[PATCH v4 07/13] lib: test_hmm add ioctl to get zone device type

[Andrew Morton]

On Tue, 31 May 2022 10:56:23 -0500 Alex Sierra <alex.sierra at amd.com> wrote:

> new ioctl cmd added to query zone device type. This will be
> used once the test_hmm adds zone device coherent type.
> 
> @@ -1026,6 +1027,15 @@ static int dmirror_snapshot(struct dmirror *dmirror,
>  	return ret;
>  }
>  
> +static int dmirror_get_device_type(struct dmirror *dmirror,
> +			    struct hmm_dmirror_cmd *cmd)
> +{
> +	mutex_lock(&dmirror->mutex);
> +	cmd->zone_device_type = dmirror->mdevice->zone_device_type;
> +	mutex_unlock(&dmirror->mutex);

What does the locking here do?

Presumably cmd->zone_device_type can become out of date the instant the
mutex is released, so what was the point in taking the mutex?

And does it make sense to return potentially out-of-date info to
userspace?  Perhaps this interface simply shouldn't exist?