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Date: Wed, 18 Jan 2017 18:48:25 +0800
From: Fam Zheng <famz@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 13/14] raw-posix: Implement image locking
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To: Max Reitz <mreitz@redhat.com>
Cc: qemu-devel@nongnu.org, "Daniel P. Berrange" <berrange@redhat.com>, Kevin Wolf <kwolf@redhat.com>, qemu-block@nongnu.org, rjones@redhat.com

On Fri, 12/02 03:58, Max Reitz wrote:
> On 31.10.2016 16:38, Fam Zheng wrote:
> > This implements open flag sensible image locking for local file
> > and host device protocol.
> > 
> > virtlockd in libvirt locks the first byte, so we start looking at the
> > file bytes from 1.
> > 
> > Quoting what was proposed by Kevin Wolf <kwolf@redhat.com>, there are
> > four locking modes by combining two bits (BDRV_O_RDWR and
> > BDRV_O_SHARE_RW), and implemented by taking two locks:
> > 
> > Lock bytes:
> > 
> > * byte 1: I can't allow other processes to write to the image
> > * byte 2: I am writing to the image
> > 
> > Lock modes:
> > 
> > * shared writer (BDRV_O_RDWR | BDRV_O_SHARE_RW): Take shared lock on
> >   byte 2. Test whether byte 1 is locked using an exclusive lock, and
> >   fail if so.
> 
> Testing whether something is locked would be easier by using F_OFD_GETLK
> instead of actually creating an exclusive lock and then releasing it.

My attempt to do this shows it doesn't work: fcntl forces the tested lock type
to read lock for some unknown reason, so it cannot be used to test other shared
lockers.