tty: Simplify flip buffer list with 0-sized sentinel

Use a 0-sized sentinel to avoid assigning the head ptr from
the driver side thread. This also eliminates testing head/tail
for NULL.

When the sentinel is first 'consumed' by the buffer work
(or by tty_buffer_flush()), it is detached from the list but not
freed nor added to the free list. Both buffer work and
tty_buffer_flush() continue to preserve at least 1 flip buffer
to which head & tail is pointed.

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

diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 069640e..231b7a8 100644 (file)
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -49,13 +49,16 @@ void tty_buffer_free_all(struct tty_port *port)
 
        while ((p = buf->head) != NULL) {
                buf->head = p->next;
-               kfree(p);
+               if (p->size > 0)
+                       kfree(p);
        }
        llist = llist_del_all(&buf->free);
        llist_for_each_entry_safe(p, next, llist, free)
                kfree(p);
 
-       buf->tail = NULL;
+       tty_buffer_reset(&buf->sentinel, 0);
+       buf->head = &buf->sentinel;
+       buf->tail = &buf->sentinel;
        buf->memory_used = 0;
 }
 
@@ -120,7 +123,7 @@ static void tty_buffer_free(struct tty_port *port, struct tty_buffer *b)
 
        if (b->size > MIN_TTYB_SIZE)
                kfree(b);
-       else
+       else if (b->size > 0)
                llist_add(&b->free, &buf->free);
 }
 
@@ -140,8 +143,6 @@ static void __tty_buffer_flush(struct tty_port *port)
        struct tty_bufhead *buf = &port->buf;
        struct tty_buffer *next;
 
-       if (unlikely(buf->head == NULL))
-               return;
        while ((next = buf->head->next) != NULL) {
                tty_buffer_free(port, buf->head);
                buf->head = next;
@@ -200,23 +201,14 @@ int tty_buffer_request_room(struct tty_port *port, size_t size)
        int left;
        unsigned long flags;
        spin_lock_irqsave(&buf->lock, flags);
-       /* OPTIMISATION: We could keep a per tty "zero" sized buffer to
-          remove this conditional if its worth it. This would be invisible
-          to the callers */
        b = buf->tail;
-       if (b != NULL)
-               left = b->size - b->used;
-       else
-               left = 0;
+       left = b->size - b->used;
 
        if (left < size) {
                /* This is the slow path - looking for new buffers to use */
                if ((n = tty_buffer_alloc(port, size)) != NULL) {
-                       if (b != NULL) {
-                               b->next = n;
-                               b->commit = b->used;
-                       } else
-                               buf->head = n;
+                       b->next = n;
+                       b->commit = b->used;
                        buf->tail = n;
                } else
                        size = left;
@@ -247,10 +239,8 @@ int tty_insert_flip_string_fixed_flag(struct tty_port *port,
                int goal = min_t(size_t, size - copied, TTY_BUFFER_PAGE);
                int space = tty_buffer_request_room(port, goal);
                struct tty_buffer *tb = port->buf.tail;
-               /* If there is no space then tb may be NULL */
-               if (unlikely(space == 0)) {
+               if (unlikely(space == 0))
                        break;
-               }
                memcpy(char_buf_ptr(tb, tb->used), chars, space);
                memset(flag_buf_ptr(tb, tb->used), flag, space);
                tb->used += space;
@@ -285,10 +275,8 @@ int tty_insert_flip_string_flags(struct tty_port *port,
                int goal = min_t(size_t, size - copied, TTY_BUFFER_PAGE);
                int space = tty_buffer_request_room(port, goal);
                struct tty_buffer *tb = port->buf.tail;
-               /* If there is no space then tb may be NULL */
-               if (unlikely(space == 0)) {
+               if (unlikely(space == 0))
                        break;
-               }
                memcpy(char_buf_ptr(tb, tb->used), chars, space);
                memcpy(flag_buf_ptr(tb, tb->used), flags, space);
                tb->used += space;
@@ -322,8 +310,7 @@ void tty_schedule_flip(struct tty_port *port)
        WARN_ON(port->low_latency);
 
        spin_lock_irqsave(&buf->lock, flags);
-       if (buf->tail != NULL)
-               buf->tail->commit = buf->tail->used;
+       buf->tail->commit = buf->tail->used;
        spin_unlock_irqrestore(&buf->lock, flags);
        schedule_work(&buf->work);
 }
@@ -438,8 +425,8 @@ static void flush_to_ldisc(struct work_struct *work)
        spin_lock_irqsave(&buf->lock, flags);
 
        if (!test_and_set_bit(TTYP_FLUSHING, &port->iflags)) {
-               struct tty_buffer *head;
-               while ((head = buf->head) != NULL) {
+               while (1) {
+                       struct tty_buffer *head = buf->head;
                        int count;
 
                        count = head->commit - head->read;
@@ -509,8 +496,7 @@ void tty_flip_buffer_push(struct tty_port *port)
        unsigned long flags;
 
        spin_lock_irqsave(&buf->lock, flags);
-       if (buf->tail != NULL)
-               buf->tail->commit = buf->tail->used;
+       buf->tail->commit = buf->tail->used;
        spin_unlock_irqrestore(&buf->lock, flags);
 
        if (port->low_latency)
@@ -535,8 +521,9 @@ void tty_buffer_init(struct tty_port *port)
        struct tty_bufhead *buf = &port->buf;
 
        spin_lock_init(&buf->lock);
-       buf->head = NULL;
-       buf->tail = NULL;
+       tty_buffer_reset(&buf->sentinel, 0);
+       buf->head = &buf->sentinel;
+       buf->tail = &buf->sentinel;
        init_llist_head(&buf->free);
        buf->memory_used = 0;
        INIT_WORK(&buf->work, flush_to_ldisc);

diff --git a/include/linux/tty.h b/include/linux/tty.h
index 5043b12..2e93eb8 100644 (file)
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -67,6 +67,7 @@ static inline char *flag_buf_ptr(struct tty_buffer *b, int ofs)
 struct tty_bufhead {
        struct work_struct work;
        spinlock_t lock;
+       struct tty_buffer sentinel;
        struct tty_buffer *head;        /* Queue head */
        struct tty_buffer *tail;        /* Active buffer */
        struct llist_head free;         /* Free queue head */