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Subject: Re: [PATCH v2] s390/vfio-ap: fix memory leak in mdev remove callback
To:     Halil Pasic <pasic@linux.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>
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>
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 <20210517211030.368ca64b.pasic@linux.ibm.com>
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 <9ebd5fd8-b093-e5bc-e680-88fa7a9b085c@linux.ibm.com>
 <494af62b-dc9a-ef2c-1869-d8f5ed239504@de.ibm.com>
 <20210518173351.39646b45.pasic@linux.ibm.com>
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 <20210519012709.3bcc30e7.pasic@linux.ibm.com>
From:   Christian Borntraeger <borntraeger@de.ibm.com>
Message-ID: <250189ed-bded-5261-d8f3-f75787be7aeb@de.ibm.com>
Date:   Wed, 19 May 2021 10:17:49 +0200
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On 19.05.21 01:27, Halil Pasic wrote:
> 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.

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.