Messages in this thread | | | From | Dan Williams <> | Date | Tue, 30 Mar 2021 09:05:29 -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 8:47 AM Jason Gunthorpe <jgg@nvidia.com> wrote: > > On Tue, Mar 30, 2021 at 08:37:19AM -0700, Dan Williams wrote: > > 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. > > 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.
Agree.
The data being protected is the value of dev->kobj.state_in_sysfs. The read-side is allowed to keep running, and the syncrhonize_rcu() guarantees that any read-side that saw state_in_sysfs == 1 has had a chance to complete. Future reads terminate the ioctl at the device_is_registered() check.
> 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.
Agree.
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