tty: Track flip buffer memory limit atomically
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@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 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 @@
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 @@
/* 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 @@
found:
tty_buffer_reset(p, size);
- port->buf.memory_used += size;
+ atomic_add(size, &port->buf.memory_used);
return p;
}
@@ -118,8 +144,7 @@
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 @@
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);
}