On 08/07/2013 06:49 PM, Sylwester Nawrocki wrote:
> Hi Hans,
>
> 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.
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.
> 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,
Hans
>
>>> Signed-off-by: Sylwester Nawrocki <s.nawrocki@xxxxxxxxxxx>
>>> Signed-off-by: Kyungmin Park <kyungmin.park@xxxxxxxxxxx>
>>> ---
>>> drivers/media/v4l2-core/v4l2-dev.c | 8 ++------
>>> 1 file changed, 2 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/drivers/media/v4l2-core/v4l2-dev.c b/drivers/media/v4l2-core/v4l2-dev.c
>>> index c8859d6..1743119 100644
>>> --- a/drivers/media/v4l2-core/v4l2-dev.c
>>> +++ b/drivers/media/v4l2-core/v4l2-dev.c
>>> @@ -444,13 +444,9 @@ static int v4l2_open(struct inode *inode, struct file *filp)
>>> /* and increase the device refcount */
>>> video_get(vdev);
>>> mutex_unlock(&videodev_lock);
>>> - if (vdev->fops->open) {
>>> - if (video_is_registered(vdev))
>>> - ret = vdev->fops->open(filp);
>>> - else
>>> - ret = -ENODEV;
>>> - }
>>>
>>> + if (vdev->fops->open)
>>> + ret = vdev->fops->open(filp);
>>> if (vdev->debug)
>>> printk(KERN_DEBUG "%s: open (%d)\n",
>>> video_device_node_name(vdev), ret);
>>> --
>>> 1.7.9.5
>>>
>>
>> A long story, but the patch is good, although the commit message needs work :-)
>>
>> Regards,
>>
>> Hans
>
>
> Regards,
> Sylwester
>
--
To unsubscribe from this list: send the line "unsubscribe linux-media" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
