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Wed, 19 May 2021 08:46:16 +0000 (GMT) Received: from d06av25.portsmouth.uk.ibm.com (unknown [127.0.0.1]) by IMSVA (Postfix) with ESMTP id 081F211C307; Wed, 19 May 2021 08:17:50 +0000 (GMT) Received: from oc7455500831.ibm.com (unknown [9.171.89.97]) by d06av25.portsmouth.uk.ibm.com (Postfix) with ESMTP; Wed, 19 May 2021 08:17:49 +0000 (GMT) Subject: Re: [PATCH v2] s390/vfio-ap: fix memory leak in mdev remove callback To: Halil Pasic Cc: Tony Krowiak , 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 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> <20210519012709.3bcc30e7.pasic@linux.ibm.com> From: Christian Borntraeger Message-ID: <250189ed-bded-5261-d8f3-f75787be7aeb@de.ibm.com> Date: Wed, 19 May 2021 10:17:49 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.10.0 MIME-Version: 1.0 In-Reply-To: <20210519012709.3bcc30e7.pasic@linux.ibm.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-GUID: -pRYk4RQyfIs3-_W5GkkstdLTwAmRRRt X-Proofpoint-ORIG-GUID: Rtnd7wG_q_PxxiCdyVU6eOCYewUvxU0r X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:6.0.391,18.0.761 definitions=2021-05-19_04:2021-05-18,2021-05-19 signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 lowpriorityscore=0 phishscore=0 impostorscore=0 spamscore=0 bulkscore=0 malwarescore=0 suspectscore=0 mlxscore=0 clxscore=1011 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2104190000 definitions=main-2105190064 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 19.05.21 01:27, Halil Pasic wrote: > On Tue, 18 May 2021 19:01:42 +0200 > Christian Borntraeger wrote: > >> On 18.05.21 17:33, Halil Pasic wrote: >>> On Tue, 18 May 2021 15:59:36 +0200 >>> Christian Borntraeger 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.