tty: Use lockless flip buffer free list

In preparation for lockless flip buffers, make the flip buffer
free list lockless.

NB: using llist is not the optimal solution, as the driver and
buffer work may contend over the llist head unnecessarily. However,
test measurements indicate this contention is low.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 12 insertions, 17 deletions

diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 0259a76..069640e 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -44,16 +44,17 @@ static void tty_buffer_reset(struct tty_buffer *p, size_t size)
 void tty_buffer_free_all(struct tty_port *port)
 {
 	struct tty_bufhead *buf = &port->buf;
-	struct tty_buffer *p;
+	struct tty_buffer *p, *next;
+	struct llist_node *llist;
 
 	while ((p = buf->head) != NULL) {
 		buf->head = p->next;
 		kfree(p);
 	}
-	while ((p = buf->free) != NULL) {
-		buf->free = p->next;
+	llist = llist_del_all(&buf->free);
+	llist_for_each_entry_safe(p, next, llist, free)
 		kfree(p);
-	}
+
 	buf->tail = NULL;
 	buf->memory_used = 0;
 }
@@ -68,22 +69,20 @@ void tty_buffer_free_all(struct tty_port *port)
  *	allocation behaviour.
  *	Return NULL if out of memory or the allocation would exceed the
  *	per device queue
- *
- *	Locking: Caller must hold tty->buf.lock
  */
 
 static struct tty_buffer *tty_buffer_alloc(struct tty_port *port, size_t size)
 {
-	struct tty_buffer **tbh = &port->buf.free;
+	struct llist_node *free;
 	struct tty_buffer *p;
 
 	/* Round the buffer size out */
 	size = __ALIGN_MASK(size, TTYB_ALIGN_MASK);
 
 	if (size <= MIN_TTYB_SIZE) {
-		if (*tbh) {
-			p = *tbh;
-			*tbh = p->next;
+		free = llist_del_first(&port->buf.free);
+		if (free) {
+			p = llist_entry(free, struct tty_buffer, free);
 			goto found;
 		}
 	}
@@ -109,8 +108,6 @@ found:
  *
  *	Free a tty buffer, or add it to the free list according to our
  *	internal strategy
- *
- *	Locking: Caller must hold tty->buf.lock
  */
 
 static void tty_buffer_free(struct tty_port *port, struct tty_buffer *b)
@@ -123,10 +120,8 @@ static void tty_buffer_free(struct tty_port *port, struct tty_buffer *b)
 
 	if (b->size > MIN_TTYB_SIZE)
 		kfree(b);
-	else {
-		b->next = buf->free;
-		buf->free = b;
-	}
+	else
+		llist_add(&b->free, &buf->free);
 }
 
 /**
@@ -542,7 +537,7 @@ void tty_buffer_init(struct tty_port *port)
 	spin_lock_init(&buf->lock);
 	buf->head = NULL;
 	buf->tail = NULL;
-	buf->free = NULL;
+	init_llist_head(&buf->free);
 	buf->memory_used = 0;
 	INIT_WORK(&buf->work, flush_to_ldisc);
 }