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charset="us-ascii" Content-ID: <8E3B45B3499AF14E90DB72AA1E22C56E@namprd15.prod.outlook.com> Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-MS-Exchange-CrossTenant-Network-Message-Id: 5aca90cb-af4a-48e0-7ce0-08d7176aef5e X-MS-Exchange-CrossTenant-originalarrivaltime: 02 Aug 2019 17:00:34.8162 (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: DM6PR15MB3689 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=861 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1906280000 definitions=main-1908020176 X-FB-Internal: deliver Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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_char= ge") > > > 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 been > > 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 reference > 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 you > generate a sequence of events that would lead to use-after-free? 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. In theory, it could be the last reference, and the memcg can be destroyed remotely, so we end up trying to call css_tryget() over freed memory. The race is theoretical, but as I wrote in the thread, I think that it's still worth fixing, because the current code looks confusing (and this confirms my feelings). Thanks!