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

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Date:   Wed, 14 Jun 2017 09:20:48 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Binoy Jayan <binoy.jayan@...aro.org>
Cc:     David Herrmann <dh.herrmann@...il.com>,
        Benjamin Tissoires <benjamin.tissoires@...hat.com>,
        "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 Wed, Jun 14, 2017 at 7:22 AM, Binoy Jayan <binoy.jayan@...aro.org> wrote:
> Hi,
>
> On 14 June 2017 at 01:55, Arnd Bergmann <arnd@...db.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.

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