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

[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, tries to 
acquire F_WRLCK.  Calling F_SETLK with F_WRLCK will drop the F_RDLCK and 
upgrade to F_WRLCK.
6) F_SETLK fails because another process is holding F_RDLCK, error is 
printed to user

Now assume that there is only one user and commit succeeded, then we'd 
drop back down to F_RDLCK (after an fsync) which means that afterwards, 
people could open up the image again.

Regards,

Anthony Liguori