Re: [Qemu-devel] [PATCH for-2.7 v2 05/17] raw-posix: Implement .bdrv_lockf

["Denis V. Lunev" <den@openvz.org>]

On 04/18/2016 12:34 PM, Daniel P. Berrange wrote:
> On Mon, Apr 18, 2016 at 09:12:44AM +0800, Fam Zheng wrote:
>> On Sat, 04/16 16:29, Denis V. Lunev wrote:
>>> On 04/15/2016 06:27 AM, Fam Zheng wrote:
>>>> virtlockd in libvirt locks the first byte, we lock byte 1 to avoid
>>>> the intervene.
>>>>
>>>> Suggested-by: "Daniel P. Berrange" <berrange@redhat.com>
>>>> Signed-off-by: Fam Zheng <famz@redhat.com>
>>>> ---
>>>>   block/raw-posix.c | 35 +++++++++++++++++++++++++++++++++++
>>>>   1 file changed, 35 insertions(+)
>
>>>> @@ -1960,6 +1993,8 @@ BlockDriver bdrv_file = {
>>>>       .bdrv_detach_aio_context = raw_detach_aio_context,
>>>>       .bdrv_attach_aio_context = raw_attach_aio_context,
>>>> +    .bdrv_lockf = raw_lockf,
>>>> +
>>>>       .create_opts = &raw_create_opts,
>>>>   };
>>> would it be better to use
>>>
>>>         int flock(int fd, int operation);
>>>
>>> DESCRIPTION
>>>         Apply or remove an advisory lock on the open file specified by fd.
>>> The
>>>         argument operation is one of the following:
>>>
>>>             LOCK_SH  Place a shared lock.  More than one  process may  hold
>>> a
>>>                      shared lock for a given file at a given time.
>>>
>>>             LOCK_EX  Place  an  exclusive  lock.   Only one process may hold
>>> an
>>>                      exclusive lock for a given file at a given time.
>>>
>>>             LOCK_UN  Remove an existing lock held by this process.
>>>
>>> for this purpose?
>>>
>>> Sorry that missed this point in the initial review...
>>> We will not intersect with libvirt for the case.
>> As noted in the cover letter, flock() is nop on NFS mount points on Linux, so
>> fcntl is safer.
> Actually on Modern linux flock is implemented in terms of fcntl
> on Linux. Flock does not do byte-range locking so by using
> flock on NFS, IIUC you'll get a fcntl lock for the entire file
> range on the server, which will clash with the fcntl lock that
> virtlogd has acquired. On older linux flock is a no-op on NFS.
> Other UNIX have varying degrees of usable for flock on NFS. So
> in general fcntl is a better bet for NFS compatibility and will
> not clash with libvirt.
>
> The only minor issue is that some versions of OCFS do not support
> fcntl locks at all, only flock.
>
> Regards,
> Daniel
thank you for clarification

this is not a bid deal at all :) This approach would work fine.

Den