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Message-ID: <e08fb14bec5faf5f092fdf6d89020a416311080d.camel@analog.com>
Date: Mon, 27 Apr 2020 10:04:43 +0000
From: "Ardelean, Alexandru" <alexandru.Ardelean@...log.com>
To: "jic23@...nel.org" <jic23@...nel.org>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
"pmeerw@...erw.net" <pmeerw@...erw.net>,
"lars@...afoo.de" <lars@...afoo.de>
Subject: Re: [PATCH v5 5/6] iio: core: add simple centralized mechanism for
ioctl() handlers
On Mon, 2020-04-27 at 06:43 +0000, Ardelean, Alexandru wrote:
> [External]
>
> On Sun, 2020-04-26 at 11:21 +0100, Jonathan Cameron wrote:
> > [External]
> >
> > On Sun, 26 Apr 2020 11:10:37 +0100
> > Jonathan Cameron <jic23@...nel.org> wrote:
> >
> > > On Sun, 26 Apr 2020 10:38:16 +0300
> > > Alexandru Ardelean <alexandru.ardelean@...log.com> wrote:
> > >
> > > > The aim of this is to reduce the organization violation of ioctl() calls
> > > > in
> > > > IIO core. Currently, since the chardev is split across files, event
> > > > ioctl()
> > > > calls need to be called in buffer ioctl() calls.
> > > >
> > > > The 'industrialio-core.c' file will provide a 'iio_device_ioctl()' which
> > > > will iterate over a list of ioctls registered with the IIO device. These
> > > > can be event ioctl() or buffer ioctl() calls, or something else.
> > > > This is needed, since there is currently one chardev per IIO device and
> > > > that is used for both event handling and reading from the buffer.
> > > >
> > > > Each ioctl() will have to return a IIO_IOCTL_UNHANDLED code (which is
> > > > positive 1), if the ioctl() did not handle the call in any. This
> > > > eliminates
> > > > any potential ambiguities; if we were to have used error codes it would
> > > > have been uncertain whether they were actual errors, or whether
> > > > the registered ioctl() doesn't service the command.
> > > >
> > > > If any ioctl() returns 0, it was considered that it was serviced
> > > > successfully and the loop will exit.
> > > >
> > > > One assumption for all registered ioctl() handlers is that they are
> > > > statically allocated, so the iio_device_unregister() which just remove
> > > > all
> > > > of them from the device's ioctl() handler list.
> > > >
> > > > Also, something that is a bit hard to do [at this point] and may not be
> > > > worth the effort of doing, is to check whether registered ioctl()
> > > > calls/commands overlap. This should be unlikely to happen, and should
> > > > get
> > > > caught at review time. Though, new ioctl() calls would likely not be
> > > > added
> > > > too often.
> > > >
> > > > Signed-off-by: Alexandru Ardelean <alexandru.ardelean@...log.com>
> > >
> > > A question on locking inline. Otherwise this looks fairly clean and simple
> > > to me.
> > >
> > > Jonathan
> > >
> > >
> > > > ---
> > > > drivers/iio/iio_core.h | 14 ++++++++++++++
> > > > drivers/iio/industrialio-core.c | 33 +++++++++++++++++++++++++++++++++
> > > > include/linux/iio/iio.h | 2 ++
> > > > 3 files changed, 49 insertions(+)
> > > >
> > > > diff --git a/drivers/iio/iio_core.h b/drivers/iio/iio_core.h
> > > > index a527a66be9e5..34c3e19229d8 100644
> > > > --- a/drivers/iio/iio_core.h
> > > > +++ b/drivers/iio/iio_core.h
> > > > @@ -17,6 +17,20 @@ struct iio_dev;
> > > >
> > > > extern struct device_type iio_device_type;
> > > >
> > > > +#define IIO_IOCTL_UNHANDLED 1
> > > > +struct iio_ioctl_handler {
> > > > + struct list_head entry;
> > > > + long (*ioctl)(struct iio_dev *indio_dev, struct file *filp,
> > > > + unsigned int cmd, unsigned long arg);
> > > > +};
> > > > +
> > > > +long iio_device_ioctl(struct iio_dev *indio_dev, struct file *filp,
> > > > + unsigned int cmd, unsigned long arg);
> > > > +
> > > > +void iio_device_ioctl_handler_register(struct iio_dev *indio_dev,
> > > > + struct iio_ioctl_handler *h);
> > > > +void iio_device_ioctl_handler_unregister(struct iio_ioctl_handler *h);
> > > > +
> > > > int __iio_add_chan_devattr(const char *postfix,
> > > > struct iio_chan_spec const *chan,
> > > > ssize_t (*func)(struct device *dev,
> > > > diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-
> > > > core.c
> > > > index aec585cc8453..79e8fa8ff70b 100644
> > > > --- a/drivers/iio/industrialio-core.c
> > > > +++ b/drivers/iio/industrialio-core.c
> > > > @@ -1531,6 +1531,7 @@ struct iio_dev *iio_device_alloc(int sizeof_priv)
> > > > }
> > > > dev_set_name(&dev->dev, "iio:device%d", dev->id);
> > > > INIT_LIST_HEAD(&dev->buffer_list);
> > > > + INIT_LIST_HEAD(&dev->ioctl_handlers);
> > > >
> > > > return dev;
> > > > }
> > > > @@ -1584,6 +1585,33 @@ struct iio_dev *devm_iio_device_alloc(struct
> > > > device
> > > > *dev, int sizeof_priv)
> > > > }
> > > > EXPORT_SYMBOL_GPL(devm_iio_device_alloc);
> > > >
> > > > +void iio_device_ioctl_handler_register(struct iio_dev *indio_dev,
> > > > + struct iio_ioctl_handler *h)
> > > > +{
> > > > + /* this assumes that all ioctl() handlers are statically
> > > > allocated */
> > > > + list_add_tail(&h->entry, &indio_dev->ioctl_handlers);
> > > > +}
> > > > +
> > > > +long iio_device_ioctl(struct iio_dev *indio_dev, struct file *filp,
> > > > + unsigned int cmd, unsigned long arg)
> > > > +{
> > > > + struct iio_ioctl_handler *h;
> > > > + int ret;
> > > > +
> > > > + if (!indio_dev->info)
> > > > + return -ENODEV;
> > > > +
> > > > + list_for_each_entry(h, &indio_dev->ioctl_handlers, entry) {
> > > > + ret = h->ioctl(indio_dev, filp, cmd, arg);
> > > > + if (ret == 0)
> > > > + return 0;
> > > > + if (ret != IIO_IOCTL_UNHANDLED)
> > > > + return ret;
> > > > + }
> > > > +
> > > > + return -EINVAL;
> > > > +}
> > > > +
> > > > static int iio_check_unique_scan_index(struct iio_dev *indio_dev)
> > > > {
> > > > int i, j;
> > > > @@ -1695,6 +1723,8 @@ EXPORT_SYMBOL(__iio_device_register);
> > > > **/
> > > > void iio_device_unregister(struct iio_dev *indio_dev)
> > > > {
> > > > + struct iio_ioctl_handler *h, *t;
> > > > +
> > > > if (indio_dev->chrdev)
> > > > cdev_device_del(indio_dev->chrdev, &indio_dev->dev);
> > > > else
> > > > @@ -1708,6 +1738,9 @@ void iio_device_unregister(struct iio_dev
> > > > *indio_dev)
> > > >
> > > > iio_disable_all_buffers(indio_dev);
> > > >
> > > > + list_for_each_entry_safe(h, t, &indio_dev->ioctl_handlers,
> > > > entry)
> > > > + list_del(&h->entry);
> > > > +
> > >
> > > Is there any chance anything is walking that list whilst we are deleting
> > > it?
> > > I think this needs to happen under a lock as does the walk.
> >
> > We may want to use the rcu methods to make the walk and delete safe whilst
> > avoiding heavy weight locking on the read path
> >
> > linux/rculist.h
> >
> > I've not thought about this in much depth though so take that advice as
> > a vague suggestion and nothing more!
>
> I'm a bit tempted to do the simple mutex approach, but I'll take a look at the
> rculist.h thingi.
>
> The point is good about the list being walked while being deleted.
I'm thinking this could re-use the 'indio_dev->info_exist_lock' [i.e add the
lock in iio_device_ioctl()].
Since, the use of this lock is to guard against the 'indio_dev->info = NULL', we
can couple this with the list deletion hardening.
iio_device_ioctl() does have a check for 'indio_dev->info == NULL'
>
> >
> > > > indio_dev->info = NULL;
> > > >
> > > > iio_device_wakeup_eventset(indio_dev);
> > > > diff --git a/include/linux/iio/iio.h b/include/linux/iio/iio.h
> > > > index 52992be44e9e..b6ca8d85629e 100644
> > > > --- a/include/linux/iio/iio.h
> > > > +++ b/include/linux/iio/iio.h
> > > > @@ -488,6 +488,7 @@ struct iio_buffer_setup_ops {
> > > > * @currentmode: [DRIVER] current operating mode
> > > > * @dev: [DRIVER] device structure, should be assigned a
> > > > parent
> > > > * and owner
> > > > + * @ioctl_handlers: [INTERN] list of registered ioctl handlers
> > > > * @event_interface: [INTERN] event chrdevs associated with interrupt
> > > > lines
> > > > * @buffer: [DRIVER] any buffer present
> > > > * @buffer_list: [INTERN] list of all buffers currently attached
> > > > @@ -529,6 +530,7 @@ struct iio_dev {
> > > > int modes;
> > > > int currentmode;
> > > > struct device dev;
> > > > + struct list_head ioctl_handlers;
> > > >
> > > > struct iio_event_interface *event_interface;
> > > >
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