Re: [PATCH v2 10/19] tools/xenstore: change per-domain node accounting interface

[Juergen Gross <jgross@xxxxxxxx>]

On 11.01.23 18:48, Julien Grall wrote:
> Hi Juergen,
>
> On 11/01/2023 08:59, Juergen Gross wrote:
> > > ... to make sure domain_nbentry_add() is not returning a negative value. Then 
> > > it would not work.
> > > A good example imagine you have a transaction removing nodes from tree but 
> > > not adding any. Then the "ret" would be negative.
> > > Meanwhile the nodes are also removed outside of the transaction. So the sum 
> > > of "d->nbentry + ret" would be negative resulting to a failure here.
> > Thanks for catching this.
> >
> > I think the correct way to handle this is to return max(d->nbentry + ret, 0) in
> > domain_nbentry_add(). The value might be imprecise, but always >= 0 and never
> > wrong outside of a transaction collision.
> I am bit confused with your proposal. If the return value is imprecise, then 
> what's the point of returning max(...) instead of simply 0?
Please have a look at the use case especially in domain_nbentry(). Returning
always 0 would clearly break quota checks.


Juergen

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