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From: Binoy Jayan <binoy.jayan@linaro.org>
Date: Wed, 14 Jun 2017 10:52:31 +0530
Message-ID: <CAHv-k_9Wxt+TkeTssLizjf5WNZAgfMy1bcXGPBfMePsNLRAY1Q@mail.gmail.com>
Subject: Re: [PATCH v2] HID: Replace semaphore driver_lock with mutex
To: Arnd Bergmann <arnd@arndb.de>
Cc: David Herrmann <dh.herrmann@gmail.com>,
        Benjamin Tissoires <benjamin.tissoires@redhat.com>,
        "open list:HID CORE LAYER" <linux-input@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Rajendra <rnayak@codeaurora.org>, Mark Brown <broonie@kernel.org>,
        Jiri Kosina <jikos@kernel.org>,
        David Herrmann <dh.herrmann@googlemail.com>,
        Andrew de los Reyes <adlr@chromium.org>
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Hi,

On 14 June 2017 at 01:55, Arnd Bergmann <arnd@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