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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);

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

Cheers,
Vita


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