[v2] HID: Replace semaphore driver_lock with mutex

On Wed, Jun 14, 2017 at 9:45 AM, David Herrmann <dh.herrmann@gmail.com> wrote:
> Hey
>
> On Wed, Jun 14, 2017 at 9:20 AM, Arnd Bergmann <arnd@arndb.de> wrote:
>> On Wed, Jun 14, 2017 at 7:22 AM, Binoy Jayan <binoy.jayan@linaro.org> wrote:
>>> Hi,
>>>
>>> On 14 June 2017 at 01:55, Arnd Bergmann <arnd@arndb.de> wrote:
>>>
>>>>> The mutex code clearly states mutex_trylock() must not be used in
>>>>> interrupt context (see kernel/locking/mutex.c), hence we used a
>>>>> semaphore here. Unless the mutex code is changed to allow this, we
>>>>> cannot switch away from semaphores.
>>>>
>>>> Right, that makes a lot of sense. I don't think changing the mutex
>>>> code is an option here, but I wonder if we can replace the semaphore
>>>> with something simpler anyway.
>>>>
>>>> From what I can tell, it currently does two things:
>>>>
>>>> 1. it acts as a simple flag to prevent  hid_input_report from derefencing
>>>>     the hid->driver pointer during initialization and exit. I think this could
>>>>     be done equally well using a simple atomic set_bit()/test_bit() or similar.
>>>>
>>>> 2. it prevents the hid->driver pointer from becoming invalid while an
>>>>     asynchronous hid_input_report() is in progress. This actually seems to
>>>>     be a reference counting problem rather than a locking problem.
>>>>     I don't immediately see how to better address it, or how exactly this
>>>>     could go wrong in practice, but I would naively expect that either
>>>>     hdev->driver->remove() needs to wait for the last user of hdev->driver
>>>>     to complete, or we need kref_get/kref_put in hid_input_report()
>>>>     to trigger the actual release function.
>
> The HID design is explained in detail in
> ./Documentation/hid/hid-transport.txt, in case you want some
> background information. The problem here is that the low-level
> transport driver has a lifetime that is independent of the hid
> device-driver. So the transport driver needs to be able to tell the
> HID layer about coming/going devices, as well as incoming traffic. At
> the same time, the HID layer can bind upper-layer hid device drivers
> *anytime* (since it is exposed via the driver core interfaces in /sys
> to bind drivers).
>
> The locking architecture is very similar to 's_active' on
> super-blocks, or 'active' on kernfs-nodes. However, the big difference
> here is that drivers can be rebound. Hence, we're not limited to just
> one driver lifetime, which is why we went with a semaphore instead.

Ok, thanks for the background information!

Does that mean that we can have a concurrent hid_device_remove()
and hid_device_probe() on the same device, using different
drivers and actually still need the driver_lock for that? I would assume
that the driver core handles that part at least.

> Also note that hid_input_report() might be called from interrupt
> context, hence it can never call into kref_put() or similar (unless we
> want to guarantee that unbinding can run in interrupt context).

I was thinking that we could do most of the unbinding in
hid_device_remove() and only do a small part (the final kfree
at the minimum) in the release() callback that gets called from
kref_put(), but I guess that also isn't easy to retrofit.

> If we really want to get rid of the semaphore, a spinlock might do
> fine as well. Then again, some hid device drivers might expect their
> transport driver to *not* run in irq context, and thus break under a
> spinlock. So if you want to fix this, we need to audit the hid device
> drivers.

Do you mean the hdrv->report or hdrv->raw_event might not work
in atomic context, or the probe/release callbacks might not work
there?

If it's only the former, we could do something like the (untested,
needs rebasing etc) patch below, which only holds the spinlock
during hid_input_report(). We test the io_started flag under the
lock, which makes the flag sufficiently atomic, and the release
function will wait for any concurrent hid_input_report() to complete
before resetting the flag.

For reference only, do not apply.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

+       spinlock_t driver_input_lock;
 /* protects the current driver */
        struct device dev;
 /* device */
        struct hid_driver *driver;

@@ -854,12 +854,12 @@ __u32 hid_field_extract(const struct hid_device
*hid, __u8 *report,
  * incoming packets to be delivered to the driver.
  */
 static inline void hid_device_io_start(struct hid_device *hid) {
+       spin_lock(&hid->driver_input_lock);
        if (hid->io_started) {
                dev_warn(&hid->dev, "io already started\n");
-               return;
        }
        hid->io_started = true;
-       up(&hid->driver_input_lock);
+       spin_unlock(&hid->driver_input_lock);
 }

 /**
@@ -874,12 +874,12 @@ static inline void hid_device_io_start(struct
hid_device *hid) {
  * by the thread calling probe or remove.
  */
 static inline void hid_device_io_stop(struct hid_device *hid) {
+       spin_lock(&hid->driver_input_lock);
        if (!hid->io_started) {
                dev_warn(&hid->dev, "io already stopped\n");
-               return;
        }
        hid->io_started = false;
-       down(&hid->driver_input_lock);
+       spin_unlock(&hid->driver_input_lock);
 }

 /**
--
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[v2] HID: Replace semaphore driver_lock with mutex

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