Messages in this thread | | | From | Dan Williams <> | Date | Tue, 30 Mar 2021 08:37:19 -0700 | Subject | Re: [PATCH v2 2/4] cxl/mem: Fix synchronization mechanism for device removal vs ioctl operations |
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On Tue, Mar 30, 2021 at 4:16 AM Jason Gunthorpe <jgg@nvidia.com> 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.
[1]: https://lore.kernel.org/lkml/20180408031113.GO3948@linux.vnet.ibm.com/
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