Re: [Qemu-devel] One more thing about block device locking

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

On Fri, Apr 23, 2010 at 09:07:28AM +0400, Michael Tokarev wrote:
> So we'll have to either
> trial and error, or open "normally", check
> if it's a block device and re-open with that
> flag set.

Perhaps I'm missing something, but why can't you stat(2) the name
first to see if it's a block device (ie. S_ISBLK(st_mode)) then add
the O_EXCL flag or not as appropriate?

Anyway, the problem with this is it's not helpful (in fact: actively
bad) for libguestfs.  We want to allow people to open existing devices
read-only, even if one qemu process has them open for writing.
Consider, for example, virt-df or virt-cat which allow you to read
information out from live VMs.

This is why we should have exclusive|shared modes for locking (or if
you prefer write|read -- it's the same thing with a different name
AFAICT).

Rich.

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