From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:49597) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dezGX-0007FD-JP for qemu-devel@nongnu.org; Tue, 08 Aug 2017 03:49:26 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1dezGT-0007ZM-J9 for qemu-devel@nongnu.org; Tue, 08 Aug 2017 03:49:25 -0400 Received: from mx1.redhat.com ([209.132.183.28]:57124) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1dezGT-0007Yk-DM for qemu-devel@nongnu.org; Tue, 08 Aug 2017 03:49:21 -0400 Received: from smtp.corp.redhat.com (int-mx03.intmail.prod.int.phx2.redhat.com [10.5.11.13]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 21F7B61463 for ; Tue, 8 Aug 2017 07:49:20 +0000 (UTC) Date: Tue, 8 Aug 2017 15:49:15 +0800 From: Peter Xu Message-ID: <20170808074915.GB13486@pxdev.xzpeter.org> References: <1502175607-9531-1-git-send-email-peterx@redhat.com> <3f0e8fe6-babd-df82-c690-de0f1ffdd460@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <3f0e8fe6-babd-df82-c690-de0f1ffdd460@redhat.com> Subject: Re: [Qemu-devel] [PATCH for-2.11] rcu: init globals only once List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Paolo Bonzini Cc: qemu-devel@nongnu.org, Eric Blake On Tue, Aug 08, 2017 at 09:26:43AM +0200, Paolo Bonzini wrote: > On 08/08/2017 09:00, Peter Xu wrote: > > We were calling rcu_init_complete() twice in the child processes when > > fork happened. However the pthread library does not really suggest to do > > it that way: > > > > http://pubs.opengroup.org/onlinepubs/7908799/xsh/pthread_mutex_init.html > > > > "Attempting to initialise an already initialised mutex results in > > undefined behaviour." > > > > Actually, IMHO we can do it in a more natural way: Firstly, we only init > > the RCU globals once in rcu_init(). Then, in rcu_init_child(), we unlock > > all the locks held in rcu_init_lock() just like what we do in the parent > > process, then do the rest of RCU re-init (e.g., create the RCU thread). > > This doesn't work for error-checking mutexes: rcu_init_child has a > different PID than the parent, so the mutexes aren't unlocked. It's > also true that right now we don't use error-checking mutexes (commit > 24fa90499f, "qemu-thread: do not use PTHREAD_MUTEX_ERRORCHECK", > 2015-03-10); however, that's also a bit sad. > > The reason for the undefined behavior is probably that some operating > systems allocate memory in pthread_mutex_init, and initializing twice > causes a memory leak. One such operating system is OpenBSD. :( Good to know. :) I thought pthread_atfork() was designed to solve such a locking problem (in child hanlder, we unlock all the held locks). If PTHREAD_MUTEX_ERRORCHECK cannot coop well with it, not sure whether that means we should just avoid using PTHREAD_MUTEX_ERRORCHECK in such a use case (but we should be able to use the error checks in other mutexes that do not need extra fork handling)? Another idea is: can we just destroy the mutex first then re-init it in subprocess? A quick glance in libpthread code shows that at least pthread_mutex_destroy() won't check PTHREAD_MUTEX_ERRORCHECK. Thanks, > > Eric, you chimed in on the patch that became commit 24fa90499f, what do > you suggest? > > Paolo -- Peter Xu