From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:54616) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1bwUOb-0000l2-PG for qemu-devel@nongnu.org; Tue, 18 Oct 2016 09:25:34 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1bwUOY-0001sB-Gd for qemu-devel@nongnu.org; Tue, 18 Oct 2016 09:25:33 -0400 Received: from mx1.redhat.com ([209.132.183.28]:49160) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1bwUOY-0001s1-9G for qemu-devel@nongnu.org; Tue, 18 Oct 2016 09:25:30 -0400 Received: from int-mx09.intmail.prod.int.phx2.redhat.com (int-mx09.intmail.prod.int.phx2.redhat.com [10.5.11.22]) (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 96D84C04B311 for ; Tue, 18 Oct 2016 13:25:29 +0000 (UTC) Date: Tue, 18 Oct 2016 14:25:25 +0100 From: "Dr. David Alan Gilbert" Message-ID: <20161018132524.GG2190@work-vm> References: <20161012191502.GC16187@work-vm> <20161018100409.GH4349@redhat.com> <20161018113202.GE2190@work-vm> <20161018120121.GN4349@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline In-Reply-To: <20161018120121.GN4349@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 12:32:02PM +0100, Dr. David Alan Gilbert wrote: > > * Daniel P. Berrange (berrange@redhat.com) wrote: > > > On Wed, Oct 12, 2016 at 08:15:02PM +0100, Dr. David Alan Gilbert wr= ote: > > > > 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 fd. > > > > That makes them tricky to integrate into qemu, where > > > > the chardev's hide a whole bunch of non-fd things; in particular > > > > tls, mux, ringbuffers etc. > > > >=20 > > > > If we could get away with just a FILE* then we could use fopencoo= kie, > > > > but that's GNU only. > > > >=20 > > > > Is there any sane way of shepherding all chardev's into having an > > > > fd? > > >=20 > > > The entire chardev abstraction model exists precisely because we ca= nnot > > > make all chardevs look like a single fd. Even those which are fd ba= sed > > > may have separate FDs for input and output. > >=20 > > Note that editline takes separate in/out streams, but it does want th= ose streams > > to be FILE*'s. > >=20 > > > IMHO the only viable approach would be to enhance linenoise/editlin= e 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 wh= at we > > had for a very long time; I'd assume it would take a long time before > > any mods we made to the libraries would come around to be generally u= seful. > >=20 > > > BTW, what is the actual thread issue you are facing ? Chardevs at l= east > > > 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 thread saf= e. > > 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 the mux co= me > > 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 that will > race on contents of CharDriverState". That's too fuzzy to be useful to > developers though, so I think the only sensible option right now is to > say any "top level" CharDriverState should only be touch from one threa= d > at a time. IOW, if you have a mux, that that rule would apply to the > mux itself and the various children it owns as if they were a single > unnit. OK; I think we're probably saved by the big lock at the moment, so that all device emulation that outputs text is probably holding it and the mon= itor is also. What about something like an error_report from a different thre= ad while something is happening in the monitor? > > My actual thoughts for threads came from a few sides: > > a) Maybe I could have a shim thread that fed the editline fd from a= chardev > > b) I'd eventually like multiple monitor threads. >=20 > Can you expand on what you mean by multiple monitor threads ? Presumabl= y > you're meaning a single monitor instance, with multiple threads process= ing > commands concurrently ? If so, I think that ought to be fine even with > the current thread rules around chardevs. The processing of individual > monitor commands doesn't interact with the CharDriverState AFAIR, as we > have clean separation between parsing the incoming command, running the > command, and formatting the outgoing response. IOW, for a single monito= r > it is still sufficient to have a single thread deal with all I/O for th= e > chardev - only the command execution needs to be delegated to other > threads, and those wouldn't be touching the chardev at all. Hmm, I'd thought of the other way around - multiple individual monitors e= ach running one command; ie each connection for a monitor would be it's own t= hread. Dave >=20 > 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