linux-kernel - Re: [PATCHv3 03/16] pps: fix race in PPS

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Message-ID: <20100805051929.GA14413@pc11.op.pod.cz>
Date:	Thu, 5 Aug 2010 07:19:29 +0200
From:	Vitezslav Samel <vitezslav@...el.cz>
To:	Alexander Gordeev <lasaine@....cs.msu.su>
Cc:	linux-kernel@...r.kernel.org,
	"Nikita V. Youshchenko" <yoush@...msu.su>,
	linuxpps@...enneenne.com, Rodolfo Giometti <giometti@...eenne.com>,
	john stultz <johnstul@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Joonwoo Park <joonwpark81@...il.com>
Subject: Re: [PATCHv3 03/16] pps: fix race in PPS_FETCH handler

On Thu, Aug 05, 2010 at 01:06:40AM +0400, 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..3f89f5e 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);

  What happens if pps->last_ev overflows? Seems to me it would freeze
pps.

	Cheers,
		Vita
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