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X-MS-Exchange-AntiSpam-MessageData: =?us-ascii?Q?izjxZYSF3hNShbR9p7hhwa8h0EWQfkjMcTvjE/X7HNwLrpCY5Y/KklRzU8rh?= =?us-ascii?Q?mI1zUAOnWBCfvqgxfbyhpc/mAva1rZovwGCQJn039Qv/DbVLOERJwXS1vFQH?= =?us-ascii?Q?R0Z9QUKSD2BeeNYhCEorkUqxipWliICIWDU1ESHZYQ5AEvoiOpOjKYRNQnVU?= =?us-ascii?Q?PUSlx77ZR/T3nm/7Rz1zXpKG7hG3rcpsEKSeU1KERv3KdUqahVxb4uz5hzgb?= =?us-ascii?Q?RtuLRKFVv/jmeXCaS/rPVfdkLToNo4Z+EK8BtEvMFEYwimK2/UPW2gI53Dfj?= =?us-ascii?Q?8CLJrsLFWlIvgBPvAOdFD7svJwhQhjdK9MjGwXQ+Fnw2umbLGfJsl4agIEBz?= =?us-ascii?Q?uuP2xsVEE6CHwX+BiPWi5UBLp/U86xM7a29qYs+uU3N+5MoN6rapBA19mmQ0?= =?us-ascii?Q?i4NgYve5qNz99xpEuf7yKcRUZ0Qt1R5FoD6pO0L8nHyVhvl73tgGg6ItWGyj?= =?us-ascii?Q?09B+a7kK8uvpD6bqSmng0n6sUVQsetPt7CpbOSJfmG2HcXpjQ1MIcJz0R5sP?= =?us-ascii?Q?21UQITWY212cOn4cTed3P4mFHh7RTPlB4kl3h7gb2YvA2wS9B0P2L83Ov6KU?= =?us-ascii?Q?bXP4J0bqJsmuYbwtz2ik99Se3E1nTOf76wXrqq/Er3sfvzOrJAhm6OJTOKos?= =?us-ascii?Q?7URUNGvppLb1rNzbRFFl79E5XiDCGyeWbioAZwiVyuNNIZYcjhlklXsn1Geo?= =?us-ascii?Q?IZHLsS11LjoLStvwgVJreAnjpb5eAiq/tzDsLGj/S0pM7HKqXI+LWBNFE5oj?= =?us-ascii?Q?UV6Ny+jHcxVcPoeECeknQa5J44hs3lv7n2azW5PZB7Dx+XlD/YB/3eBnu9VO?= =?us-ascii?Q?wbQKSlQF0ktiiOlJnFIhR4CfD7nUcfcQzdpkezae7Jh1IvS/JAdIi2QSjC2h?= =?us-ascii?Q?ZuH0SOi5Zp3bueI7Ah1hPigUIdFWC8V+R9xslf7et+EGLswabZkXT5HZOErt?= =?us-ascii?Q?0irxY37RC50lSTlXG6dASYkynD5yHHYn3MA+ILR8lj4F2So2QXgxEx3Ao8V8?= =?us-ascii?Q?H7cxK4tolXjyRY17EGOcpfMRJYdTzbyIMj3zQ8GNahNBZyMPbZR85Wr87x/U?= =?us-ascii?Q?0VBsU5FeDriJ1i4sRl8//yVGvMtlcqPsgDFUnrAiGMquGuHBOBjSNRMBCyC3?= =?us-ascii?Q?4a+z078L6gd+eftxBzAHs1IM6Sh2/nwwTIhmIVjCG5ZAdAgOHd5q0tJi8Of1?= =?us-ascii?Q?mjnUlewwI3OwJUzg5QoZgLgGvNESVgFcGYes/QCKIUwBhxZaXAZBc6y6DDb4?= =?us-ascii?Q?vyVUc0P4N600ACcuLKdqZ4iRbeYUUrCpISAxtd+EYpqs9z0i/qxPxOMS2p2N?= =?us-ascii?Q?KqZGlmXMRPppEx42ZTIXXZnvgpSQmcjyAGovqObhd75feg=3D=3D?= X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-Network-Message-Id: d6cd0ada-4a9c-450e-88e1-08d8f393168c X-MS-Exchange-CrossTenant-AuthSource: DM6PR12MB3834.namprd12.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-OriginalArrivalTime: 30 Mar 2021 15:47:14.2692 (UTC) X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-MailboxType: HOSTED X-MS-Exchange-CrossTenant-UserPrincipalName: DRoJcRVW6YGV4Ezz6e/2vOOQD+LARZGN8YVs6x8CTSrN/dHY1rLCNmrt/HXFjMA8 X-MS-Exchange-Transport-CrossTenantHeadersStamped: DM5PR12MB1882 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Mar 30, 2021 at 08:37:19AM -0700, Dan Williams wrote: > On Tue, Mar 30, 2021 at 4:16 AM Jason Gunthorpe wrote: > > > > On Mon, Mar 29, 2021 at 07:47:49PM -0700, Dan Williams wrote: > > > > > @@ -1155,21 +1175,12 @@ static void cxlmdev_unregister(void *_cxlmd) > > > struct cxl_memdev *cxlmd = _cxlmd; > > > struct device *dev = &cxlmd->dev; > > > > > > - percpu_ref_kill(&cxlmd->ops_active); > > > cdev_device_del(&cxlmd->cdev, dev); > > > - wait_for_completion(&cxlmd->ops_dead); > > > + synchronize_srcu(&cxl_memdev_srcu); > > > > This needs some kind of rcu protected pointer for SRCU to to > > work.. The write side has to null the pointer and the read side has to > > copy the pointer to the stack and check for NULL. > > > > Otherwise the read side can't detect when the write side is shutting > > down. > > > > Basically you must use rcu_derference(), rcu_assign_pointer(), etc > > when working with RCU. > > If the shutdown path was not using synchronize_rcu() then I would > agree with you. This usage of srcu is also used to protect dax device > shutdown after talking through rwsem vs srcu in this thread with Jan > and Paul [1]. The syncrhonize_rcu() guarantees that all read-side > critical sections have had at least one chance to quiesce. > > So this could either use rcu pointer accessors and call_srcu to free > the object in a quiescent state, or it can use synchronize_srcu() > relative to a condition that aborts usage of the pointer. synchronize_rcu doesn't stop the read side from running it. It only guarentees that all running or future read sides will see the *write* performed prior to synchronize_rcu. If you can't clearly point to the *data* under RCU protection it is being used wrong. Same as if you can't point to the *data* being protected by a rwsem it is probably being used wrong. We are not locking code. Jason