Re: [Qemu-devel] [PATCH] qga: ignore EBUSY when freezing a filesystem

[Peter Lieven]

Am 03.02.2017 um 09:22 schrieb Christian Theune:
> Hi,
>
>> On 3 Feb 2017, at 09:20, Peter Lieven <address@hidden 
>> <mailto:address@hidden>> wrote:
>>
>> If you try to THAW an unfrozen FS you get EINVAL.
>>
>> The current code thaws until an error is returned.
>>
>>
>> So it should work as is.
>>
>>
>> If you feel uncomfortable with the EBUSY approach. The other idea would
>>
>> be to track all devices which have been successfully frozen and skip 
>> consecutive
>>
>> tries to freeze them.
>
> Which will be subject to race conditions as other processes may call 
> freeze/thaw on any device. Those are completely out of our control.

That will always be out of control. If we freeze the FS and another process 
thaws it, we snapshot an unfrozen fs and believe it is frozen.

I think the EBUSY approach is fine. If I look at the documentation of the thaw 
in the code it seems that the behaviour of the linux kernel changed. It seems
that it was possible to call FIFREEZE multiple times on an FS and this required 
multiple FITHAWs to unfreeze it.

Peter