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SubjectRe: [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@lvk.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);
>
> kill_fasync(&pps->async_queue, SIGIO, POLL_IN);
> }
> diff --git a/drivers/pps/pps.c b/drivers/pps/pps.c
> index c76afb9..cb24147 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 */
> --
> 1.7.1

Yes. It was added when we passed from ioctl to unlocked_ioctl...

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

--

GNU/Linux Solutions e-mail: giometti@enneenne.com
Linux Device Driver giometti@linux.it
Embedded Systems phone: +39 349 2432127
UNIX programming skype: rodolfo.giometti
Freelance ICT Italia - Consulente ICT Italia - www.consulenti-ict.it


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