From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:55866) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fg14h-0002qr-UH for qemu-devel@nongnu.org; Thu, 19 Jul 2018 01:02:00 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fg14e-0007sf-Ry for qemu-devel@nongnu.org; Thu, 19 Jul 2018 01:01:59 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:39480 helo=mx1.redhat.com) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1fg14e-0007sQ-Mf for qemu-devel@nongnu.org; Thu, 19 Jul 2018 01:01:56 -0400 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 0142340711BF for ; Thu, 19 Jul 2018 05:01:56 +0000 (UTC) Date: Thu, 19 Jul 2018 13:01:45 +0800 From: Peter Xu Message-ID: <20180719050145.GD4071@xz-mi> References: <20180620071040.28729-1-peterx@redhat.com> <87y3e8lfks.fsf@dusky.pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <87y3e8lfks.fsf@dusky.pond.sub.org> Subject: Re: [Qemu-devel] [PATCH v4] monitor: let cur_mon be per-thread List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Markus Armbruster Cc: qemu-devel@nongnu.org, "Dr. David Alan Gilbert" , Stefan Hajnoczi , =?utf-8?Q?Marc-Andr=C3=A9?= Lureau On Wed, Jul 18, 2018 at 05:38:11PM +0200, Markus Armbruster wrote: > Peter Xu writes: > > > After the Out-Of-Band work, the monitor iothread may be accessing the > > cur_mon as well (via monitor_qmp_dispatch_one()). Let's convert the > > cur_mon variable to be a per-thread variable to make sure there won't be > > a race between threads when accessing the variable. > > Hmm... why hasn't the OOB work created such a race already? > > A monitor reads, parses, dispatches and executes commands, formats and > sends replies. > > Before OOB, all of that ran in the main thread. Any access of cur_mon > should therefore be from the main thread. No races. > > OOB moves read, parse, format and send to an I/O thread. Dispatch and > execute remain in the main thread. *Except* for commands executed OOB, > dispatch and execute move to the I/O thread, too. > > Why is this not racy? I guess it relies on careful non-use of cur_mon > in any part that may now execute in the I/O thread. Scary... I think it's because cur_mon is not really used in out-of-band command executions - now we only have a few out-of-band enabled commands, and IIUC none of them is using cur_mon (for example, in qmp_migrate_recover() we don't even call error_report, and the code path is quite straight forward to make sure of that). So IIUC cur_mon variable is still only touched by main thread for now hence we should be safe. However that condition might change in the future when we add more out-of-band capable commands. (not to mention that I don't even know whether there are real users of out-of-band if we haven't yet started to support that for libvirt...) > > Should this go into 3.0 to reduce the risk of bugs? Yes I think it would be good to have that even for 3.0, since it still can be seen as a bug fix of existing code. Regards, -- Peter Xu