[RFC PATCH 4/4] selftests/rseq: Implement rseq_mutex test program

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

Example use of the rseq sched state.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---
 tools/testing/selftests/rseq/.gitignore   |   1 +
 tools/testing/selftests/rseq/Makefile     |   3 +-
 tools/testing/selftests/rseq/rseq_mutex.c | 120 ++++++++++++++++++++++
 3 files changed, 123 insertions(+), 1 deletion(-)
 create mode 100644 tools/testing/selftests/rseq/rseq_mutex.c

diff --git a/tools/testing/selftests/rseq/.gitignore b/tools/testing/selftests/rseq/.gitignore
index a8db9f7a7cec..38d5b2fe5905 100644
--- a/tools/testing/selftests/rseq/.gitignore
+++ b/tools/testing/selftests/rseq/.gitignore
@@ -10,3 +10,4 @@ param_test_mm_cid
 param_test_mm_cid_benchmark
 param_test_mm_cid_compare_twice
 sched_state_test
+rseq_mutex
diff --git a/tools/testing/selftests/rseq/Makefile b/tools/testing/selftests/rseq/Makefile
index 7c8f4f2be74c..a9d7ceb5b79b 100644
--- a/tools/testing/selftests/rseq/Makefile
+++ b/tools/testing/selftests/rseq/Makefile
@@ -14,7 +14,8 @@ OVERRIDE_TARGETS = 1
 
 TEST_GEN_PROGS = basic_test basic_percpu_ops_test basic_percpu_ops_mm_cid_test param_test \
 		param_test_benchmark param_test_compare_twice param_test_mm_cid \
-		param_test_mm_cid_benchmark param_test_mm_cid_compare_twice sched_state_test
+		param_test_mm_cid_benchmark param_test_mm_cid_compare_twice sched_state_test \
+		rseq_mutex
 
 TEST_GEN_PROGS_EXTENDED = librseq.so
 
diff --git a/tools/testing/selftests/rseq/rseq_mutex.c b/tools/testing/selftests/rseq/rseq_mutex.c
new file mode 100644
index 000000000000..7e580fc6eebf
--- /dev/null
+++ b/tools/testing/selftests/rseq/rseq_mutex.c
@@ -0,0 +1,120 @@
+// SPDX-License-Identifier: LGPL-2.1
+
+#define _GNU_SOURCE
+#include <assert.h>
+#include <sched.h>
+#include <signal.h>
+#include <stdio.h>
+#include <string.h>
+#include <sys/time.h>
+#include <poll.h>
+
+#include "rseq.h"
+
+#define RSEQ_MUTEX_MAX_BUSY_LOOP	100
+
+struct rseq_mutex {
+	/*
+	 * NULL: unlocked. When non-NULL, owner points to per-thread rseq_abi
+	 * of owner thread.
+	 */
+	struct rseq_abi *owner;
+};
+
+static struct rseq_mutex lock = { .owner = NULL };
+
+static int testvar;
+
+static void rseq_lock_slowpath(struct rseq_mutex *lock)
+{
+	int i = 0;
+
+	for (;;) {
+		struct rseq_abi *expected = NULL, *self = rseq_get_abi();
+
+		if (__atomic_compare_exchange_n(&lock->owner, &expected, self, false,
+						    __ATOMIC_ACQUIRE, __ATOMIC_RELAXED))
+			break;
+		//TODO: use rseq critical section to protect dereference of owner thread's
+		//rseq_abi, combined with rseq fence at thread reclaim.
+		if ((RSEQ_ACCESS_ONCE(expected->sched_state) & RSEQ_SCHED_STATE_ON_CPU) &&
+		    i < RSEQ_MUTEX_MAX_BUSY_LOOP) {
+			rseq_barrier();			/* busy-wait, e.g. cpu_relax(). */
+			i++;
+		} else {
+			//TODO: Enqueue waiter in a wait-queue, and integrate
+			//with sys_futex rather than waiting for 10ms.
+			(void) poll(NULL, 0, 10);	/* wait 10ms */
+		}
+	}
+}
+
+static void rseq_lock(struct rseq_mutex *lock)
+{
+	struct rseq_abi *expected = NULL, *self = rseq_get_abi();
+
+	if (rseq_likely(__atomic_compare_exchange_n(&lock->owner, &expected, self, false,
+						    __ATOMIC_ACQUIRE, __ATOMIC_RELAXED)))
+		return;
+	rseq_lock_slowpath(lock);
+}
+
+static void rseq_unlock(struct rseq_mutex *lock)
+{
+	__atomic_store_n(&lock->owner, NULL, __ATOMIC_RELEASE);
+	//TODO: integrate with sys_futex and wakeup oldest waiter.
+}
+
+static
+void *test_thread(void *arg)
+{
+	int i;
+
+	if (rseq_register_current_thread()) {
+		fprintf(stderr, "Error: rseq_register_current_thread(...) failed(%d): %s\n",
+			errno, strerror(errno));
+		abort();
+	}
+
+	for (i = 0; i < 1000; i++) {
+		int var;
+
+		rseq_lock(&lock);
+		var = RSEQ_READ_ONCE(testvar);
+		if (var) {
+			fprintf(stderr, "Unexpected value %d\n", var);
+			abort();
+		}
+		RSEQ_WRITE_ONCE(testvar, 1);
+		if (!(i % 10))
+			(void) poll(NULL, 0, 10);
+		else
+			rseq_barrier();
+		RSEQ_WRITE_ONCE(testvar, 0);
+		rseq_unlock(&lock);
+	}
+
+	if (rseq_unregister_current_thread()) {
+		fprintf(stderr, "Error: rseq_unregister_current_thread(...) failed(%d): %s\n",
+			errno, strerror(errno));
+		abort();
+	}
+	return NULL;
+}
+
+int main(int argc, char **argv)
+{
+	int nr_threads = 5;
+	pthread_t test_thread_id[nr_threads];
+	int i;
+
+	for (i = 0; i < nr_threads; i++) {
+		pthread_create(&test_thread_id[i], NULL, test_thread, NULL);
+	}
+
+	for (i = 0; i < nr_threads; i++) {
+		pthread_join(test_thread_id[i], NULL);
+	}
+
+	return 0;
+}
-- 
2.25.1