From owner-freebsd-hackers@FreeBSD.ORG Fri Jul 18 00:18:10 2008 Return-Path: Delivered-To: freebsd-hackers@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id 0E977106567A for ; Fri, 18 Jul 2008 00:18:10 +0000 (UTC) (envelope-from jhb@freebsd.org) Received: from server.baldwin.cx (bigknife-pt.tunnel.tserv9.chi1.ipv6.he.net [IPv6:2001:470:1f10:75::2]) by mx1.freebsd.org (Postfix) with ESMTP id 5E8358FC14 for ; Fri, 18 Jul 2008 00:18:09 +0000 (UTC) (envelope-from jhb@freebsd.org) Received: from zion.baldwin.cx (zion.baldwin.cx [IPv6:2001:470:1f11:75:2a0:d2ff:fe18:8b38]) (authenticated bits=0) by server.baldwin.cx (8.14.2/8.14.2) with ESMTP id m6I0HuYS038965; Thu, 17 Jul 2008 20:18:03 -0400 (EDT) (envelope-from jhb@freebsd.org) From: John Baldwin To: "Michael B Allen" Date: Thu, 17 Jul 2008 20:15:11 -0400 User-Agent: KMail/1.9.7 References: <78c6bd860807121611w4f6ab44brbebfffea9929682a@mail.gmail.com> <200807171005.53148.jhb@freebsd.org> <78c6bd860807171042o54627c78nfcc0c19717b75f1e@mail.gmail.com> In-Reply-To: <78c6bd860807171042o54627c78nfcc0c19717b75f1e@mail.gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Content-Disposition: inline Message-Id: <200807172015.11460.jhb@freebsd.org> X-Greylist: Sender succeeded SMTP AUTH authentication, not delayed by milter-greylist-2.0.2 (server.baldwin.cx [IPv6:2001:470:1f11:75::1]); Thu, 17 Jul 2008 20:18:03 -0400 (EDT) X-Virus-Scanned: ClamAV 0.93.1/7742/Thu Jul 17 19:22:26 2008 on server.baldwin.cx X-Virus-Status: Clean X-Spam-Status: No, score=-2.6 required=4.2 tests=AWL,BAYES_00,NO_RELAYS autolearn=ham version=3.1.3 X-Spam-Checker-Version: SpamAssassin 3.1.3 (2006-06-01) on server.baldwin.cx Cc: freebsd-hackers@freebsd.org Subject: Re: Pls sanity check my semtimedop(2) implementation X-BeenThere: freebsd-hackers@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Technical Discussions relating to FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Fri, 18 Jul 2008 00:18:10 -0000 On Thursday 17 July 2008 01:42:31 pm Michael B Allen wrote: > On Thu, Jul 17, 2008 at 10:05 AM, John Baldwin wrote: > > On Saturday 12 July 2008 07:11:26 pm Michael B Allen wrote: > >> Hi, > >> > >> Below is a semtimedop(2) implementation that I'm using for FreeBSD. I > >> was hoping someone could look it over and tell me if they think the > >> implementation is sound. > >> > >> The code seems to work ok but when stressing the FreeBSD build of my app > >> I have managed to provoke errors related to concurrency (usually when a > >> SIGALRM goes off). The Linux build works flawlessesly so I'm wondering > >> about this one critical function that is different. > >> > >> Do you think it would make any difference if I used > >> ITIMER_VIRTUAL / SIGVTALRM instead of ITIMER_REAL / SIGALRM? > >> > >> Or perhaps I should be using a different implementation entirely? > > > > What specific races are you seeing? The timer is firing too early, too > > late? > > It's very difficult to tell. I can only trigger the issue very > occasionally running my torture test such that any diagnostic logging > changes the results. > > And at this point I'm not sure my semtimedop implementation is > responsible. I have not seen the issue since fixing the race pointed > out by Mikko (although I have not tried very hard to provoke it). > > For now, I'm satisfied since I do not think the issue will be > triggered in the wild. I hate to use signals for anything but as much > as I try, there's just no other way to implement semtimedop within a > single largely self-contained function. In the future I will likely > use another process in the application that uses select(2) as an > "event service" to post on semaphores after a certain time period. > Unfortunately, right now, that service ultimately calls semtimedop so > I'll save it for a rainy day. > > Although if you implemented semtimedop(2) into the FreeBSD API that > would work too :-) POSIX semaphores (sem_open(3), sem_create(3), etc.) do have a sem_timedwait(3). However, POSIX semaphores have several bugs in 6.x and 7.x (they should work a lot better in 8). If you want I can give you a patch for 6.x or 7.x that backports the 8.x POSIX semaphores. -- John Baldwin