diff --git a/kernel/mars_client.c b/kernel/mars_client.c
index e9cf6d3a..92680549 100644
--- a/kernel/mars_client.c
+++ b/kernel/mars_client.c
@@ -238,17 +238,16 @@ static
 void _hash_insert(struct client_output *output, struct client_mref_aspect *mref_a)
 {
 	struct mref_object *mref = mref_a->object;
-	unsigned long flags;
 	int hash_index;
 
-	traced_lock(&output->lock, flags);
+	mutex_lock(&output->mutex);
 	list_del(&mref_a->io_head);
 	list_add_tail(&mref_a->io_head, &output->mref_list);
 	list_del(&mref_a->hash_head);
 	mref->ref_id = ++output->last_id;
 	hash_index = mref->ref_id % CLIENT_HASH_MAX;
 	list_add_tail(&mref_a->hash_head, &output->hash_table[hash_index]);
-	traced_unlock(&output->lock, flags);
+	mutex_unlock(&output->mutex);
 }
 
 static void client_ref_io(struct client_output *output, struct mref_object *mref)
@@ -300,7 +299,6 @@ int receiver_thread(void *data)
 		struct list_head *tmp;
 		struct client_mref_aspect *mref_a = NULL;
 		struct mref_object *mref = NULL;
-		unsigned long flags;
 
 		if (output->recv_error) {
 			/* The protocol may be out of sync.
@@ -333,13 +331,13 @@ int receiver_thread(void *data)
 		{
 			int hash_index = cmd.cmd_int1 % CLIENT_HASH_MAX;
 
-			traced_lock(&output->lock, flags);
+			mutex_lock(&output->mutex);
 			for (tmp = output->hash_table[hash_index].next; tmp != &output->hash_table[hash_index]; tmp = tmp->next) {
 				struct mref_object *tmp_mref;
 				mref_a = container_of(tmp, struct client_mref_aspect, hash_head);
 				tmp_mref = mref_a->object;
 				if (unlikely(!tmp_mref)) {
-					traced_unlock(&output->lock, flags);
+					mutex_unlock(&output->mutex);
 					MARS_ERR("bad internal mref pointer\n");
 					status = -EBADR;
 					goto done;
@@ -351,7 +349,7 @@ int receiver_thread(void *data)
 					break;
 				}
 			}
-			traced_unlock(&output->lock, flags);
+			mutex_unlock(&output->mutex);
 
 			if (unlikely(!mref)) {
 				MARS_WRN("got unknown id = %d for callback\n", cmd.cmd_int1);
@@ -415,9 +413,7 @@ int receiver_thread(void *data)
 static
 void _do_resubmit(struct client_output *output)
 {
-	unsigned long flags;
-
-	traced_lock(&output->lock, flags);
+	mutex_lock(&output->mutex);
 	if (!list_empty(&output->wait_list)) {
 		struct list_head *first = output->wait_list.next;
 		struct list_head *last = output->wait_list.prev;
@@ -428,7 +424,7 @@ void _do_resubmit(struct client_output *output)
 		INIT_LIST_HEAD(&output->wait_list);
 		MARS_IO("done re-submit %p %p\n", first, last);
 	}
-	traced_unlock(&output->lock, flags);
+	mutex_unlock(&output->mutex);
 }
 
 static
@@ -440,7 +436,6 @@ void _do_timeout(struct client_output *output, struct list_head *anchor, bool fo
 	LIST_HEAD(tmp_list);
 	int rounds = 0;
 	long io_timeout = brick->power.io_timeout;
-	unsigned long flags;
 
 	if (io_timeout <= 0)
 		io_timeout = global_net_io_timeout;
@@ -453,7 +448,7 @@ void _do_timeout(struct client_output *output, struct list_head *anchor, bool fo
 	
 	io_timeout *= HZ;
 	
-	traced_lock(&output->lock, flags);
+	mutex_lock(&output->mutex);
 	for (tmp = anchor->next, next = tmp->next; tmp != anchor; tmp = next, next = tmp->next) {
 		struct client_mref_aspect *mref_a;
 
@@ -468,7 +463,7 @@ void _do_timeout(struct client_output *output, struct list_head *anchor, bool fo
 		list_del_init(&mref_a->io_head);
 		list_add_tail(&mref_a->tmp_head, &tmp_list);
 	}
-	traced_unlock(&output->lock, flags);
+	mutex_unlock(&output->mutex);
 
 	while (!list_empty(&tmp_list)) {
 		struct client_mref_aspect *mref_a;
@@ -501,7 +496,6 @@ static int sender_thread(void *data)
 {
 	struct client_output *output = data;
 	struct client_brick *brick = output->brick;
-	unsigned long flags;
 	bool do_kill = false;
 	int status = 0;
 
@@ -569,16 +563,16 @@ static int sender_thread(void *data)
 
 		/* Grab the next mref from the queue
 		 */
-		traced_lock(&output->lock, flags);
+		mutex_lock(&output->mutex);
 		if (list_empty(&output->mref_list)) {
-			traced_unlock(&output->lock, flags);
+			mutex_unlock(&output->mutex);
 			continue;
 		}
 		tmp = output->mref_list.next;
 		list_del(tmp);
 		list_add(tmp, &output->wait_list);
 		mref_a = container_of(tmp, struct client_mref_aspect, io_head);
-		traced_unlock(&output->lock, flags);
+		mutex_unlock(&output->mutex);
 
 		mref = mref_a->object;
 
@@ -739,7 +733,7 @@ static int client_output_construct(struct client_output *output)
 	for (i = 0; i < CLIENT_HASH_MAX; i++) {
 		INIT_LIST_HEAD(&output->hash_table[i]);
 	}
-	spin_lock_init(&output->lock);
+	mutex_init(&output->mutex);
 	INIT_LIST_HEAD(&output->mref_list);
 	INIT_LIST_HEAD(&output->wait_list);
 	init_waitqueue_head(&output->event);
diff --git a/kernel/mars_client.h b/kernel/mars_client.h
index f821905f..1b1365d1 100644
--- a/kernel/mars_client.h
+++ b/kernel/mars_client.h
@@ -64,7 +64,7 @@ struct client_output {
 	MARS_OUTPUT(client);
 	atomic_t fly_count;
 	atomic_t timeout_count;
-	spinlock_t lock;
+	struct mutex mutex;
 	struct list_head mref_list;
 	struct list_head wait_list;
 	wait_queue_head_t event;