linux-kernel - Re: [PATCH 3/4] input: evdev: Indicate buffer overrun with SYN

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Message-ID: <20110325081442.GA5664@polaris.bitmath.org>
Date:	Fri, 25 Mar 2011 09:14:42 +0100
From:	"Henrik Rydberg" <rydberg@...omail.se>
To:	Jeff Brown <jeffbrown@...roid.com>
Cc:	linux-input@...r.kernel.org, linux-kernel@...r.kernel.org,
	Jeff Brown <jeffbrown@...gle.com>
Subject: Re: [PATCH 3/4] input: evdev: Indicate buffer overrun with
 SYN_DROPPED.

Hi Jeff,

> diff --git a/drivers/input/evdev.c b/drivers/input/evdev.c
> index 7f42d3a..203ed70 100644
> --- a/drivers/input/evdev.c
> +++ b/drivers/input/evdev.c
> @@ -57,14 +57,20 @@ static void evdev_pass_event(struct evdev_client *client,
>  {
>  	/*
>  	 * Interrupts are disabled, just acquire the lock.
> -	 * Make sure we don't leave with the client buffer
> -	 * "empty" by having client->head == client->tail.
> +	 * When the client buffer is full, replace the tail with SYN_DROPPED
> +	 * to let the client know that events were dropped.  Ensure that the
> +	 * head and tail never coincide so the buffer does not appear "empty".
>  	 */
>  	spin_lock(&client->buffer_lock);
> -	do {
> -		client->buffer[client->head++] = *event;
> -		client->head &= client->bufsize - 1;
> -	} while (client->head == client->tail);
> +	client->buffer[client->head++] = *event;
> +	client->head &= client->bufsize - 1;
> +	if (client->head == client->tail) {
> +		client->tail = (client->tail + 1) & (client->bufsize - 1);
> +		client->buffer[client->tail].time = event->time;
> +		client->buffer[client->tail].type = EV_SYN;
> +		client->buffer[client->tail].code = SYN_DROPPED;
> +		client->buffer[client->tail].value = 0;
> +	}

It looks like this will eventually fill the entire buffer with
SYN_DROPPED events if the client does not catch up.

Thanks,
Henrik
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