sparc64: Avoid irqsave/restore on vio.lock if in_softirq()

For NAPIfied drivers , there is no need to synchronize by doing irqsave/restore on vio.lock in the I/O path.

---
 arch/sparc/kernel/viohs.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

index 526fcb5..92e2a32 100644
@@ -747,10 +747,11 @@ EXPORT_SYMBOL(vio_ldc_free);
 
 void vio_port_up(struct vio_driver_state *vio)
 {
-	unsigned long flags;
+	unsigned long flags = 0;
 	int err, state;
 
-	spin_lock_irqsave(&vio->lock, flags);
+	if (!in_softirq())
+		spin_lock_irqsave(&vio->lock, flags);
 
 	state = ldc_state(vio->lp);
 

@@ -777,7 +778,8 @@ void vio_port_up(struct vio_driver_state *vio)
 		mod_timer(&vio->timer, expires);
 	}
 
-	spin_unlock_irqrestore(&vio->lock, flags);
+	if (!in_softirq())
+		spin_unlock_irqrestore(&vio->lock, flags);
 }
 EXPORT_SYMBOL(vio_port_up);
 

1.8.4.2

From: Sowmini Varadhan <[email protected]>
Date: Wed, 22 Oct 2014 18:12:45 -0400

Conditional locking is broken locking, and the bug you introduce here is a good example of why that is.

If the vio->timer has to be triggered when vio_port_up() is invoked, it will next run fron in_softirq() context regardless of whether the user is sunvnet or sunvdc. So it will elide the locking regardless of who is using this vio context.

Never, ever, use conditional locking.

This locking is harmless overhead in a slow path, just leave it alone for now.