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Date: Thu, 14 Apr 2016 10:24:18 +0800
From: Fam Zheng <famz@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH for-2.7 05/15] raw-posix: Implement
 .bdrv_lockf
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To: "Daniel P. Berrange" <berrange@redhat.com>
Cc: qemu-devel@nongnu.org, Kevin Wolf <kwolf@redhat.com>, Max Reitz <mreitz@redhat.com>, Jeff Cody <jcody@redhat.com>, Markus Armbruster <armbru@redhat.com>, Eric Blake <eblake@redhat.com>, John Snow <jsnow@redhat.com>, qemu-block@nongnu.org, pbonzini@redhat.com, den@openvz.org

On Wed, 04/13 10:21, Daniel P. Berrange wrote:
> On Wed, Apr 13, 2016 at 05:09:54PM +0800, Fam Zheng wrote:
> > Because virtlockd in libvirt already uses the fcntl lock on the image file, we
> > have to workaround this by locking a digest-mapped temporary file.
> > 
> > Signed-off-by: Fam Zheng <famz@redhat.com>
> > ---
> >  block/raw-posix.c | 97 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 97 insertions(+)
> > 
> > diff --git a/block/raw-posix.c b/block/raw-posix.c
> > index 906d5c9..277f20d 100644
> > --- a/block/raw-posix.c
> > +++ b/block/raw-posix.c
> > @@ -35,6 +35,7 @@
> >  #include "raw-aio.h"
> >  #include "qapi/util.h"
> >  #include "qapi/qmp/qstring.h"
> > +#include "glib.h"
> >  
> >  #if defined(__APPLE__) && (__MACH__)
> >  #include <paths.h>
> > @@ -149,6 +150,9 @@ typedef struct BDRVRawState {
> >      bool discard_zeroes:1;
> >      bool has_fallocate;
> >      bool needs_alignment;
> > +    bool image_locked;
> > +    int lock_file_fd;
> > +    char *lock_file_name;
> >  } BDRVRawState;
> >  
> >  typedef struct BDRVRawReopenState {
> > @@ -397,6 +401,87 @@ static void raw_attach_aio_context(BlockDriverState *bs,
> >  #endif
> >  }
> >  
> > +static int raw_do_lockf(int fd, BdrvLockfCmd cmd)
> > +{
> > +    int ret;
> > +    struct flock fl = (struct flock) {
> > +        .l_start = 0,
> > +        .l_whence  = SEEK_SET,
> > +        .l_len = 0,
> > +    };
> 
> If you change this to
> 
>    .l_start = 1,
>    .l_len = 1,
> 
> then you would be telling a selective lock at byte 1 which would
> not interfere with anything virtlockd currently does, and also
> leave the other bytes unlocked for future use by QEMU or libvirt
> as needed.

That's brillent, thanks a lot!

Fam