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

On Tue, 31 May 2022 10:56:23 -0500 Alex Sierra alex.sierra@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zone_device_type can become out of date the instant the mutex is released, so what was the point in taking the mutex?

And does it make sense to return potentially out-of-date info to userspace?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?