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Fri, 2 Aug 2019 17:36:42 +0000 From: Roman Gushchin To: Michal Hocko CC: Andrew Morton , "linux-mm@kvack.org" , Johannes Weiner , "linux-kernel@vger.kernel.org" , Kernel Team Subject: Re: [PATCH] mm: memcontrol: switch to rcu protection in drain_all_stock() Thread-Topic: [PATCH] mm: memcontrol: switch to rcu protection in drain_all_stock() Thread-Index: AQHVSMHRid8xy6v2VESi4846kGYnRKbngEUAgAAPfICAABD8gIAAeVaAgAAGFgA= Date: Fri, 2 Aug 2019 17:36:42 +0000 Message-ID: <20190802173638.GC28431@tower.DHCP.thefacebook.com> References: <20190801233513.137917-1-guro@fb.com> <20190802080422.GA6461@dhcp22.suse.cz> <20190802085947.GC6461@dhcp22.suse.cz> <20190802170030.GB28431@tower.DHCP.thefacebook.com> <20190802171451.GN6461@dhcp22.suse.cz> In-Reply-To: <20190802171451.GN6461@dhcp22.suse.cz> Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-clientproxiedby: MWHPR17CA0063.namprd17.prod.outlook.com (2603:10b6:300:93::25) To DM6PR15MB2635.namprd15.prod.outlook.com (2603:10b6:5:1a6::24) x-ms-exchange-messagesentrepresentingtype: 1 x-originating-ip: [2620:10d:c090:200::a1bd] x-ms-publictraffictype: Email x-ms-office365-filtering-correlation-id: 970329ea-ab62-4131-dfec-08d7176ffb51 x-microsoft-antispam: BCL:0;PCL:0;RULEID:(2390118)(7020095)(4652040)(8989299)(4534185)(4627221)(201703031133081)(201702281549075)(8990200)(5600148)(711020)(4605104)(1401327)(2017052603328)(7193020);SRVR:DM6PR15MB2745; 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charset="us-ascii" Content-ID: Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-MS-Exchange-CrossTenant-Network-Message-Id: 970329ea-ab62-4131-dfec-08d7176ffb51 X-MS-Exchange-CrossTenant-originalarrivaltime: 02 Aug 2019 17:36:42.4318 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: 8ae927fe-1255-47a7-a2af-5f3a069daaa2 X-MS-Exchange-CrossTenant-mailboxtype: HOSTED X-MS-Exchange-CrossTenant-userprincipalname: guro@fb.com X-MS-Exchange-Transport-CrossTenantHeadersStamped: DM6PR15MB2745 X-OriginatorOrg: fb.com X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2019-08-02_07:,, signatures=0 X-Proofpoint-Spam-Details: rule=fb_default_notspam policy=fb_default score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=772 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1906280000 definitions=main-1908020184 X-FB-Internal: deliver X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Fri, Aug 02, 2019 at 07:14:51PM +0200, Michal Hocko wrote: > On Fri 02-08-19 17:00:34, Roman Gushchin wrote: > > On Fri, Aug 02, 2019 at 10:59:47AM +0200, Michal Hocko wrote: > > > On Fri 02-08-19 10:04:22, Michal Hocko wrote: > > > > On Thu 01-08-19 16:35:13, Roman Gushchin wrote: > > > > > Commit 72f0184c8a00 ("mm, memcg: remove hotplug locking from try_= charge") > > > > > introduced css_tryget()/css_put() calls in drain_all_stock(), > > > > > which are supposed to protect the target memory cgroup from being > > > > > released during the mem_cgroup_is_descendant() call. > > > > >=20 > > > > > However, it's not completely safe. In theory, memcg can go away > > > > > between reading stock->cached pointer and calling css_tryget(). > > > >=20 > > > > I have to remember how is this whole thing supposed to work, it's b= een > > > > some time since I've looked into that. > > >=20 > > > OK, I guess I remember now and I do not see how the race is possible. > > > Stock cache is keeping its memcg alive because it elevates the refere= nce > > > counting for each cached charge. And that should keep the whole chain= up > > > to the root (of draining) alive, no? Or do I miss something, could yo= u > > > generate a sequence of events that would lead to use-after-free? > >=20 > > Right, but it's true when you reading a local percpu stock. > > But here we read a remote stock->cached pointer, which can be cleared > > by a remote concurrent drain_local_stock() execution. >=20 > OK, I can see how refill_stock can race with drain_all_stock. I am not > sure I see drain_local_stock race because that should be triggered only > from drain_all_stock and only one cpu is allowed to do that. Maybe we > might have scheduled a work from the previous run? Exactly. Previously executed drain_all_stock() -> schedule_work -> drain_local_stock() on a remote cpu races with checking memcg pointer from drain_all_stock. >=20 > In any case, please document the race in the changelog please. This code > is indeed tricky and a comment would help as well. Sure, will send out v2 soon. Thanks!