linux-kernel - Re: [PATCH v2] HID: Replace semaphore driver

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Date:   Tue, 13 Jun 2017 22:25:43 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     David Herrmann <dh.herrmann@...il.com>
Cc:     Benjamin Tissoires <benjamin.tissoires@...hat.com>,
        Binoy Jayan <binoy.jayan@...aro.org>,
        "open list:HID CORE LAYER" <linux-input@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Rajendra <rnayak@...eaurora.org>,
        Mark Brown <broonie@...nel.org>,
        Jiri Kosina <jikos@...nel.org>,
        David Herrmann <dh.herrmann@...glemail.com>,
        Andrew de los Reyes <adlr@...omium.org>
Subject: Re: [PATCH v2] HID: Replace semaphore driver_lock with mutex

On Tue, Jun 13, 2017 at 5:43 PM, David Herrmann <dh.herrmann@...il.com> wrote:
> Hi
>
> On Tue, Jun 13, 2017 at 11:56 AM, Benjamin Tissoires
> <benjamin.tissoires@...hat.com> 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