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

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

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(+)
>
> diff --git a/block/raw-posix.c b/block/raw-posix.c
> index 906d5c9..3a2c17f 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>
> @@ -397,6 +398,38 @@ static void raw_attach_aio_context(BlockDriverState *bs,
>   #endif
>   }
>   
> +static int raw_lockf(BlockDriverState *bs, BdrvLockfCmd cmd)
> +{
> +
> +    BDRVRawState *s = bs->opaque;
> +    int ret;
> +    struct flock fl = (struct flock) {
> +        .l_whence  = SEEK_SET,
> +        /* Locking byte 1 avoids interfereing with virtlockd. */
> +        .l_start = 1,
> +        .l_len = 1,
> +    };
> +
> +    switch (cmd) {
> +    case BDRV_LOCKF_RWLOCK:
> +        fl.l_type = F_WRLCK;
> +        break;
> +    case BDRV_LOCKF_ROLOCK:
> +        fl.l_type = F_RDLCK;
> +        break;
> +    case BDRV_LOCKF_UNLOCK:
> +        fl.l_type = F_UNLCK;
> +        break;
> +    default:
> +        abort();
> +    }
> +    ret = fcntl(s->fd, F_SETLK, &fl);
> +    if (ret) {
> +        ret = -errno;
> +    }
> +    return ret;
> +}
> +
>   #ifdef CONFIG_LINUX_AIO
>   static int raw_set_aio(void **aio_ctx, int *use_aio, int bdrv_flags)
>   {
> @@ -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.

Den