From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:33319) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bwZWT-0004gM-0g for qemu-devel@nongnu.org; Tue, 18 Oct 2016 14:54:05 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1bwZWP-0007WG-7x for qemu-devel@nongnu.org; Tue, 18 Oct 2016 14:54:01 -0400 Received: from mx1.redhat.com ([209.132.183.28]:44914) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1bwZWP-0007Vm-0F for qemu-devel@nongnu.org; Tue, 18 Oct 2016 14:53:57 -0400 Received: from int-mx14.intmail.prod.int.phx2.redhat.com (int-mx14.intmail.prod.int.phx2.redhat.com [10.5.11.27]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 5DDAB804E5 for ; Tue, 18 Oct 2016 18:53:55 +0000 (UTC) Date: Tue, 18 Oct 2016 19:53:51 +0100 From: "Dr. David Alan Gilbert" Message-ID: <20161018185351.GB22395@work-vm> References: <20161012191502.GC16187@work-vm> <20161018100409.GH4349@redhat.com> <20161018113202.GE2190@work-vm> <20161018120121.GN4349@redhat.com> <20161018132524.GG2190@work-vm> <20161018133528.GD12728@redhat.com> <20161018135213.GI2190@work-vm> <20161018140141.GF12728@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <20161018140141.GF12728@redhat.com> Content-Transfer-Encoding: quoted-printable Subject: Re: [Qemu-devel] chardev's and fd's in monitors List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: "Daniel P. Berrange" Cc: qemu-devel@nongnu.org, armbru@redhat.com * Daniel P. Berrange (berrange@redhat.com) wrote: > On Tue, Oct 18, 2016 at 02:52:13PM +0100, Dr. David Alan Gilbert wrote: > > * Daniel P. Berrange (berrange@redhat.com) wrote: > > > On Tue, Oct 18, 2016 at 02:25:25PM +0100, Dr. David Alan Gilbert wr= ote: > > > > * Daniel P. Berrange (berrange@redhat.com) wrote: > > > > > On Tue, Oct 18, 2016 at 12:32:02PM +0100, Dr. David Alan Gilber= t wrote: > > > > > > * Daniel P. Berrange (berrange@redhat.com) wrote: > > > > > > > On Wed, Oct 12, 2016 at 08:15:02PM +0100, Dr. David Alan Gi= lbert wrote: > > > > > > > > Hi, > > > > > > > > I had a look at a couple of readline like libraries; > > > > > > > > editline and linenoise. A difficulty with using them is = that > > > > > > > > they both want fd's or FILE*'s; editline takes either but > > > > > > > > from a brief look I think it's expecting to extract the f= d. > > > > > > > > That makes them tricky to integrate into qemu, where > > > > > > > > the chardev's hide a whole bunch of non-fd things; in par= ticular > > > > > > > > tls, mux, ringbuffers etc. > > > > > > > >=20 > > > > > > > > If we could get away with just a FILE* then we could use = fopencookie, > > > > > > > > but that's GNU only. > > > > > > > >=20 > > > > > > > > Is there any sane way of shepherding all chardev's into h= aving an > > > > > > > > fd? > > > > > > >=20 > > > > > > > The entire chardev abstraction model exists precisely becau= se we cannot > > > > > > > make all chardevs look like a single fd. Even those which a= re fd based > > > > > > > may have separate FDs for input and output. > > > > > >=20 > > > > > > Note that editline takes separate in/out streams, but it does= want those streams > > > > > > to be FILE*'s. > > > > > >=20 > > > > > > > IMHO the only viable approach would be to enhance linenoise= /editline to > > > > > > > not assume use of fd* or FILE * abstractions. > > > > > >=20 > > > > > > I think if it came to that then we'd probably end up sticking= with what we > > > > > > had for a very long time; I'd assume it would take a long tim= e before > > > > > > any mods we made to the libraries would come around to be gen= erally useful. > > > > > >=20 > > > > > > > BTW, what is the actual thread issue you are facing ? Chard= evs at least > > > > > > > ought to be usable from a separate thread, as long as each = distinct > > > > > > > chardev object instance was only used from one thread at a = time ? > > > > > >=20 > > > > > > Marc-Andr=E9 pointed that out; I hadn't realised they were th= read safe. > > > > > > But what are the rules? You say 'only used from one thread at= a time' - > > > > > > what happens if we have a mux and the different streams to th= e mux come > > > > > > from different threads? > > > > >=20 > > > > > Well there is no mutex locking on the CharDriverState objects, = so the > > > > > exact rule is "you mustn't do anything from multiple threads th= at will > > > > > race on contents of CharDriverState". That's too fuzzy to be us= eful to > > > > > developers though, so I think the only sensible option right no= w is to > > > > > say any "top level" CharDriverState should only be touch from o= ne thread > > > > > at a time. IOW, if you have a mux, that that rule would apply t= o the > > > > > mux itself and the various children it owns as if they were a s= ingle > > > > > unnit. > > > >=20 > > > > OK; I think we're probably saved by the big lock at the moment, s= o that > > > > all device emulation that outputs text is probably holding it and= the monitor > > > > is also. What about something like an error_report from a differ= ent thread > > > > while something is happening in the monitor? > > >=20 > > > If we moved execution of monitor commands to separate thread from t= he > > > thread handling monitor I/O, then we'd have to modify error_report = so > > > that it queued the text in some manner, such that it was only then > > > fed back to the client once the command thread completed. Alternati= vely > > > we'd have to introduced locking in the Monitor object, that seriali= zed > > > access to the underling CharDriverState I/O funcs. > >=20 > > I already use error_report's in places in migration threads of variou= s > > types; I'm not sure if that's a problem. >=20 > Unless those places are protected by the big qemu lock, that sounds > not good. error_report calls into error_vprintf which checks the > 'cur_mon' global "Monitor" pointer. This variable is updated at > runtime - eg in qmp_human_monitor_command(), monitor_qmp_read(), > monitor_read(), etc. So if migration threads outside the BQL are > calling error_report() that could well cause problems. If you > are lucky messages will merely end up going to stderr instead of > the monitor, but in worst case I wouldn't be surprised if there > is a crash possibility in some race conditions. Hmm that's going to be interesting to fix; I certainly use error_report all over in postcopy, and the postcopy code uses device load code in its threads that are shared by the normal load paths. I doubt any of the rest of the threaded code is clean from them either; does block code used in the iothreads ever end up with an error_report? Can't we take the bql in the inside of error_report? Dave > Regards, > Daniel > --=20 > |: http://berrange.com -o- http://www.flickr.com/photos/dberran= ge/ :| > |: http://libvirt.org -o- http://virt-manager.= org :| > |: http://entangle-photo.org -o- http://search.cpan.org/~danbe= rr/ :| -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK