pps: fix race in PPS_FETCH handler

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>
Acked-by: Rodolfo Giometti <giometti@linux.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 1 insertions, 1 deletions

diff --git a/include/linux/pps_kernel.h b/include/linux/pps_kernel.h
index 23f63c3263bb..65194fe498bb 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 */