[PATCH V2 1/2] nvme-core: use xarray for ctrl ns tracking

[Chaitanya Kulkarni]

On 7/1/20 11:29 AM, Keith Busch wrote:
> On Wed, Jul 01, 2020 at 06:19:50PM +0000, Chaitanya Kulkarni wrote:
>> On 7/1/20 6:12 AM, Christoph Hellwig wrote:
>>> [willy: a comment/request on the xa_load API below]
>>> On Tue, Jun 30, 2020 at 07:25:16PM -0700, Chaitanya Kulkarni wrote:
>>>> +	if (le32_to_cpu(id->nn) > 1) {
>>>> +		dev_warn(ctrl->device,
>>>> +		"NVME_IOCTL_IO_CMD not supported when multiple namespaces present!\n");
>>>> +		goto out;
>>>>    	}
>>> This code doesn't make any sense at all.  Why does a patch changing
>>> data structures add new calls that go out on the wire?
>>>
>> Yes, this should not be here, I'll remove that and only keep the code to
>> check multiple namespaces and if needed this needs to be a separate
>> patch.
> This isn't actually checking for multiple namespaces, though. It's just
> getting the highest nsid, which doesn't tell us how many namespaces the
> driver is bound to.
> 
hmmm, I'll try and fix it in a next version.
>>>> -	down_write(&ns->ctrl->namespaces_rwsem);
>>>> -	list_del_init(&ns->list);
>>>> -	up_write(&ns->ctrl->namespaces_rwsem);
>>>> +	xa_lock(xa);
>>>> +	__xa_erase(xa, ns->head->ns_id);
>>>> +	free = refcount_dec_and_test(&ns->kref.refcount) ? true : false;
>>>> +	xa_unlock(xa);
>>>>    
>>>>    	nvme_mpath_check_last_path(ns);
>>>> -	nvme_put_ns(ns);
>>>> +	if (free)
>>>> +		__nvme_free_ns(ns);
>>> This looks very strange to me.  Shoudn't this be a normal xa_erase
>>> followed by a normal nvme_put_ns?  For certain the driver code has
>>> no business poking into the kref internals.
>>>
>> There is a race when kref is manipulated in nvme_find_get_ns() and
>> nvme _ns_remove() pointed by Keith which needs ns->kref to be guarded
>> with locks. Let me know I'll share a detail scenario.

> That wasn't the race I was pointing out. In your earlier internal
> implementation, you had the xa_erase done after the final put, and I was
> warning against that. The erase needs to happen where you have it, but I
> don't see a need for protecting the kref like this.
> 

Okay I've misunderstood it and follow the same pattern here due to my 
previous implementation. Maybe we can get rid of it, let me see.