Re: [Qemu-devel] [Qemu-block] [PATCH v10 14/16] file-posix: Implement image locking

["Richard W.M. Jones" <rjones@redhat.com>]

On Thu, Jan 19, 2017 at 10:19:29AM -0600, Eric Blake wrote:
> On 01/19/2017 09:49 AM, Daniel P. Berrange wrote:
> > On Thu, Jan 19, 2017 at 10:38:14PM +0800, 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:
> >>
> 
> >> +/* Posix file locking bytes. Libvirt takes byte 0, so start from byte 1. */
> >> +#define RAW_LOCK_BYTE_MIN             1
> >> +#define RAW_LOCK_BYTE_NO_OTHER_WRITER 1
> >> +#define RAW_LOCK_BYTE_WRITE           2
> > 
> > ...would you mind if QEMU started from say byte 10, leaving the first 10
> > reserved for libvirt uses. This lets libvirt have a continuous space for
> > its own usage if we want to use more bytes
> 
> Thankfully, fcntl() locks can be taken beyond end-of-file, so I think
> we're okay making an arbitrarily larger range of bytes reserved for each
> process, even in the unlikely corner case of passing files smaller than
> 512 bytes.

Not so unlikely.  libguestfs actually detects and works around this
case because qemu was (possibly still is) very broken when you pass
small files.

https://github.com/libguestfs/libguestfs/blob/master/src/drives.c#L395-L441

Rich.

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