pps: fix race in PPS_FETCH handler

3003d55b · Alexander Gordeev <[email protected]> · 2011-01-12 17:00:50 -08:00

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 <[email protected]>
Acked-by: Rodolfo Giometti <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Commit: 3003d55b59aa98aeaff2773df69732b27c0cbf6a
Parents: 86d921f9

Changes

3 files changed · +10 −6

File	Status	Add	Del	Content
drivers/pps/kapi.c	modified	+2	−2	View file
drivers/pps/pps.c	modified	+7	−3	View file
include/linux/pps_kernel.h	modified	+1	−1	View file

drivers/pps/kapi.c

View file

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);
 	}

drivers/pps/pps.c

View file

diff --git a/drivers/pps/pps.c b/drivers/pps/pps.c
index c76afb9..dc7e66c 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;
 			}

include/linux/pps_kernel.h

View file

diff --git a/include/linux/pps_kernel.h b/include/linux/pps_kernel.h
index 23f63c3..65194fe 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 */