Commit 809850b7 authored by Peter Hurley's avatar Peter Hurley Committed by Greg Kroah-Hartman
Browse files

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: default avatarPeter Hurley <peter@hurleysoftware.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 2cf7b67e

drivers/tty/tty_buffer.c

+12 −17
Original line number Diff line number Diff line
@@ -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 @@ static struct tty_buffer *tty_buffer_alloc(struct tty_port *port, size_t size) 
 *

 *	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);

}

include/linux/llist.h

+23 −0
Original line number Diff line number Diff line
@@ -124,6 +124,29 @@ static inline void init_llist_head(struct llist_head *list) 
	     &(pos)->member != NULL;					\

	     (pos) = llist_entry((pos)->member.next, typeof(*(pos)), member))



/**

 * llist_for_each_entry_safe - iterate over some deleted entries of lock-less list of given type

 *			       safe against removal of list entry

 * @pos:	the type * to use as a loop cursor.

 * @n:		another type * to use as temporary storage

 * @node:	the first entry of deleted list entries.

 * @member:	the name of the llist_node with the struct.

 *

 * In general, some entries of the lock-less list can be traversed

 * safely only after being removed from list, so start with an entry

 * instead of list head.

 *

 * If being used on entries deleted from lock-less list directly, the

 * traverse order is from the newest to the oldest added entry.  If

 * you want to traverse from the oldest to the newest, you must

 * reverse the order by yourself before traversing.

 */

#define llist_for_each_entry_safe(pos, n, node, member)			       \

	for (pos = llist_entry((node), typeof(*pos), member);		       \

	     &pos->member != NULL &&					       \

	        (n = llist_entry(pos->member.next, typeof(*n), member), true); \

	     pos = n)



/**

 * llist_empty - tests whether a lock-less list is empty

 * @head:	the list to test

include/linux/tty.h

+6 −2
Original line number Diff line number Diff line
@@ -11,6 +11,7 @@ 
#include <linux/tty_flags.h>

#include <uapi/linux/tty.h>

#include <linux/rwsem.h>

#include <linux/llist.h>
@@ -30,7 +31,10 @@ 
#define __DISABLED_CHAR '\0'



struct tty_buffer {

	union {

		struct tty_buffer *next;

		struct llist_node free;

	};

	int used;

	int size;

	int commit;
@@ -65,7 +69,7 @@ struct tty_bufhead { 
	spinlock_t lock;

	struct tty_buffer *head;	/* Queue head */

	struct tty_buffer *tail;	/* Active buffer */

	struct tty_buffer *free;	/* Free queue head */

	struct llist_head free;		/* Free queue head */

	int memory_used;		/* Buffer space used excluding

								free queue */

};