Re: [PATCH v2] s390/vfio-ap: fix memory leak in mdev remove callback

[Halil Pasic <pasic@linux.ibm.com>]

On Tue, 18 May 2021 19:01:42 +0200
Christian Borntraeger <borntraeger@de.ibm.com> wrote:

> On 18.05.21 17:33, Halil Pasic wrote:
> > On Tue, 18 May 2021 15:59:36 +0200
> > Christian Borntraeger <borntraeger@de.ibm.com> wrote:
[..]
> >>>>
> >>>> Would it help, if the code in priv.c would read the hook once
> >>>> and then only work on the copy? We could protect that with rcu
> >>>> and do a synchronize rcu in vfio_ap_mdev_unset_kvm after
> >>>> unsetting the pointer?  
> > 
> > Unfortunately just "the hook" is ambiguous in this context. We
> > have kvm->arch.crypto.pqap_hook that is supposed to point to
> > a struct kvm_s390_module_hook member of struct ap_matrix_mdev
> > which is also called pqap_hook. And struct kvm_s390_module_hook
> > has function pointer member named "hook".  
> 
> I was referring to the full struct.
> >   
> >>>
> >>> I'll look into this.  
> >>
> >> I think it could work. in priv.c use rcu_readlock, save the
> >> pointer, do the check and call, call rcu_read_unlock.
> >> In vfio_ap use rcu_assign_pointer to set the pointer and
> >> after setting it to zero call sychronize_rcu.  
> > 
> > In my opinion, we should make the accesses to the
> > kvm->arch.crypto.pqap_hook pointer properly synchronized. I'm
> > not sure if that is what you are proposing. How do we usually
> > do synchronisation on the stuff that lives in kvm->arch?
> >   
> 
> RCU is a method of synchronization. We  make sure that structure
> pqap_hook is still valid as long as we are inside the rcu read
> lock. So the idea is: clear pointer, wait until all old readers
> have finished and the proceed with getting rid of the structure.

Yes I know that RCU is a method of synchronization, but I'm not
very familiar with it. I'm a little confused by "read the hook
once and then work on a copy". I guess, I would have to read up
on the RCU again to get clarity. I intend to brush up my RCU knowledge
once the patch comes along. I would be glad to have your help when
reviewing an RCU based solution for this.   

Regards,
Halil