From: Jason Gunthorpe <jgg@nvidia.com>
To: Dan Williams <dan.j.williams@intel.com>
Cc: linux-cxl@vger.kernel.org,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Vishal L Verma <vishal.l.verma@intel.com>,
"Weiny, Ira" <ira.weiny@intel.com>,
"Schofield, Alison" <alison.schofield@intel.com>
Subject: Re: [PATCH v2 2/4] cxl/mem: Fix synchronization mechanism for device removal vs ioctl operations
Date: Tue, 30 Mar 2021 12:47:12 -0300 [thread overview]
Message-ID: <20210330154712.GR2356281@nvidia.com> (raw)
In-Reply-To: <CAPcyv4iGByX1+CoUnc3SJahvoT0NGNnbkcDLyEkSJ8YFC9PBUg@mail.gmail.com>
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.
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
next prev parent reply other threads:[~2021-03-30 15:48 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-30 2:47 [PATCH v2 0/4] cxl/mem: Fix memdev device setup Dan Williams
2021-03-30 2:47 ` [PATCH v2 1/4] cxl/mem: Use sysfs_emit() for attribute show routines Dan Williams
2021-03-30 2:47 ` [PATCH v2 2/4] cxl/mem: Fix synchronization mechanism for device removal vs ioctl operations Dan Williams
2021-03-30 11:16 ` Jason Gunthorpe
2021-03-30 15:37 ` Dan Williams
2021-03-30 15:47 ` Jason Gunthorpe [this message]
2021-03-30 16:05 ` Dan Williams
2021-03-30 17:02 ` Jason Gunthorpe
2021-03-30 17:31 ` Dan Williams
2021-03-30 17:54 ` Jason Gunthorpe
2021-03-30 19:00 ` Dan Williams
2021-03-30 19:26 ` Jason Gunthorpe
2021-03-30 19:43 ` Dan Williams
2021-03-30 19:51 ` Jason Gunthorpe
2021-03-30 21:00 ` Dan Williams
2021-03-30 22:09 ` Jason Gunthorpe
2021-03-30 2:47 ` [PATCH v2 3/4] cxl/mem: Do not rely on device_add() side effects for dev_set_name() failures Dan Williams
2021-03-30 2:48 ` [PATCH v2 4/4] cxl/mem: Disable cxl device power management Dan Williams
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