This corresponds to the NT syscall NtReleaseMutant().

This syscall decrements the mutex's recursion count by one, and returns the
previous value. If the mutex is not owned by the given owner ID, the function
instead fails and returns -EPERM.

Signed-off-by: Elizabeth Figura <zfigura@codeweavers.com>
---
 drivers/misc/ntsync.c       | 64 +++++++++++++++++++++++++++++++++++++
 include/uapi/linux/ntsync.h |  1 +
 2 files changed, 65 insertions(+)

diff --git a/drivers/misc/ntsync.c b/drivers/misc/ntsync.c
index 6f7086d0440a..222ebead8eba 100644
--- a/drivers/misc/ntsync.c
+++ b/drivers/misc/ntsync.c
@@ -312,6 +312,68 @@ static int ntsync_sem_post(struct ntsync_obj *sem, void __user *argp)
 	return ret;
 }
 
+/*
+ * Actually change the mutex state, returning -EPERM if not the owner.
+ */
+static int unlock_mutex_state(struct ntsync_obj *mutex,
+			      const struct ntsync_mutex_args *args)
+{
+	lockdep_assert_held(&mutex->lock);
+
+	if (mutex->u.mutex.owner != args->owner)
+		return -EPERM;
+
+	if (!--mutex->u.mutex.count)
+		mutex->u.mutex.owner = 0;
+	return 0;
+}
+
+static int ntsync_mutex_unlock(struct ntsync_obj *mutex, void __user *argp)
+{
+	struct ntsync_mutex_args __user *user_args = argp;
+	struct ntsync_device *dev = mutex->dev;
+	struct ntsync_mutex_args args;
+	__u32 prev_count;
+	int ret;
+
+	if (copy_from_user(&args, argp, sizeof(args)))
+		return -EFAULT;
+	if (!args.owner)
+		return -EINVAL;
+
+	if (mutex->type != NTSYNC_TYPE_MUTEX)
+		return -EINVAL;
+
+	if (atomic_read(&mutex->all_hint) > 0) {
+		spin_lock(&dev->wait_all_lock);
+		spin_lock_nest_lock(&mutex->lock, &dev->wait_all_lock);
+
+		prev_count = mutex->u.mutex.count;
+		ret = unlock_mutex_state(mutex, &args);
+		if (!ret) {
+			try_wake_all_obj(dev, mutex);
+			try_wake_any_mutex(mutex);
+		}
+
+		spin_unlock(&mutex->lock);
+		spin_unlock(&dev->wait_all_lock);
+	} else {
+		spin_lock(&mutex->lock);
+
+		prev_count = mutex->u.mutex.count;
+		ret = unlock_mutex_state(mutex, &args);
+		if (!ret)
+			try_wake_any_mutex(mutex);
+
+		spin_unlock(&mutex->lock);
+	}
+
+	if (!ret && put_user(prev_count, &user_args->count))
+		ret = -EFAULT;
+
+	return ret;
+}
+
 static int ntsync_obj_release(struct inode *inode, struct file *file)
 {
 	struct ntsync_obj *obj = file->private_data;
@@ -331,6 +393,8 @@ static long ntsync_obj_ioctl(struct file *file, unsigned int cmd,
 	switch (cmd) {
 	case NTSYNC_IOC_SEM_POST:
 		return ntsync_sem_post(obj, argp);
+	case NTSYNC_IOC_MUTEX_UNLOCK:
+		return ntsync_mutex_unlock(obj, argp);
 	default:
 		return -ENOIOCTLCMD;
 	}
diff --git a/include/uapi/linux/ntsync.h b/include/uapi/linux/ntsync.h
index 8ac9d419c360..265503d441b1 100644
--- a/include/uapi/linux/ntsync.h
+++ b/include/uapi/linux/ntsync.h
@@ -39,5 +39,6 @@ struct ntsync_wait_args {
 #define NTSYNC_IOC_CREATE_MUTEX		_IOWR('N', 0x84, struct ntsync_sem_args)
 
 #define NTSYNC_IOC_SEM_POST		_IOWR('N', 0x81, __u32)
+#define NTSYNC_IOC_MUTEX_UNLOCK		_IOWR('N', 0x85, struct ntsync_mutex_args)
 
 #endif
-- 
2.43.0
