From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:34932) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1fg3uX-0008I7-AF for qemu-devel@nongnu.org; Thu, 19 Jul 2018 04:03:55 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1fg3uA-0007lQ-6X for qemu-devel@nongnu.org; Thu, 19 Jul 2018 04:03:41 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:59290 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 1fg3uA-0007ki-07 for qemu-devel@nongnu.org; Thu, 19 Jul 2018 04:03:18 -0400 Received: from smtp.corp.redhat.com (int-mx04.intmail.prod.int.rdu2.redhat.com [10.11.54.4]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 7BAC887AAA for ; Thu, 19 Jul 2018 08:03:17 +0000 (UTC) Date: Thu, 19 Jul 2018 16:03:06 +0800 From: Peter Xu Message-ID: <20180719080306.GF4071@xz-mi> References: <20180620071040.28729-1-peterx@redhat.com> <87y3e8lfks.fsf@dusky.pond.sub.org> <20180719050145.GD4071@xz-mi> <87va9bitdp.fsf@dusky.pond.sub.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <87va9bitdp.fsf@dusky.pond.sub.org> Content-Transfer-Encoding: quoted-printable 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: =?utf-8?Q?Marc-Andr=C3=A9?= Lureau , qemu-devel@nongnu.org, Stefan Hajnoczi , "Dr. David Alan Gilbert" On Thu, Jul 19, 2018 at 09:20:34AM +0200, Markus Armbruster wrote: > Peter Xu writes: >=20 > > On Wed, Jul 18, 2018 at 05:38:11PM +0200, Markus Armbruster wrote: > >> Peter Xu writes: > >>=20 > >> > 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 t= he > >> > cur_mon variable to be a per-thread variable to make sure there wo= n't be > >> > a race between threads when accessing the variable. > >>=20 > >> Hmm... why hasn't the OOB work created such a race already? > >>=20 > >> A monitor reads, parses, dispatches and executes commands, formats a= nd > >> sends replies. > >>=20 > >> Before OOB, all of that ran in the main thread. Any access of cur_m= on > >> should therefore be from the main thread. No races. > >>=20 > >> OOB moves read, parse, format and send to an I/O thread. Dispatch a= nd > >> execute remain in the main thread. *Except* for commands executed O= OB, > >> dispatch and execute move to the I/O thread, too. > >>=20 > >> Why is this not racy? I guess it relies on careful non-use of cur_m= on > >> 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 comman= d > > 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_mo= n > > 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 o= f > > out-of-band if we haven't yet started to support that for libvirt...= ) >=20 > It's not just the actual OOB commands (there are just two), it's also > the monitor code to read, parse, format and send. My understanding is that read, parse, format, send will not touch cur_mon (it was touched before but some patches in the out-of-band series should have removed the last users when parsing). So IIUC only the dispatcher would touch that now. I didn't consider the callers like net_init_socket() and I'm only considering the monitor code (and those callers should be only in the main thread too after all). >=20 > >> 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 stil= l > > can be seen as a bug fix of existing code. >=20 > Agreed. >=20 > > Regards, > > > >> > Note that thread variables are not initialized to a valid value wh= en new > >> > thread is created. >=20 > Confusing. It sounds like @cur_mon's initial value would be > indeterminate, like an automatic variable's. Not true. Variables with > thread storage duration are initialized when the thread is created. > Since @cur_mon's declaration lacks an initializer, it'll be initialized > to a null pointer. Your sentence is correct when you consider that nul= l > pointer not a valid value. Yes that's what I meant. So how about this? Note that the per-thread @cur_mon variable is not initialized to point to a valid Monitor struct when a new thread is created (the default value will be NULL). Please feel free to tune it up. >=20 > >> > However for our case we don't need to set it u= p, > >> > since the cur_mon variable is only used in such a pattern: > >> >=20 > >> > old_mon =3D cur_mon; > >> > cur_mon =3D xxx; > >> > (do something, read cur_mon if necessary in the stack) [1] > >> > cur_mon =3D old_mon; > >> >=20 > >> > It plays a role as stack variable, so no need to be initialized at= all. > >> > We only need to make sure the variable won't be changed unexpected= ly by > >> > other threads. >=20 > Do we need this paragraph? The commit doesn't mess with @cur_mon's > initial value at all... I was trying to explain why we don't need to initialize that variable for each thread. A common idea (at least that's what I have had in mind) is that when we create a new thread we should possibly inherit that @cur_mon variable in a copy-on-write fashion for that new thread. But that's not really necessary for the use case like above (as long as we don't create thread during [1], and that's what we do). If you think the patch explains itself better without these lines, please feel free to drop it. >=20 > >> > Reviewed-by: Eric Blake > >> > Reviewed-by: Marc-Andr=C3=A9 Lureau > >> > Reviewed-by: Stefan Hajnoczi > >> > [peterx: touch up commit message a bit] > >> > Signed-off-by: Peter Xu Thanks, --=20 Peter Xu