From owner-freebsd-hackers@FreeBSD.ORG  Fri Jun  3 13:27:18 2005
Return-Path: <owner-freebsd-hackers@FreeBSD.ORG>
X-Original-To: freebsd-hackers@freebsd.org
Delivered-To: freebsd-hackers@freebsd.org
Received: from mx1.FreeBSD.org (mx1.freebsd.org [216.136.204.125])
	by hub.freebsd.org (Postfix) with ESMTP id 50B7616A41C
	for <freebsd-hackers@freebsd.org>; Fri,  3 Jun 2005 13:27:18 +0000 (GMT)
	(envelope-from NKoch@demig.de)
Received: from server.absolute-media.de (server.absolute-media.de
	[213.239.231.9])
	by mx1.FreeBSD.org (Postfix) with ESMTP id E431543D49
	for <freebsd-hackers@freebsd.org>; Fri,  3 Jun 2005 13:27:17 +0000 (GMT)
	(envelope-from NKoch@demig.de)
Received: from localhost (unknown [127.0.0.1])
	by server.absolute-media.de (Postfix) with ESMTP id 642C97F30D
	for <freebsd-hackers@freebsd.org>;
	Fri,  3 Jun 2005 15:27:15 +0200 (CEST)
Received: from server.absolute-media.de ([127.0.0.1])
	by localhost (server [127.0.0.1]) (amavisd-new, port 10024) with ESMTP
	id 23756-10 for <freebsd-hackers@freebsd.org>;
	Fri,  3 Jun 2005 15:27:10 +0200 (CEST)
Received: from firewall.demig (p50838965.dip0.t-ipconnect.de [80.131.137.101])
	by server.absolute-media.de (Postfix) with ESMTP id 97FB57982E
	for <freebsd-hackers@freebsd.org>;
	Fri,  3 Jun 2005 15:27:10 +0200 (CEST)
Received: from ws-ew-3 (ws-ew-3.w2kdemig [192.168.1.72])
	by firewall.demig (8.13.4/8.13.1) with SMTP id j53DOKjg047520
	for <freebsd-hackers@freebsd.org>; Fri, 3 Jun 2005 15:24:20 +0200 (CEST)
	(envelope-from NKoch@demig.de)
From: "Norbert Koch" <NKoch@demig.de>
To: "Freebsd-Hackers@Freebsd. Org" <freebsd-hackers@freebsd.org>
Date: Fri, 3 Jun 2005 15:24:19 +0200
Message-ID: <000001c5683f$8c7c1e60$4801a8c0@ws-ew-3.W2KDEMIG>
MIME-Version: 1.0
Content-Type: text/plain;
	charset="iso-8859-1"
Content-Transfer-Encoding: 7bit
X-Priority: 3 (Normal)
X-MSMail-Priority: Normal
X-Mailer: Microsoft Outlook 8.5, Build 4.71.2173.0
Importance: Normal
X-MimeOLE: Produced By Microsoft MimeOLE V4.72.2120.0
X-Virus-Scanned: by amavisd-new
X-Virus-Scanned: by amavisd-new at absolute-media.de
Subject: synchronization question about /sys/dev/vkbd/vkbd.c
X-BeenThere: freebsd-hackers@freebsd.org
X-Mailman-Version: 2.1.5
Precedence: list
List-Id: Technical Discussions relating to FreeBSD
	<freebsd-hackers.freebsd.org>
List-Unsubscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-hackers>, 
	<mailto:freebsd-hackers-request@freebsd.org?subject=unsubscribe>
List-Archive: <http://lists.freebsd.org/pipermail/freebsd-hackers>
List-Post: <mailto:freebsd-hackers@freebsd.org>
List-Help: <mailto:freebsd-hackers-request@freebsd.org?subject=help>
List-Subscribe: <http://lists.freebsd.org/mailman/listinfo/freebsd-hackers>,
	<mailto:freebsd-hackers-request@freebsd.org?subject=subscribe>
X-List-Received-Date: Fri, 03 Jun 2005 13:27:18 -0000

Hello.

When looking at /sys/dev/vkbd/vkbd.c I found
one thing, that I do not understand.

There are three places, where a flag TASK is used:

1. in vkbd_dev_close():

  while(state->ks_flag & TASK) VKBD_SLEEP (...);

2. in vkbd_dev_write()

  VKBD_LOCK ();
  ...
  if (!(state->ks_flags & TASK) && task_enqueue(...))
    state->ks_flags |= TASK;
  ...
  VKBD_UNLOCK ();

3. in vkbd_dev_intr()

  ...
  /* call intr */
  ...
  VKBD_LOCK();
  state->ks_flags &= ~TASK;
  wakeup(...);
  VKBD_UNLOCK();

As I understand:
vkbd_dev_write() writes data into its queue
and wants vkbd_dev_intr() to process the queue.

My question is:
Is it not possible, that vkbd_dev_intr() could be
interrupted at any position before the VKBD_LOCK()
and then vkbd_dev_write() called?
If yes, how should vkbd_dev_write() know, that it should
call task_enqueue(), as TASK is still set?

Why not always call task_enqueue() unconditionally here
and leave TASK only to synchronize the close call?

Thank you for any help,

Norbert Koch