linux-kernel - Re: [PATCHv4 03/17] pps: fix race in PPS

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Date:	Sat, 20 Nov 2010 16:20:29 +0100
From:	Rodolfo Giometti <giometti@...eenne.com>
To:	Alexander Gordeev <lasaine@....cs.msu.su>
Cc:	linux-kernel@...r.kernel.org,
	"Nikita V. Youshchenko" <yoush@...msu.su>,
	linuxpps@...enneenne.com, Tejun Heo <tj@...nel.org>
Subject: Re: [PATCHv4 03/17] pps: fix race in PPS_FETCH handler

On Thu, Nov 18, 2010 at 07:00:56PM +0300, Alexander Gordeev wrote:
> There was a race in PPS_FETCH ioctl handler when several processes want
> to obtain PPS data simultaneously using sleeping PPS_FETCH. They all
> sleep most of the time in the system call.
> With the old approach when the first process waiting on the pps queue
> is waken up it makes new system call right away and zeroes pps->go. So
> other processes continue to sleep. This is a clear race condition
> because of the global 'go' variable.
> With the new approach pps->last_ev holds some value increasing at each
> PPS event. PPS_FETCH ioctl handler saves current value to the local
> variable at the very beginning so it can safely check that there is a
> new event by just comparing both variables.
> 
> Signed-off-by: Alexander Gordeev <lasaine@....cs.msu.su>
> ---
>  drivers/pps/kapi.c         |    4 ++--
>  drivers/pps/pps.c          |   10 +++++++---
>  include/linux/pps_kernel.h |    2 +-
>  3 files changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/pps/kapi.c b/drivers/pps/kapi.c
> index 55f3961..3f89f5eb 100644
> --- a/drivers/pps/kapi.c
> +++ b/drivers/pps/kapi.c
> @@ -326,8 +326,8 @@ void pps_event(int source, struct pps_ktime *ts, int event, void *data)
>  
>  	/* Wake up if captured something */
>  	if (captured) {
> -		pps->go = ~0;
> -		wake_up_interruptible(&pps->queue);
> +		pps->last_ev++;
> +		wake_up_interruptible_all(&pps->queue);
>  
>  		kill_fasync(&pps->async_queue, SIGIO, POLL_IN);
>  	}
> diff --git a/drivers/pps/pps.c b/drivers/pps/pps.c
> index c76afb9..dc7e66c 100644
> --- a/drivers/pps/pps.c
> +++ b/drivers/pps/pps.c
> @@ -136,6 +136,7 @@ static long pps_cdev_ioctl(struct file *file,
>  
>  	case PPS_FETCH: {
>  		struct pps_fdata fdata;
> +		unsigned int ev;
>  
>  		pr_debug("PPS_FETCH: source %d\n", pps->id);
>  
> @@ -143,11 +144,12 @@ static long pps_cdev_ioctl(struct file *file,
>  		if (err)
>  			return -EFAULT;
>  
> -		pps->go = 0;
> +		ev = pps->last_ev;
>  
>  		/* Manage the timeout */
>  		if (fdata.timeout.flags & PPS_TIME_INVALID)
> -			err = wait_event_interruptible(pps->queue, pps->go);
> +			err = wait_event_interruptible(pps->queue,
> +					ev != pps->last_ev);
>  		else {
>  			unsigned long ticks;
>  
> @@ -159,7 +161,9 @@ static long pps_cdev_ioctl(struct file *file,
>  
>  			if (ticks != 0) {
>  				err = wait_event_interruptible_timeout(
> -						pps->queue, pps->go, ticks);
> +						pps->queue,
> +						ev != pps->last_ev,
> +						ticks);
>  				if (err == 0)
>  					return -ETIMEDOUT;
>  			}
> diff --git a/include/linux/pps_kernel.h b/include/linux/pps_kernel.h
> index c930d11..c3aed4b 100644
> --- a/include/linux/pps_kernel.h
> +++ b/include/linux/pps_kernel.h
> @@ -55,7 +55,7 @@ struct pps_device {
>  	struct pps_ktime clear_tu;
>  	int current_mode;			/* PPS mode at event time */
>  
> -	int go;					/* PPS event is arrived? */
> +	unsigned int last_ev;			/* last PPS event id */
>  	wait_queue_head_t queue;		/* PPS event queue */
>  
>  	unsigned int id;			/* PPS source unique ID */
> -- 
> 1.7.2.3
> 

Acked-by: Rodolfo Giometti <giometti@...ux.it>

-- 

GNU/Linux Solutions                  e-mail: giometti@...eenne.com
Linux Device Driver                          giometti@...ux.it
Embedded Systems                     phone:  +39 349 2432127
UNIX programming                     skype:  rodolfo.giometti
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