Re: [Xen-devel] [PATCH 2/5] xen/domain: Break __domain_destroy() out of domain_create() and complete_domain_destroy()

[Wei Liu <wei.liu2@xxxxxxxxxx>]

On Mon, Sep 03, 2018 at 05:58:02PM +0100, Andrew Cooper wrote:
> On 03/09/18 17:54, Wei Liu wrote:
> > On Mon, Sep 03, 2018 at 03:46:57PM +0100, Andrew Cooper wrote:
> >> This is the first step in making the destroy path idepotent, and using it 
> >> in
> > "idempotent".
> >
> >> place of the ad-hoc cleanup paths in the create path.
> >>
> >> To begin with, the trivial free operations are broken out.  The rest of the
> >> cleanup code will be moved as it is demonstrated (or made) to be 
> >> idempotent.
> >>
> >> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> >> ---
> >> CC: Jan Beulich <JBeulich@xxxxxxxx>
> >> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
> >> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> >> CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> >> CC: Julien Grall <julien.grall@xxxxxxx>
> >> ---
> >>  xen/common/domain.c | 29 ++++++++++++++++++++++-------
> >>  1 file changed, 22 insertions(+), 7 deletions(-)
> >>
> >> diff --git a/xen/common/domain.c b/xen/common/domain.c
> >> index 43ab926..2253c2d 100644
> >> --- a/xen/common/domain.c
> >> +++ b/xen/common/domain.c
> >> @@ -260,6 +260,23 @@ static int __init parse_extra_guest_irqs(const char 
> >> *s)
> >>  }
> >>  custom_param("extra_guest_irqs", parse_extra_guest_irqs);
> >>  
> >> +/*
> >> + * Destroy a domain once all references to it have been dropped.  Used 
> >> either
> >> + * from the RCU path, or from the domain_create() error path before the 
> >> domain
> >> + * is inserted into the domlist.
> >> + */
> >> +static void __domain_destroy(struct domain *d)
> >> +{
> >> +    BUG_ON(!d->is_dying);
> >> +    BUG_ON(atomic_read(&d->refcnt) != DOMAIN_DESTROYED);
> >> +
> >> +    xfree(d->pbuf);
> > With this changed to XFREE here:
> 
> This is the one place where it doesn't matter.  d goes fully out of
> scope before the end of this function.

That's fair enough.

> 
> >
> > Reviewed-by: Wei Liu <wei.liu2@xxxxxxxxxx>
> >
> >> +
> >> +    free_cpumask_var(d->dirty_cpumask);
> > On making things idempotent: this function seems to be a candidate.
> 
> I don't understand.  One implementation is xfree() under the hood, and
> the other is a no-op because no allocation took place.

I mean it would probably be useful to make free_cpumask_var idempotent
by using XFREE so multiple calls to it will not free dangling pointer.

Wei.

> 
> ~Andrew

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