From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759825Ab1IKCiu (ORCPT ); Sat, 10 Sep 2011 22:38:50 -0400 Received: from zeniv.linux.org.uk ([195.92.253.2]:43904 "EHLO ZenIV.linux.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758946Ab1IKCit (ORCPT ); Sat, 10 Sep 2011 22:38:49 -0400 Date: Sun, 11 Sep 2011 03:38:40 +0100 From: Al Viro To: Vladislav Bolkhovitin Cc: Peter Zijlstra , Ingo Molnar , linux-kernel@vger.kernel.org Subject: Re: Lockdep and rw_semaphores Message-ID: <20110911023840.GZ2203@ZenIV.linux.org.uk> References: <4E6C1016.8030703@vlnb.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <4E6C1016.8030703@vlnb.net> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Sep 10, 2011 at 09:34:14PM -0400, Vladislav Bolkhovitin wrote: > Hello, > > Looks like lockdep somehow over-restrictive for rw_semaphores in case when they > are taken for read (down_read()) and requires them to follow the same inner-outer > rules as for plain locks. > > For instance, code like: > > DECLARE_RWSEM(QQ_sem); > DECLARE_RWSEM(QQ1_sem); > > thread1: > > down_read(&QQ_sem); > down_read(&QQ1_sem); > thread2: > > down_read(&QQ1_sem); > down_read(&QQ_sem); > Is it by design or just something overlooked? I don't see how reverse order of > down_read()'s can lead to any deadlock. Or am I missing anything? thread1: got QQ thread2: got QQ1 thread3: tries to do down_write() on QQ, gets blocked thread4: tries to do down_write() on QQ1, gets blocked Now we have thread1 that can't get QQ1 once the threads trying to get it exclusive get a shot at it. Thread2 is blocked in the same way on QQ. And neither is going to release the (shared) lock they are holding, so thread3 and thread4 are not going to get anywhere either. IOW, ordering *is* needed there. Note that for the same reason trying to grab the same lock shared twice is a deadlock: A: already holds X shared B: blocks trying to grab it exclusive A: tries to grab it shared again and gets stuck, since there is a pending down_write() and we are guaranteed that writer will get served as soon as all current readers are through; no new readers are allowed to starve it.