Re: [Xen-devel] [PATCH v2 09/18] libxl: disallow attaching the same device more than once

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Wed, Aug 27, 2014 at 02:48:30AM +0100, Ian Campbell wrote:
> On Wed, 2014-07-30 at 19:23 +0100, Wei Liu wrote:
> > Originally the code allowed users to attach the same device more than
> > once. It just stupidly overwrites xenstore entries. This is bogus as
> > frontend will be very confused.
> > 
> > Introduce a helper function to check if the device to be written to
> > xenstore already exists. A new error code is also introduced.
> > 
> > Signed-off-by: Wei Liu <wei.liu2@xxxxxxxxxx>
> > ---
> >  tools/libxl/libxl.c          |   50 
> > ++++++++++++++++++++++++++++++++++++++++++
> >  tools/libxl/libxl_device.c   |   19 ++++++++++++++++
> >  tools/libxl/libxl_internal.h |    3 +++
> >  tools/libxl/libxl_pci.c      |   12 ++++++++++
> >  tools/libxl/libxl_types.idl  |    1 +
> >  5 files changed, 85 insertions(+)
> > 
> > diff --git a/tools/libxl/libxl.c b/tools/libxl/libxl.c
> > index 01dffa0..e6fe238 100644
> > --- a/tools/libxl/libxl.c
> > +++ b/tools/libxl/libxl.c
> > @@ -1906,6 +1906,15 @@ void libxl__device_vtpm_add(libxl__egc *egc, 
> > uint32_t domid,
> >      rc = libxl__device_from_vtpm(gc, domid, vtpm, device);
> >      if ( rc != 0 ) goto out;
> >  
> > +    rc = libxl__device_exists(gc, XBT_NULL, device);
> 
> Do we hold any locks from here until the code which actually creates the
> device? I don't think we have an active transaction either which would
> serve a similar purpose.
> 

At this point we hold libxl context lock, so it should be safe against
other threads in the same process.

But I can see there's problem WRT to other processes trying to
manipulate device. I think we need to have a xs transaction here. What
do you think?

Wei.

> Otherwise can't another thread come and create another device after the
> check and similarly confuse us?
> 
> (likewise for the other cases).
> 
> Ian.
> 

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