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Date: Tue, 12 Jan 2016 19:33:32 +0800
From: Fam Zheng <famz@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 1/5] block: added lock image option and
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To: Kevin Wolf <kwolf@redhat.com>
Cc: qemu-devel@nongnu.org, Max Reitz <mreitz@redhat.com>, Olga Krishtal <okrishtal@virtuozzo.com>, "Denis V. Lunev" <den@openvz.org>

On Tue, 01/12 11:10, Kevin Wolf wrote:
> 
> The problem is that libvirt already takes a lock, as Dan mentioned in
> another reply in this thread, so we can't enable locking in qemu by
> default. It would always fail when run under libvirt.
> 
> Unless I'm seriously mistaken, this means that flock() inside qemu is
> dead.

Yes, I see the problem with libvirt, but can we instead do these?

  1) Do a soft flock() in QEMU invocation. If it fails, sliently ignore.
  2) Do a hard flock() in qemu-img invocation. If it fails, report and exit.

This way, if libvirt is holding flock, we can assume libvirt is actually
"using" the image: 1) just works as before, but 2) will not break the qcow2.
That is still a slight improvement, and does solve the reckless "qemu-img
snapshot create" user's problem.

Fam