From owner-svn-src-head@FreeBSD.ORG Mon Dec 14 16:46:55 2009 Return-Path: Delivered-To: svn-src-head@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [IPv6:2001:4f8:fff6::34]) by hub.freebsd.org (Postfix) with ESMTP id BD02810656AE; Mon, 14 Dec 2009 16:46:55 +0000 (UTC) (envelope-from jhb@freebsd.org) Received: from cyrus.watson.org (cyrus.watson.org [65.122.17.42]) by mx1.freebsd.org (Postfix) with ESMTP id 8CD888FC1E; Mon, 14 Dec 2009 16:46:55 +0000 (UTC) Received: from bigwig.baldwin.cx (66.111.2.69.static.nyinternet.net [66.111.2.69]) by cyrus.watson.org (Postfix) with ESMTPSA id 2B93246B38; Mon, 14 Dec 2009 11:46:55 -0500 (EST) Received: from jhbbsd.localnet (unknown [209.249.190.9]) by bigwig.baldwin.cx (Postfix) with ESMTPA id 5AF718A025; Mon, 14 Dec 2009 11:46:54 -0500 (EST) From: John Baldwin To: Attilio Rao Date: Mon, 14 Dec 2009 10:13:32 -0500 User-Agent: KMail/1.12.1 (FreeBSD/7.2-CBSD-20091103; KDE/4.3.1; amd64; ; ) References: <200912122131.nBCLV71f064304@svn.freebsd.org> In-Reply-To: <200912122131.nBCLV71f064304@svn.freebsd.org> MIME-Version: 1.0 Content-Type: Text/Plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <200912141013.32839.jhb@freebsd.org> X-Greylist: Sender succeeded SMTP AUTH, not delayed by milter-greylist-4.0.1 (bigwig.baldwin.cx); Mon, 14 Dec 2009 11:46:54 -0500 (EST) X-Virus-Scanned: clamav-milter 0.95.1 at bigwig.baldwin.cx X-Virus-Status: Clean X-Spam-Status: No, score=-2.5 required=4.2 tests=AWL,BAYES_00,RDNS_NONE autolearn=no version=3.2.5 X-Spam-Checker-Version: SpamAssassin 3.2.5 (2008-06-10) on bigwig.baldwin.cx Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org Subject: Re: svn commit: r200447 - in head: share/man/man9 sys/kern sys/sys X-BeenThere: svn-src-head@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: SVN commit messages for the src tree for head/-current List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 14 Dec 2009 16:46:55 -0000 On Saturday 12 December 2009 4:31:07 pm Attilio Rao wrote: > Author: attilio > Date: Sat Dec 12 21:31:07 2009 > New Revision: 200447 > URL: http://svn.freebsd.org/changeset/base/200447 > > Log: > In current code, threads performing an interruptible sleep (on both > sxlock, via the sx_{s, x}lock_sig() interface, or plain lockmgr), will > leave the waiters flag on forcing the owner to do a wakeup even when if > the waiter queue is empty. > That operation may lead to a deadlock in the case of doing a fake wakeup > on the "preferred" (based on the wakeup algorithm) queue while the other > queue has real waiters on it, because nobody is going to wakeup the 2nd > queue waiters and they will sleep indefinitively. > > A similar bug, is present, for lockmgr in the case the waiters are > sleeping with LK_SLEEPFAIL on. In this case, even if the waiters queue > is not empty, the waiters won't progress after being awake but they will > just fail, still not taking care of the 2nd queue waiters (as instead the > lock owned doing the wakeup would expect). > > In order to fix this bug in a cheap way (without adding too much locking > and complicating too much the semantic) add a sleepqueue interface which > does report the actual number of waiters on a specified queue of a > waitchannel (sleepq_sleepcnt()) and use it in order to determine if the > exclusive waiters (or shared waiters) are actually present on the lockmgr > (or sx) before to give them precedence in the wakeup algorithm. > This fix alone, however doesn't solve the LK_SLEEPFAIL bug. In order to > cope with it, add the tracking of how many exclusive LK_SLEEPFAIL waiters > a lockmgr has and if all the waiters on the exclusive waiters queue are > LK_SLEEPFAIL just wake both queues. > > The sleepq_sleepcnt() introduction and ABI breakage require > __FreeBSD_version bumping. Hmm, do you need an actual count of waiters or would a 'sleepq_empty()' (similar to turnstile_empty()) method be sufficient? -- John Baldwin