[PATCH v2 2/3] pps: convert pps_device lock to raw_spinlock for PREEMPT_RT

[Michael Byczkowski]

On PREEMPT_RT, spinlock_t becomes a sleeping mutex, which allows
pps_event() to be preempted mid-update by other RT threads. This
introduces jitter in the PPS event recording path.

Convert pps_device.lock to raw_spinlock_t so the timestamp and
sequence number updates remain non-preemptible on RT.

On non-RT kernels, raw_spinlock_t compiles to identical code as
spinlock_t — no behavioral change.

Signed-off-by: Michael Byczkowski <by@xxxxxxxxxxxx>
Acked-by: Rodolfo Giometti <giometti@xxxxxxxxxxxx>
Tested-by: Michael Byczkowski <by@xxxxxxxxxxxx>
Tested-by: Calvin Owens <calvin@xxxxxxxxxx>
---
 drivers/pps/kapi.c         |  6 +++---
 drivers/pps/pps.c          | 16 ++++++++--------
 include/linux/pps_kernel.h |  2 +-
 3 files changed, 12 insertions(+), 12 deletions(-)

diff --git a/drivers/pps/kapi.c b/drivers/pps/kapi.c
index 1bf0335a1b41..dc7fac75ec27 100644
--- a/drivers/pps/kapi.c
+++ b/drivers/pps/kapi.c
@@ -102,7 +102,7 @@ struct pps_device *pps_register_source(struct pps_source_info *info,
 		pps->info.echo = pps_echo_client_default;
 
 	init_waitqueue_head(&pps->queue);
-	spin_lock_init(&pps->lock);
+	raw_spin_lock_init(&pps->lock);
 
 	/* Create the char device */
 	err = pps_register_cdev(pps);
@@ -167,7 +167,7 @@ void pps_event(struct pps_device *pps, struct pps_event_time *ts, int event,
 
 	timespec_to_pps_ktime(&ts_real, ts->ts_real);
 
-	spin_lock_irqsave(&pps->lock, flags);
+	raw_spin_lock_irqsave(&pps->lock, flags);
 
 	/* Must call the echo function? */
 	if ((pps->params.mode & (PPS_ECHOASSERT | PPS_ECHOCLEAR)))
@@ -214,6 +214,6 @@ void pps_event(struct pps_device *pps, struct pps_event_time *ts, int event,
 		kill_fasync(&pps->async_queue, SIGIO, POLL_IN);
 	}
 
-	spin_unlock_irqrestore(&pps->lock, flags);
+	raw_spin_unlock_irqrestore(&pps->lock, flags);
 }
 EXPORT_SYMBOL(pps_event);
diff --git a/drivers/pps/pps.c b/drivers/pps/pps.c
index c6b8b6478276..ad96425208a1 100644
--- a/drivers/pps/pps.c
+++ b/drivers/pps/pps.c
@@ -103,12 +103,12 @@ static long pps_cdev_ioctl(struct file *file,
 	case PPS_GETPARAMS:
 		dev_dbg(&pps->dev, "PPS_GETPARAMS\n");
 
-		spin_lock_irq(&pps->lock);
+		raw_spin_lock_irq(&pps->lock);
 
 		/* Get the current parameters */
 		params = pps->params;
 
-		spin_unlock_irq(&pps->lock);
+		raw_spin_unlock_irq(&pps->lock);
 
 		err = copy_to_user(uarg, &params, sizeof(struct pps_kparams));
 		if (err)
@@ -139,7 +139,7 @@ static long pps_cdev_ioctl(struct file *file,
 			return -EINVAL;
 		}
 
-		spin_lock_irq(&pps->lock);
+		raw_spin_lock_irq(&pps->lock);
 
 		/* Save the new parameters */
 		pps->params = params;
@@ -163,7 +163,7 @@ static long pps_cdev_ioctl(struct file *file,
 		pps->params.assert_off_tu.flags = 0;
 		pps->params.clear_off_tu.flags = 0;
 
-		spin_unlock_irq(&pps->lock);
+		raw_spin_unlock_irq(&pps->lock);
 
 		break;
 
@@ -190,7 +190,7 @@ static long pps_cdev_ioctl(struct file *file,
 			return err;
 
 		/* Return the fetched timestamp and save last fetched event  */
-		spin_lock_irq(&pps->lock);
+		raw_spin_lock_irq(&pps->lock);
 
 		pps->last_fetched_ev = pps->last_ev;
 
@@ -200,7 +200,7 @@ static long pps_cdev_ioctl(struct file *file,
 		fdata.info.clear_tu = pps->clear_tu;
 		fdata.info.current_mode = pps->current_mode;
 
-		spin_unlock_irq(&pps->lock);
+		raw_spin_unlock_irq(&pps->lock);
 
 		err = copy_to_user(uarg, &fdata, sizeof(struct pps_fdata));
 		if (err)
@@ -278,7 +278,7 @@ static long pps_cdev_compat_ioctl(struct file *file,
 			return err;
 
 		/* Return the fetched timestamp and save last fetched event  */
-		spin_lock_irq(&pps->lock);
+		raw_spin_lock_irq(&pps->lock);
 
 		pps->last_fetched_ev = pps->last_ev;
 
@@ -291,7 +291,7 @@ static long pps_cdev_compat_ioctl(struct file *file,
 		memcpy(&compat.info.clear_tu, &pps->clear_tu,
 				sizeof(struct pps_ktime_compat));
 
-		spin_unlock_irq(&pps->lock);
+		raw_spin_unlock_irq(&pps->lock);
 
 		return copy_to_user(uarg, &compat,
 				sizeof(struct pps_fdata_compat)) ? -EFAULT : 0;
diff --git a/include/linux/pps_kernel.h b/include/linux/pps_kernel.h
index aab0aebb529e..f2fe504071ed 100644
--- a/include/linux/pps_kernel.h
+++ b/include/linux/pps_kernel.h
@@ -59,7 +59,7 @@ struct pps_device {
 	void const *lookup_cookie;		/* For pps_lookup_dev() only */
 	struct device dev;
 	struct fasync_struct *async_queue;	/* fasync method */
-	spinlock_t lock;
+	raw_spinlock_t lock;
 };
 
 /*
-- 
2.47.3