* Daniel P. Berrange (berra...@redhat.com) wrote: > On Tue, Oct 18, 2016 at 02:52:13PM +0100, Dr. David Alan Gilbert wrote: > > * Daniel P. Berrange (berra...@redhat.com) wrote: > > > On Tue, Oct 18, 2016 at 02:25:25PM +0100, Dr. David Alan Gilbert wrote: > > > > * Daniel P. Berrange (berra...@redhat.com) wrote: > > > > > On Tue, Oct 18, 2016 at 12:32:02PM +0100, Dr. David Alan Gilbert > > > > > wrote: > > > > > > * Daniel P. Berrange (berra...@redhat.com) wrote: > > > > > > > On Wed, Oct 12, 2016 at 08:15:02PM +0100, Dr. David Alan Gilbert > > > > > > > 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 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. > > > > > > > > > > > > > > > > If we could get away with just a FILE* then we could use > > > > > > > > fopencookie, > > > > > > > > but that's GNU only. > > > > > > > > > > > > > > > > Is there any sane way of shepherding all chardev's into having > > > > > > > > an > > > > > > > > fd? > > > > > > > > > > > > > > The entire chardev abstraction model exists precisely because we > > > > > > > cannot > > > > > > > make all chardevs look like a single fd. Even those which are fd > > > > > > > based > > > > > > > may have separate FDs for input and output. > > > > > > > > > > > > Note that editline takes separate in/out streams, but it does want > > > > > > those streams > > > > > > to be FILE*'s. > > > > > > > > > > > > > IMHO the only viable approach would be to enhance > > > > > > > linenoise/editline to > > > > > > > not assume use of fd* or FILE * abstractions. > > > > > > > > > > > > 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 time > > > > > > before > > > > > > any mods we made to the libraries would come around to be generally > > > > > > useful. > > > > > > > > > > > > > BTW, what is the actual thread issue you are facing ? Chardevs 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 ? > > > > > > > > > > > > Marc-André pointed that out; I hadn't realised they were thread > > > > > > 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 the mux > > > > > > come > > > > > > from different threads? > > > > > > > > > > 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 > > > > > thread > > > > > 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 > > > > monitor > > > > is also. What about something like an error_report from a different > > > > thread > > > > while something is happening in the monitor? > > > > > > If we moved execution of monitor commands to separate thread from the > > > 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. Alternatively > > > we'd have to introduced locking in the Monitor object, that serialized > > > access to the underling CharDriverState I/O funcs. > > > > I already use error_report's in places in migration threads of various > > types; I'm not sure if that's a problem. > > 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 > -- > |: http://berrange.com -o- http://www.flickr.com/photos/dberrange/ :| > |: http://libvirt.org -o- http://virt-manager.org :| > |: http://entangle-photo.org -o- http://search.cpan.org/~danberr/ :| -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
