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

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Message-ID: <20110405120327.GA19300@polaris.bitmath.org>
Date:	Tue, 5 Apr 2011 14:03:27 +0200
From:	"Henrik Rydberg" <rydberg@...omail.se>
To:	Jeffrey Brown <jeffbrown@...roid.com>
Cc:	Dmitry Torokhov <dmitry.torokhov@...il.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.

> >> 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.

> >> 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.

Thanks,
Henrik
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