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

On Wed, Jun 14, 2017 at 7:22 AM, Binoy Jayan [off-list ref] wrote:

Hi,

On 14 June 2017 at 01:55, Arnd Bergmann [off-list ref] wrote:

quoted

quoted

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.

Thank you everyone for the comments. I'll resend the patch with Benjamin's
comments incorporated and address the changes in the second semaphore later.

I hope that David or someone else can provide some more feedback on
my interpretation above first so we can decide how this should be
handled. Right now, I wouldn't know how to address point 2 above.

        Arnd