linux-kernel - Re: [PATCH] platform/x86: quickstart: Fix race condition when reporting input event

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Date: Mon, 8 Apr 2024 13:33:29 +0300 (EEST)
From: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To: Hans de Goede <hdegoede@...hat.com>
cc: Armin Wolf <W_Armin@....de>, "Rafael J. Wysocki" <rafael@...nel.org>, 
    linux-acpi@...r.kernel.org, platform-driver-x86@...r.kernel.org, 
    LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] platform/x86: quickstart: Fix race condition when
 reporting input event

On Sun, 7 Apr 2024, Hans de Goede wrote:
> On 4/6/24 8:57 PM, Armin Wolf wrote:
> > Am 27.03.24 um 22:45 schrieb Armin Wolf:
> > 
> >> Since commit e2ffcda16290 ("ACPI: OSL: Allow Notify () handlers to run
> >> on all CPUs"), the ACPI core allows multiple notify calls to be active
> >> at the same time. This means that two instances of quickstart_notify()
> >> running at the same time can mess which each others input sequences.
> >>
> >> Fix this by protecting the input sequence with a mutex.
> >>
> >> Compile-tested only.
> > 
> > Any thoughts on this?
> 
> I wonder if we need this at all ?
> 
> The input_event() / input_report_key() / input_sync() functions
> which underpin sparse_keymap_report_event() all are safe to be called
> from multiple threads at the same time AFAIK.
> 
> The only thing which can then still go "wrong" if we have
> 2 sparse_keymap_report_event() functions racing for the same
> quickstart button and thus for the same keycode is that we may
> end up with:
> 
> input_report_key(dev, keycode, 1);
> input_report_key(dev, keycode, 1); /* This is a no-op */
> input_sync(); /* + another input_sync() somewhere which is a no-op */
> input_report_key(dev, keycode, 0);
> input_report_key(dev, keycode, 0); /* This is a no-op */
> input_sync(); /* + another input_sync() somewhere which is a no-op */
> 
> IOW if 2 racing notifiers hit the perfect race conditions then
> only 1 key press is reported, instead of 2 which seems like
> it is not a problem since arguably if the same event gets
> reported twice at the exact same time it probably really
> is only a single button press.
> 
> Also I think it is highly unlikely we will actually see
> 2 notifiers for this racing in practice.
> 
> So I don't think we need this at all. But if others feel strongly
> about adding this I can still merge it... ?

Hi,

I know you already merged this and I agree it's not very likely race but 
still it can turn two presses into one which seems unwanted side-effect, 
even if it's unlikely to occur in practice.

-- 
 i.