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

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Date:   Wed, 19 May 2021 10:17:49 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     Halil Pasic <pasic@...ux.ibm.com>
Cc:     Tony Krowiak <akrowiak@...ux.ibm.com>, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org, cohuck@...hat.com,
        pasic@...ux.vnet.ibm.com, jjherne@...ux.ibm.com, jgg@...dia.com,
        alex.williamson@...hat.com, kwankhede@...dia.com,
        stable@...r.kernel.org, Tony Krowiak <akrowiak@...y.rr.com>
Subject: Re: [PATCH v2] s390/vfio-ap: fix memory leak in mdev remove callback



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

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.