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Date: Wed, 13 Apr 2016 10:21:06 +0100
From: "Daniel P. Berrange" <berrange@redhat.com>
Message-ID: <20160413092106.GD8847@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: Fam Zheng <famz@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, 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.


Regards,
Daniel
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