Re: [Xen-devel] [PATCH v1 7/8]: PVH privcmd changes

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Thu, 2012-10-04 at 19:20 +0100, Mukesh Rathor wrote:
> On Thu, 4 Oct 2012 09:50:42 +0100
> Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> 
> > On Wed, 2012-10-03 at 23:31 +0100, Mukesh Rathor wrote:
> > > On Wed, 3 Oct 2012 14:21:35 +0100
> > > Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> > > 
> > > > On Fri, 2012-09-21 at 20:21 +0100, Mukesh Rathor wrote:
> > > > > +static int pvh_privcmd_resv_pfns(struct vm_area_struct *vma,
> > > > > int numpgs)
> > > > ...
> > > > > +       pvhp->pi_num_pgs = numpgs;
> > > > > +       BUG_ON(vma->vm_private_data != (void *)1);
> > > > > +       vma->vm_private_data = pvhp; 
> > > > 
> > > > How does this interact with:
> > > > 
> > > > static int privcmd_enforce_singleshot_mapping(struct
> > > > vm_area_struct *vma) {
> > > >         return (xchg(&vma->vm_private_data, (void *)1) == NULL);
> > > > }
> > > > 
> > > > If someone tries to map a second time then won't this correct the
> > > > pvhp in vm_private_data by resetting it to 1? Then when the
> > > > original mapping is torn down things all fall apart?
> > > > 
> > > > Perhaps we need a cmpxchg here? Or to rework the callers a little
> > > > bit perhaps.
> > > 
> > > Right, that's why I had it originally checking for auto xlated and
> > > doing something different. I think that is better than to change
> > > this and change again. I'll change it back to just putting the ptr
> > > here.
> > 
> > Won't that break because on the second call you will pass in the
> > freshly allocated pointer and overwrite the exiting (useful) one with
> > it?
> 
> No, for xlate, I just check for NULL. I didn't think it was big 
> deal to special case xlate in this case. We got so many if xlate 
> cases already thru the code. It leaves the semantics easy to 
> understand: NULL == avail. 1 == locked PV. PTR == Locked PVH. I'll add
> a comment this time :).

The transition from NULL => Locked PVH still needs to be done atomically
and without clobbering any existing non-NULL value, otherwise it doesn't
actually protect against multiple mappings like it is supposed to.

Ian.



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