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>

2 files changed, 19 insertions, 31 deletions

diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 069640e5b9c0..231b7a8710f1 100644
--- 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 5043b12f23ea..2e93eb831c61 100644
--- 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 */