Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Wed, 19 May 2021 01:27:09 +0200
From:   Halil Pasic <pasic@linux.ibm.com>
To:     Christian Borntraeger <borntraeger@de.ibm.com>
Cc:     Tony Krowiak <akrowiak@linux.ibm.com>, linux-s390@vger.kernel.org,
        linux-kernel@vger.kernel.org, cohuck@redhat.com,
        pasic@linux.vnet.ibm.com, jjherne@linux.ibm.com, jgg@nvidia.com,
        alex.williamson@redhat.com, kwankhede@nvidia.com,
        stable@vger.kernel.org, Tony Krowiak <akrowiak@stny.rr.com>
Subject: Re: [PATCH v2] s390/vfio-ap: fix memory leak in mdev remove
 callback
Message-ID: <20210519012709.3bcc30e7.pasic@linux.ibm.com>
In-Reply-To: <ca5f1c72-09a3-d270-44a0-bda54c554f67@de.ibm.com>
References: <20210510214837.359717-1-akrowiak@linux.ibm.com>
        <20210512203536.4209c29c.pasic@linux.ibm.com>
        <4c156ab8-da49-4867-f29c-9712c2628d44@linux.ibm.com>
        <20210513194541.58d1628a.pasic@linux.ibm.com>
        <243086e2-08a0-71ed-eb7e-618a62b007e4@linux.ibm.com>
        <20210514021500.60ad2a22.pasic@linux.ibm.com>
        <594374f6-8cf6-4c22-0bac-3b224c55bbb6@linux.ibm.com>
        <20210517211030.368ca64b.pasic@linux.ibm.com>
        <966a60ad-bdde-68d0-ae2f-06121c6ad970@de.ibm.com>
        <9ebd5fd8-b093-e5bc-e680-88fa7a9b085c@linux.ibm.com>
        <494af62b-dc9a-ef2c-1869-d8f5ed239504@de.ibm.com>
        <20210518173351.39646b45.pasic@linux.ibm.com>
        <ca5f1c72-09a3-d270-44a0-bda54c554f67@de.ibm.com>
Organization: IBM
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 8bit
Precedence: bulk

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