Re: [Qemu-devel] [PATCH] Disk image shared and exclusive locks.

[Kevin Wolf]

Am 07.12.2009 15:53, schrieb Anthony Liguori:
> Daniel P. Berrange wrote:
>> It is safe if you assume that no one else has tried to modify the disk
>> since you opened it, otherwise you'd be commiting changes against a 
>> base state which no longer exists.
> 
> 1) first user opens cow1.qcow, acquires F_WRLCK
> 2) first user opens base.qcow, acquires F_RDLCK
> 3) second user opens cow2.qcow, acquires F_WRLCK
> 4) second user opens base.qcow, acquires F_RDLCK
> 
> 5) second user attempts to commit cow2.qcow to base.qcow

This is broken regardless of locking (except for very... special use
that I don't want to even think about, I'd call it criminal). Better
don't commit to a backing file that is used by other COW images.

Kevin