Messages in this thread | | | Date | Thu, 5 Aug 2010 11:15:32 +0200 | From | Rodolfo Giometti <> | Subject | Re: [PATCHv3 03/16] pps: fix race in PPS_FETCH handler |
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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>
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