linux-kernel - Re: [PATCH v2 4/4] input: evdev: Make device readable only when it contains a complete packet.

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Date:	Tue, 5 Apr 2011 09:32:39 -0700
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	Henrik Rydberg <rydberg@...omail.se>
Cc:	Jeffrey Brown <jeffbrown@...roid.com>, djkurtz@...gle.com,
	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 4/4] input: evdev: Make device readable only when it
 contains a complete packet.

On Tue, Apr 05, 2011 at 02:03:27PM +0200, Henrik Rydberg wrote:
> > >> Should use client->head here so that the SYN_DROPPED is readable.
> > >
> > > It is readable, but we do not want to signal on it.
> > 
> > I think we do want to signal on it.  We should signal whenever the
> > device becomes readable.
> > 
> > Signaling on dropped is useful in the case where a misbehaving device
> > driver fails to ever call input_sync.  If that happens, we might
> > enqueue a dropped event and then never wake up the client which makes
> > the issue hard to diagnose.
> 
> A device that never wakes up the client seems like a detectable
> symptom. I agree with Dmitry, the dropped event is more of a note in
> passing, and as such can stay in the pipe until a real EV_SYN event
> comes along.

Also we have evbug module to report raw event stream from theinput core
without evdev involvement. It should show missing SYN_REPORT should such
a driver appear.

> 
> > >> I don't think it's safe to modify last_syn outside of the spin lock.
> > >> This should be done above.
> > >
> > > This is the only writer, plus we are running under event_lock with
> > > interrupts off, so it is safe.
> > 
> > The value will be read concurrently by evdev_fetch_next_event.  So if
> > this were safe, then we wouldn't need the spin lock at all.
> 
> The spinlock ensures atomic read/write of the event buffer. The
> position into the buffer does not need the lock.
> 
> > At the very least for the sake of consistency, I think we should keep
> > the buffer manipulations within the guarded region.
> 
> Sounds reasonable.

OK, we can pull kill_fasync inside spin_lock/unlock pair, it should
change nothing.

-- 
Dmitry
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