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
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