On Thu, Nov 23, 2017 at 11:07:51AM -0200, Mauro Carvalho Chehab wrote:
> Hi Laurent,
>
> Em Thu, 16 Nov 2017 02:33:48 +0200
> Laurent Pinchart <laurent.pinchart+rene...@ideasonboard.com> escreveu:
>
> > Device unplug being asynchronous, it naturally races with operations
> > performed by userspace through ioctls or other file operations on video
> > device nodes.
> >
> > This leads to potential access to freed memory or to other resources
> > during device access if unplug occurs during device access. To solve
> > this, we need to wait until all device access completes when unplugging
> > the device, and block all further access when the device is being
> > unplugged.
> >
> > Three new functions are introduced. The video_device_enter() and
> > video_device_exit() functions must be used to mark entry and exit from
> > all code sections where the device can be accessed. The
> > video_device_unplug() function is then used in the unplug handler to
> > mark the device as being unplugged and wait for all access to complete.
> >
> > As an example mark the ioctl handler as a device access section. Other
> > file operations need to be protected too, and blocking ioctls (such as
> > VIDIOC_DQBUF) need to be handled as well.
> >
> > Signed-off-by: Laurent Pinchart <laurent.pinchart+rene...@ideasonboard.com>
> > ---
> > drivers/media/v4l2-core/v4l2-dev.c | 57
> > ++++++++++++++++++++++++++++++++++++++
> > include/media/v4l2-dev.h | 47 +++++++++++++++++++++++++++++++
> > 2 files changed, 104 insertions(+)
> >
> > diff --git a/drivers/media/v4l2-core/v4l2-dev.c
> > b/drivers/media/v4l2-core/v4l2-dev.c
> > index c647ba648805..c73c6d49e7cf 100644
> > --- a/drivers/media/v4l2-core/v4l2-dev.c
> > +++ b/drivers/media/v4l2-core/v4l2-dev.c
> > @@ -156,6 +156,52 @@ void video_device_release_empty(struct video_device
> > *vdev)
> > }
> > EXPORT_SYMBOL(video_device_release_empty);
> >
> > +int video_device_enter(struct video_device *vdev)
> > +{
> > + bool unplugged;
> > +
> > + spin_lock(&vdev->unplug_lock);
> > + unplugged = vdev->unplugged;
> > + if (!unplugged)
> > + vdev->access_refcount++;
> > + spin_unlock(&vdev->unplug_lock);
> > +
> > + return unplugged ? -ENODEV : 0;
> > +}
> > +EXPORT_SYMBOL_GPL(video_device_enter);
> > +
> > +void video_device_exit(struct video_device *vdev)
> > +{
> > + bool wake_up;
> > +
> > + spin_lock(&vdev->unplug_lock);
> > + WARN_ON(--vdev->access_refcount < 0);
> > + wake_up = vdev->access_refcount == 0;
> > + spin_unlock(&vdev->unplug_lock);
> > +
> > + if (wake_up)
> > + wake_up(&vdev->unplug_wait);
> > +}
> > +EXPORT_SYMBOL_GPL(video_device_exit);
> > +
> > +void video_device_unplug(struct video_device *vdev)
> > +{
> > + bool unplug_blocked;
> > +
> > + spin_lock(&vdev->unplug_lock);
> > + unplug_blocked = vdev->access_refcount > 0;
> > + vdev->unplugged = true;
> > + spin_unlock(&vdev->unplug_lock);
> > +
> > + if (!unplug_blocked)
> > + return;
> > +
> > + if (!wait_event_timeout(vdev->unplug_wait, !vdev->access_refcount,
> > + msecs_to_jiffies(150000)))
> > + WARN(1, "Timeout waiting for device access to complete\n");
> > +}
> > +EXPORT_SYMBOL_GPL(video_device_unplug);
> > +
> > static inline void video_get(struct video_device *vdev)
> > {
> > get_device(&vdev->dev);
> > @@ -351,6 +397,10 @@ static long v4l2_ioctl(struct file *filp, unsigned int
> > cmd, unsigned long arg)
> > struct video_device *vdev = video_devdata(filp);
> > int ret = -ENODEV;
> >
> > + ret = video_device_enter(vdev);
> > + if (ret < 0)
> > + return ret;
> > +
> > if (vdev->fops->unlocked_ioctl) {
> > struct mutex *lock = v4l2_ioctl_get_lock(vdev, cmd);
> >
> > @@ -358,11 +408,14 @@ static long v4l2_ioctl(struct file *filp, unsigned
> > int cmd, unsigned long arg)
> > return -ERESTARTSYS;
> > if (video_is_registered(vdev))
> > ret = vdev->fops->unlocked_ioctl(filp, cmd, arg);
> > + else
> > + ret = -ENODEV;
> > if (lock)
> > mutex_unlock(lock);
> > } else
> > ret = -ENOTTY;
> >
> > + video_device_exit(vdev);
> > return ret;
> > }
> >
> > @@ -841,6 +894,10 @@ int __video_register_device(struct video_device *vdev,
> > int type, int nr,
> > if (WARN_ON(!vdev->v4l2_dev))
> > return -EINVAL;
> >
> > + /* unplug support */
> > + spin_lock_init(&vdev->unplug_lock);
> > + init_waitqueue_head(&vdev->unplug_wait);
> > +
>
> I'm c/c Greg here, as I don't think, that, the way it is, it
> belongs at V4L2 core.
>
> I mean: if this is a problem that affects all drivers, it would should,
> instead, be sitting at the driver's core.
What "problem" is trying to be solved here? One where your specific
device type races with your specific user api? Doesn't sound very
driver-core specific to me :)
As an example, what other bus/device type needs this? If you can see
others that do, then sure, move it into the core. But for just one, I
don't know if that's really needed here, do you?
thanks,
greg k-h
