Messages in this thread | | | Date | Thu, 5 Aug 2010 13:07:50 +0200 | From | Vitezslav Samel <> | Subject | Re: [PATCHv3 03/16] pps: fix race in PPS_FETCH handler |
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On Thu, Aug 05, 2010 at 02:19:51PM +0400, Alexander Gordeev wrote: > Hi Vitezslav, > > В Thu, 5 Aug 2010 07:19:29 +0200 > Vitezslav Samel <vitezslav@samel.cz> пишет: > > > 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. > > Yes, it will freeze the fds (if they don't use timeouts). But in normal > circumstances, i.e. when pps_event is called twice a second, it will > overflow after ~68 years of uninterrupted work. Well, it's the same > kind of problem as an overflow of struct timespec. I thought it's not > actually a problem. Should I use u64 instead of unsigned int or add a > runtime check somewhere?
If we're using 1PPS it's ~68 years, but someone is trying 5PPS now (it would overflow in ~13.6 years) - what if someone tries e.g. 100PPS? It's not the same as overflow of struct timespec! I think it deserves some treatment.
Cheers, Vita -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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