Lockless flip buffers require atomically updating the bytes-in-use
watermark.
The pty driver also peeks at the watermark value to limit
memory consumption to a much lower value than the default; query
the watermark with new fn, tty_buffer_space_avail().
Signed-off-by: Peter Hurley <peter@xxxxxxxxxxxxxxxxxx>
---
drivers/tty/pty.c | 10 +++-------
drivers/tty/tty_buffer.c | 37 +++++++++++++++++++++++++++++++------
include/linux/tty.h | 3 +--
include/linux/tty_flip.h | 1 +
4 files changed, 36 insertions(+), 15 deletions(-)
diff --git a/drivers/tty/pty.c b/drivers/tty/pty.c
index bbd9693..0634dd9 100644
--- a/drivers/tty/pty.c
+++ b/drivers/tty/pty.c
@@ -89,17 +89,13 @@ static void pty_unthrottle(struct tty_struct *tty)
* pty_space - report space left for writing
* @to: tty we are writing into
*
- * The tty buffers allow 64K but we sneak a peak and clip at 8K this
- * allows a lot of overspill room for echo and other fun messes to
- * be handled properly
+ * Limit the buffer space used by ptys to 8k.
*/
static int pty_space(struct tty_struct *to)
{
- int n = 8192 - to->port->buf.memory_used;
- if (n < 0)
- return 0;
- return n;
+ int n = tty_buffer_space_avail(to->port);
+ return min(n, 8192);
}
/**
diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 231b7a8..5d5a564 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -22,6 +22,31 @@
#define MIN_TTYB_SIZE 256
#define TTYB_ALIGN_MASK 255
+/*
+ * Byte threshold to limit memory consumption for flip buffers.
+ * The actual memory limit is > 2x this amount.
+ */
+#define TTYB_MEM_LIMIT 65536
+
+
+/**
+ * tty_buffer_space_avail - return unused buffer space
+ * @port - tty_port owning the flip buffer
+ *
+ * Returns the # of bytes which can be written by the driver without
+ * reaching the buffer limit.
+ *
+ * Note: this does not guarantee that memory is available to write
+ * the returned # of bytes (use tty_prepare_flip_string_xxx() to
+ * pre-allocate if memory guarantee is required).
+ */
+
+int tty_buffer_space_avail(struct tty_port *port)
+{
+ int space = TTYB_MEM_LIMIT - atomic_read(&port->buf.memory_used);
+ return max(space, 0);
+}
+
static void tty_buffer_reset(struct tty_buffer *p, size_t size)
{
p->used = 0;
@@ -59,7 +84,8 @@ void tty_buffer_free_all(struct tty_port *port)
tty_buffer_reset(&buf->sentinel, 0);
buf->head = &buf->sentinel;
buf->tail = &buf->sentinel;
- buf->memory_used = 0;
+
+ atomic_set(&buf->memory_used, 0);
}
/**
@@ -92,7 +118,7 @@ static struct tty_buffer *tty_buffer_alloc(struct tty_port *port, size_t size)
/* Should possibly check if this fails for the largest buffer we
have queued and recycle that ? */
- if (port->buf.memory_used + size > 65536)
+ if (atomic_read(&port->buf.memory_used) > TTYB_MEM_LIMIT)
return NULL;
p = kmalloc(sizeof(struct tty_buffer) + 2 * size, GFP_ATOMIC);
if (p == NULL)
@@ -100,7 +126,7 @@ static struct tty_buffer *tty_buffer_alloc(struct tty_port *port, size_t size)
found:
tty_buffer_reset(p, size);
- port->buf.memory_used += size;
+ atomic_add(size, &port->buf.memory_used);
return p;
}
@@ -118,8 +144,7 @@ static void tty_buffer_free(struct tty_port *port, struct tty_buffer *b)
struct tty_bufhead *buf = &port->buf;
/* Dumb strategy for now - should keep some stats */
- buf->memory_used -= b->size;
- WARN_ON(buf->memory_used < 0);
+ WARN_ON(atomic_sub_return(b->size, &buf->memory_used) < 0);
if (b->size > MIN_TTYB_SIZE)
kfree(b);
@@ -525,7 +550,7 @@ void tty_buffer_init(struct tty_port *port)
buf->head = &buf->sentinel;
buf->tail = &buf->sentinel;
init_llist_head(&buf->free);
- buf->memory_used = 0;
+ atomic_set(&buf->memory_used, 0);
INIT_WORK(&buf->work, flush_to_ldisc);
}
diff --git a/include/linux/tty.h b/include/linux/tty.h
index 2e93eb8..7c12454 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -71,8 +71,7 @@ struct tty_bufhead {
struct tty_buffer *head; /* Queue head */
struct tty_buffer *tail; /* Active buffer */
struct llist_head free; /* Free queue head */
- int memory_used; /* Buffer space used excluding
- free queue */
+ atomic_t memory_used; /* In-use buffers excluding free list */
};
/*
* When a break, frame error, or parity error happens, these codes are
diff --git a/include/linux/tty_flip.h b/include/linux/tty_flip.h
index ad03039..6944ed2 100644
--- a/include/linux/tty_flip.h
+++ b/include/linux/tty_flip.h
@@ -1,6 +1,7 @@
#ifndef _LINUX_TTY_FLIP_H
#define _LINUX_TTY_FLIP_H
+extern int tty_buffer_space_avail(struct tty_port *port);
extern int tty_buffer_request_room(struct tty_port *port, size_t size);
extern int tty_insert_flip_string_flags(struct tty_port *port,
const unsigned char *chars, const char *flags, size_t size);
--
1.8.1.2
--
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