd_data_t *thd_data = (thd_data_t *)arg;
+	int iter = 0;
+	data_t local_data;
+	while (iter < 1000 * 1000 - 1) {
+		bool success = seq_try_load_data(&local_data, &thd_data->data);
+		if (success) {
+			assert_data(&local_data);
+			assert_d_le(iter, local_data.arr[0],
+			    "Seq read went back in time.");
+			iter = local_data.arr[0];
+		}
+	}
+	return NULL;
+}
+
+static void *
+seq_writer_thd(void *arg) {
+	thd_data_t *thd_data = (thd_data_t *)arg;
+	data_t local_data;
+	memset(&local_data, 0, sizeof(local_data));
+	for (int i = 0; i < 1000 * 1000; i++) {
+		set_data(&local_data, i);
+		seq_store_data(&thd_data->data, &local_data);
+	}
+	return NULL;
+}
+
+TEST_BEGIN(test_seq_threaded) {
+	thd_data_t thd_data;
+	memset(&thd_data, 0, sizeof(thd_data));
+
+	thd_t reader;
+	thd_t writer;
+
+	thd_create(&reader, seq_reader_thd, &thd_data);
+	thd_create(&writer, seq_writer_thd, &thd_data);
+
+	thd_join(reader, NULL);
+	thd_join(writer, NULL);
+}
+TEST_END
+
+TEST_BEGIN(test_seq_simple) {
+	data_t data;
+	seq_data_t seq;
+	memset(&seq, 0, sizeof(seq));
+	for (int i = 0; i < 1000 * 1000; i++) {
+		set_data(&data, i);
+		seq_store_data(&seq, &data);
+		set_data(&data, 0);
+		bool success = seq_try_load_data(&data, &seq);
+		assert_b_eq(success, true, "Failed non-racing read");
+		assert_data(&data);
+	}
+}
+TEST_END
+
+int main(void) {
+	return test_no_reentrancy(
+	    test_seq_simple,
+	    test_seq_threaded);
+}