Hi, On 14 June 2017 at 01:55, Arnd Bergmann <a...@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. 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. Regards, Binoy
