Hi Hans and Sylwester,
On Wednesday 07 August 2013 19:49:53 Hans Verkuil wrote:
> On 08/07/2013 06:49 PM, Sylwester Nawrocki wrote:
> > On 08/02/2013 03:00 PM, Hans Verkuil wrote:
> >> Hi Sylwester,
> >>
> >> The patch is good, but I have some issues with the commit message itself.
> >
> > Thanks a lot for the detailed explanation, I just wrote this a bit
> > longish changelog to possibly get some feedback and to better understand
> > what is exactly going on. Currently the v4l2-core looks like a racing
> > disaster to me.
> >
> >> On 08/02/2013 02:27 PM, Sylwester Nawrocki wrote:
> >>> As it currently stands this code doesn't protect against any races
> >>> between video device open() and its unregistration. Races could be
> >>> avoided by doing the video_is_registered() check protected by the
> >>> core mutex, while the video device unregistration is also done with
> >>> this mutex held.
> >>
> >> The video_unregister_device() is called completely asynchronously,
> >> particularly in the case of usb drivers. So it was never the goal of
> >> the video_is_registered() to be fool proof, since that isn't possible,
> >> nor is that necessary.
> >>
> >> The goal was that the v4l2 core would use it for the various file
> >> operations and ioctls as a quick check whether the device was
> >> unregistered and to return the correct error. This prevents drivers from
> >> having to do the same thing.
> >
> > OK, I think I just myself used this video_is_registered() flag for some
> > more stuff, by surrounding it with mutex_lock/mutex_unlock and putting
> > more stuff in between, like media_entity_cleanup().
>
> You can't do that, because there are most likely still filehandles open
> or even ioctls being executed. Cleanup happens in the release function(s)
> when the kref goes to 0.
>
> > And this probably led me astray for a while, thinking that
> > video_is_registered() was intended to be used synchronously.
> > For example see fimc_lite_subdev_unregistered() in drivers/media/platform/
> > exynos4-is/fimc-lite.c.
> >
> > But as you said video_is_registered() is fully asynchronous.
> >
> > Actually I'm trying to fix a nasty race between deferred driver probing
> > and video device open(). The problem is that video open() callback can
> > be called after driver remove() callback was invoked.
>
> How is that possible? The video_device_register must be the last thing in
> the probe function. If it succeeds, then the probe must succeed as well.
>
> Note that I now realize that this might fail in the case of multiple device
> nodes being registered. We never had problems with that in the past, but in
> theory you can the race condition you mention in that scenario. The correct
> approach here would probably be to always return 0 in probe() if only some
> of the video_device_register calls fail.
Hmmmm... Returning success in probe when probing partly fails doesn't sound
very good to me.
Once you call video_device_register() you should be prepared to handle
userspace calls. The device node can be opened, in which case the module
refcount will be incremented, but probe() can still fail. However, the video
device becomes refcounted as soon as it's registered, so drivers should only
release resources in the release callback. This would unfortunately mean that
the devm_* helpers can't be used.
I would be surprised if this problem was specific to V4L2. It might be
something we should try to solve with the help of the device core.
> Anyway, assuming that only one device node is created, then I can't see how
> you can get a race condition here. Any open() call will increase the
> module's refcount, making it impossible to unload.
>
> As far as I can tell, once you call rmmod it should no longer be possible to
> open() an device node whose struct file_operations owner is that module
> (i.e. the owner field of the file_operations struct points to that module).
> Looking at the way fs/char_dev is implemented, that seems to be correctly
> handled by the kernel core.
>
> > This issue is actually not only related to deferred probing. It can be
> > also triggered by driver module removal or through driver's sysfs "unbind"
> > attribute.
> >
> > Let's assume following scenario:
> >
> > - a driver module is loaded
> > - driver probe is called, it registers video device,
> > - udev opens /dev/video
> > - after mutex_unlock(&videodev_lock); call in v4l2_open() in v4l2-core/
> > v4l2-dev.c something fails in probe()
>
> And that shouldn't happen. That's the crucial bit: under which scenario does
> this happen for you? If there is a control path where you do create a
> working device node, but the probe fails, then that will indeed cause all
> sorts of problems. But it shouldn't get in that situation (except I think
> in the case of multiple device nodes, which is something I need to think
> about).
>
> > and it unwinds, probe callback exits and the driver code code calls
> > dev_set_drvdata(dev, NULL); as shown below.
> >
> > static int really_probe(struct device *dev, struct device_driver *drv)
> > {
> > ...
> > pr_debug("bus: '%s': %s: probing driver %s with device %s\n",
> > drv->bus->name, __func__, drv->name, dev_name(dev));
> > ...
> > if (dev->bus->probe) {
> > ret = dev->bus->probe(dev);
> > if (ret)
> > goto probe_failed;
> >
> > } else if (drv->probe) {
> > ret = drv->probe(dev);
> > if (ret)
> > goto probe_failed;
> > }
> > ...
> > pr_debug("bus: '%s': %s: bound device %s to driver %s\n",
> > drv->bus->name, __func__, dev_name(dev), drv->name);
> >
> > goto done;
> >
> > probe_failed:
> > devres_release_all(dev);
> > driver_sysfs_remove(dev);
> > dev->driver = NULL;
> > dev_set_drvdata(dev, NULL);
> > ...
> > ret = 0;
> >
> > done:
> > ...
> > return ret;
> > }
> >
> > Now we get to
> >
> > ret = vdev->fops->open(filp);
> >
> > in v4l2_open(). This calls some driver's callback, e.g. something
> > like:
> >
> > static int fimc_lite_open(struct file *file)
> > {
> > struct fimc_lite *fimc = video_drvdata(file);
> > struct media_entity *me = &fimc->ve.vdev.entity;
> > int ret;
> >
> > mutex_lock(&fimc->lock);
> > if (!video_is_registered(&fimc->ve.vdev)) {
> > ret = -ENODEV;
> > goto unlock;
> > }
> >
> > ...
> >
> > /* Mark video pipeline ending at this video node as in use. */
> > if (ret == 0)
> > me->use_count++;
> >
> > ...
> >
> > unlock:
> > mutex_unlock(&fimc->lock);
> > return ret;
> > }
> >
> > Now what will video_drvdata(file); return ?
> >
> > static inline void *video_drvdata(struct file *file)
> > {
> > return video_get_drvdata(video_devdata(file));
> > }
> >
> > static inline void *video_get_drvdata(struct video_device *vdev)
> > {
> > return dev_get_drvdata(&vdev->dev);
> > }
> >
> > Yes, so that will be just NULL o_O, due to the dev_set_drvdata(dev, NULL);
> > in really_probe(). drvdata is cleared similarly in
> > __device_release_driver(), right after calling driver's remove handler.
> >
> > Another issue I have is that, e.g.
> > driver/media/platform/exynos4-is/fimc-lite* driver has empty video dev
> > release() callback. It should be implemented in the driver to kfree the
> > whole driver's private data structure where struct video_device is
> > embedded in (struct fimc_lite). But that freeing really needs to be
> > synchronized with driver's remove() call, since there is e.g. freed
> > interrupt which accesses the driver's private data. I can't use kref from
> > struct v4l2_device as that belongs to a different driver. A driver's
> > custom reference counting comes to mind, where vdev->release() and
> > drv->remove() would be decrementing the reference counter. But that seems
> > ugly as hell :/ And it predates devm_*.
> >
> > This is all getting a bit depressing :/ Deferred probing and the
> > asynchronous subdev handling just made those issues more visible, i.e.
> > not very good design of some parts of the v4l2-core.
>
> It's just not clear to me how exactly things go wrong for you. Ping me on
> irc tomorrow and we can discuss it further. I have reworked refcounting in
> the past (at the time it was *really* bad), so perhaps we need to rework it
> again, particularly with video nodes associated with subdevices in the mix,
> something that didn't exist at the time.
>
> >>> The history of this code is that the second video_is_registered()
> >>> call has been added in commit ee6869afc922a9849979e49bb3bbcad7948
> >>> "V4L/DVB: v4l2: add core serialization lock" together with addition
> >>>
> >>> of the core mutex support in fops:
> >>> mutex_unlock(&videodev_lock);
> >>>
> >>> - if (vdev->fops->open)
> >>> - ret = vdev->fops->open(filp);
> >>> + if (vdev->fops->open) {
> >>> + if (vdev->lock)
> >>> + mutex_lock(vdev->lock);
> >>> + if (video_is_registered(vdev))
> >>> + ret = vdev->fops->open(filp);
> >>> + else
> >>> + ret = -ENODEV;
> >>> + if (vdev->lock)
> >>> + mutex_unlock(vdev->lock);
> >>> + }
> >>
> >> The history is slightly more complicated: this commit moved the
> >> video_is_registered call from before the mutex_unlock(&videodev_lock);
> >> to just before the fops->open call.
> >>
> >> Commit ca9afe6f87b569cdf8e797395381f18ae23a2905 "v4l2-dev: fix race
> >> condition" added the video_is_registered() call to where it was
> >> originally (inside the videodev_lock critical section), but it didn't
> >> bother to remove the duplicate second video_is_registered call.
> >>
> >> So that's how v4l2_open ended up with two calls to video_is_registered.
> >
> > Apologies for simplifying the history . I'll just drop it from the
> > changelog, as it can be retrieved git. I'll try to put just concise
> > explanation why this this video_is_registered() is not needed currently.
> >
> >>> While commit cf5337358548b813479b58478539fc20ee86556c
> >>> "[media] v4l2-dev: remove V4L2_FL_LOCK_ALL_FOPS"
> >>>
> >>> removed only code touching the mutex:
> >>> mutex_unlock(&videodev_lock);
> >>> if (vdev->fops->open) {
> >>>
> >>> - if (test_bit(V4L2_FL_LOCK_ALL_FOPS, &vdev->flags) &&
> >>> - mutex_lock_interruptible(vdev->lock)) {
> >>> - ret = -ERESTARTSYS;
> >>> - goto err;
> >>> - }
> >>>
> >>> if (video_is_registered(vdev))
> >>>
> >>> ret = vdev->fops->open(filp);
> >>>
> >>> else
> >>>
> >>> ret = -ENODEV;
> >>>
> >>> - if (test_bit(V4L2_FL_LOCK_ALL_FOPS, &vdev->flags))
> >>> - mutex_unlock(vdev->lock);
> >>>
> >>> }
> >>>
> >>> Remove the remaining video_is_registered() call as it doesn't provide
> >>> any real protection and just adds unnecessary overhead.
> >>
> >> True.
> >>
> >>> The drivers need to perform the unregistration check themselves inside
> >>> their file operation handlers, while holding respective mutex.
> >>
> >> No, drivers do not need to do the unregistration check. Since
> >> unregistration is asynchronous it can happen at any time, so there
> >> really is no point in checking for it other than in the core. If the
> >> device is unregistered while in the middle of a file operation, then
> >> that means that any USB activity will return an error, and that any
> >> future file operations other than release() will be met by an error as
> >> well from the v4l2 core.
> >
> > Yes, so video_is_registered() seems not very useful to use in drivers.
>
> That's true. The main use case for it is in the v4l2 core to stop ioctls and
> fops from going through to the driver and return an error instead. So
> drivers don't need to do that themselves.
I'd like to add that the video_is_registered() call in v4l2_open() really
protects from a race condition with video_unregister_device() and makes sure
that either v4l2_open() gets a reference to the device before it gets
unregistered in video_unregister_device(), or v4l2_open() fails the registered
check and returns an error.
> > But as I've shown above it's not even used optimally in the v4l2-core.
>
> There really isn't anything else you can do with it in my view.
--
Regards,
Laurent Pinchart
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