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

[Christian Borntraeger]

On 19.05.21 01:27, Halil Pasic wrote:
> On Tue, 18 May 2021 19:01:42 +0200
> Christian Borntraeger <borntraeger@xxxxxxxxxx> wrote:
>
> > On 18.05.21 17:33, Halil Pasic wrote:
> > > On Tue, 18 May 2021 15:59:36 +0200
> > > Christian Borntraeger <borntraeger@xxxxxxxxxx> 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.

Just had a quick look. Its not trivial, as the hook function itself
takes a mutex and an rcu section must not sleep. Will have a deeper
look.