Re: [PATCH v2] HID: Replace semaphore driver_lock with mutex

[Arnd Bergmann]

On Tue, Jun 13, 2017 at 5:43 PM, David Herrmann <dh.herrmann@xxxxxxxxx> wrote:
> Hi
>
> On Tue, Jun 13, 2017 at 11:56 AM, Benjamin Tissoires
> <benjamin.tissoires@xxxxxxxxxx> wrote:
>>> > -       struct semaphore driver_lock;                                   /* protects the current driver, except during input */
>>> > +       struct mutex driver_lock;                                       /* protects the current driver, except during input */
>>> >         struct semaphore driver_input_lock;                             /* protects the current driver */
>>
>> Unless I am mistaken, this one could also be converted to a mutex (in a
>> separate patch, of course).
>
> 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.

        Arnd