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Subject: Re: [PATCH] new tty buffering locking fix
From: Paul Fulghum <paulkf@microgate.com>
To: Olaf Hering <olh@suse.de>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>, linux-kernel@vger.kernel.org
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Date: Tue, 07 Feb 2006 15:27:24 -0600
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Olaf:

Try this patch and let me know how it works.
(it is against 2.6.16-rc2-git1)

This change prevents claiming a partially filled
buffer if it has already been committed by the
driver calling a tty scheduling function, but not
yet processed by the tty layer.

Individual drivers can no longer stall committed
data by calling any of the tty buffering functions.
(example: calling tty_buffer_request_room
and not immediately adding receive data with an
associated tty scheduling call)

This remains relatively efficient. 
1. partially filled active buffer can be reused
2. already allocated empty buffer can be reused

Thanks,
Paul


--- linux-2.6.16-rc2/drivers/char/tty_io.c	2006-02-07 13:18:40.000000000 -0600
+++ b/drivers/char/tty_io.c	2006-02-07 13:46:28.000000000 -0600
@@ -324,8 +324,14 @@ int tty_buffer_request_room(struct tty_s
 	   remove this conditional if its worth it. This would be invisible
 	   to the callers */
 	if ((b = tty->buf.tail) != NULL) {
-		left = b->size - b->used;
-		b->active = 1;
+		if (!b->active) {
+			if (!b->used) {
+				left = b->size;
+				b->active = 1;
+			} else
+				left = 0;
+		} else
+			left = b->size - b->used;
 	} else
 		left = 0;