[linux-lvm] libdm cannot swap names between two child volumes

[Zdenek Kabelac]

Dne 8.6.2015 v 04:09 M.H. Tsai napsal(a):
> 2015-06-05 16:04 GMT+08:00 Zdenek Kabelac <zkabelac at redhat.com>:
>> The problem with rename is -
>>
>> you have device   'lv1'  you rename it to 'lv2' - yet
>> those who opened device with the name 'lv1' still thinks
>> the 'lv1' device exits.
>>
>> So for safety reason before you 'reuse' any existing name in-use,
>> there should be 'deactivating' such device first - so there is no 'race' in
>> name usage.
>>
>> It's even possible we miss to track full history of active renamed device.
>>
>> Since you get into strange scenarios when you start to count
>> with udev event handling and link generating here - it's getting nearly
>> impossible to synchronize this properly...
>
> Does that mean, if I can confirm that there's no program using the device name,

Hi

It doesn't really matter here what you could confirm here - there is a race 
you can't avoid - i.e. udev is completely 'independent' and may execute
trigger udev rules at any random point in time or some other command may try 
to open device in parallel (i.e. 'dd')

So the only way how to ensure there is no such race - is to deactivate such 
device (which should be possible - since as you said - noone has it open)

Also remember - activation routine is 'separate' from command code - as it 
could run on a completely different node - so you cannot 'validate' from 
command code there is no user of a device on 'activation' node unless device 
is locally active.

> then it's safe to rename an active device? The devices I want to rename are
> internal volumes. I think that there's no user space program using these names,
> except LVM.

IMHO there is no point to 'optimize' this process - I do not expect anyone is 
doing million swaps of internal LVs in a second.

Thus going through the proper sequence of steps and allowing udev to properly 
synchronize (i.e. you should not 'mix'  activation & deactivation under same 
cookie) is clearly the best way how to achieve your desired goal.

Regards

Zdenek