linux-kernel - Re: [PATCH] Input: serio_raw - ensure we don't block in non-blocking read

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Date:	Wed, 11 Apr 2012 00:23:00 -0700
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	linux-input@...r.kernel.org
Cc:	Wanlong Gao <gaowanlong@...fujitsu.com>,
	Che-Liang Chiou <clchiou@...omium.org>,
	David Herrmann <dh.herrmann@...glemail.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Input: serio_raw - ensure we don't block in non-blocking
 read

On Tue, Apr 03, 2012 at 01:24:33PM -0700, Dmitry Torokhov wrote:
> Avoid calling wait_event_interruptible() if client requested non-blocking
> read, since it is not guaranteed that another thread will not consume
> event after we checked if serio_raw->head != serio_raw->tail.
> 
> Also ensure we do not return 0 but keep waiting instead in blocking case,
> when another thread steals "our" byte.
> 
> Signed-off-by: Dmitry Torokhov <dtor@...l.ru>

Anyone? It would be nice if someone could review my patches as well,
otherwise we may end up with breakages...

Thanks.

> ---
>  drivers/input/serio/serio_raw.c |   43 ++++++++++++++++++++++----------------
>  1 files changed, 25 insertions(+), 18 deletions(-)
> 
> diff --git a/drivers/input/serio/serio_raw.c b/drivers/input/serio/serio_raw.c
> index 4494233..094d6fb 100644
> --- a/drivers/input/serio/serio_raw.c
> +++ b/drivers/input/serio/serio_raw.c
> @@ -165,31 +165,38 @@ static ssize_t serio_raw_read(struct file *file, char __user *buffer,
>  	struct serio_raw *serio_raw = client->serio_raw;
>  	char uninitialized_var(c);
>  	ssize_t read = 0;
> -	int retval;
> +	int error = 0;
>  
> -	if (serio_raw->dead)
> -		return -ENODEV;
> +	do {
> +		if (serio_raw->dead)
> +			return -ENODEV;
>  
> -	if (serio_raw->head == serio_raw->tail && (file->f_flags & O_NONBLOCK))
> -		return -EAGAIN;
> +		if (serio_raw->head == serio_raw->tail &&
> +		    (file->f_flags & O_NONBLOCK))
> +			return -EAGAIN;
>  
> -	retval = wait_event_interruptible(serio_raw->wait,
> -			serio_raw->head != serio_raw->tail || serio_raw->dead);
> -	if (retval)
> -		return retval;
> +		if (count == 0)
> +			break;
>  
> -	if (serio_raw->dead)
> -		return -ENODEV;
> +		while (read < count && serio_raw_fetch_byte(serio_raw, &c)) {
> +			if (put_user(c, buffer++)) {
> +				error = -EFAULT;
> +				goto out;
> +			}
> +			read++;
> +		}
>  
> -	while (read < count && serio_raw_fetch_byte(serio_raw, &c)) {
> -		if (put_user(c, buffer++)) {
> -			retval = -EFAULT;
> +		if (read)
>  			break;
> -		}
> -		read++;
> -	}
>  
> -	return read ?: retval;
> +		if (!(file->f_flags & O_NONBLOCK))
> +			error = wait_event_interruptible(serio_raw->wait,
> +					serio_raw->head != serio_raw->tail ||
> +					serio_raw->dead);
> +	} while (!error);
> +
> +out:
> +	return read ?: error;
>  }
>  
>  static ssize_t serio_raw_write(struct file *file, const char __user *buffer,
> -- 
> 1.7.7.6
> 

-- 
Dmitry
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