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Date: Tue, 08 Dec 2009 10:40:15 +0100
From: Kevin Wolf <kwolf@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] Disk image shared and exclusive locks.
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: Avi Kivity <avi@redhat.com>, "Richard W.M. Jones" <rjones@redhat.com>, qemu-devel@nongnu.org

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