Hi On Tue, Aug 21, 2018 at 8:29 AM Peter Xu <pet...@redhat.com> wrote: > > On Mon, Aug 20, 2018 at 05:37:55PM +0200, Marc-André Lureau wrote: > > Hi > > > > On Mon, Aug 20, 2018 at 4:48 AM Peter Xu <pet...@redhat.com> wrote: > > > > > > On Fri, Aug 17, 2018 at 03:52:20PM +0200, Marc-André Lureau wrote: > > > > Hi, > > > > > > > > In commit 25679e5d58e "chardev: tcp: postpone async connection setup" > > > > (and its follow up 99f2f54174a59), Peter moved chardev socket > > > > connection to machine_done event. However, chardev created later will > > > > no longer attempt to connect, and chardev created in tests do not have > > > > machine_done event (breaking some of vhost-user-test). > > > > > > > > The goal was to move the "connect" source to the chardev frontend > > > > context (the monitor thread context in his case). chr->gcontext is set > > > > with qemu_chr_fe_set_handlers(). But there is no guarantee that the > > > > function will be called in general, > > > > > > Could you hint a case where we didn't use qemu_chr_fe_set_handlers() > > > upon a chardev backend? I thought it was always used in chardev > > > frontends, and what the backend could do if without a frontend? > > > > Well, you don't have to have a front-end to have side effects. Connect > > will be attempted even without frontend. We may have users expecting > > that behaviour, that might be considered a break if we change it. > > > > (and unlikely, there might be frontends that are write only) > > My understanding is that qemu_chr_fe_set_handlers() is not only for > port read, but also for the rest. For example, we need to pass in the > correct IOEventHandler* to handle chardev backend events even if the > frontend only writes. > > > > > > > > > [1] > > > > > > > so we can't delay connection until > > > > then: the chardev should still attempt to connect during open(), using > > > > the main context. > > > > > > > > An alternative would be to specify the iothread during chardev > > > > creation. Setting up monitor OOB would be quite different too, it > > > > would take the same iothread as argument. > > > > > > > > 99f2f54174a595e is also a bit problematic, since it will behave > > > > differently before and after machine_done (the first case gives a > > > > chance to use a different context reliably, the second looks racy) > > > > > > > > In the end, I am not sure this is all necessary, as chardev callbacks > > > > are called after qemu_chr_fe_set_handlers(), at which point the > > > > context of sources are updated. In "char-socket: update all ioc > > > > handlers when changing context", I moved also the hup handler to the > > > > updated context. So unless the main thread is already stuck, we can > > > > setup a different context for the chardev at that time. Or not? > > > > > > IMHO the two patches that you reverted are special-cases for reasons. > > > > > > The TLS handshake is carried out with an TLS internal GSource which is > > > not owned by the chardev code, so the qemu_chr_fe_set_handlers() won't > > > update that GSource (please refer to qio_channel_tls_handshake_task). > > > > What can go wrong by using the default context for initial connection > > and TLS handshake? > > > > Presumably, you have a case where the mainloop is no longer processed > > and that will hang the chardev? > > Yeah I don't see a big problem now, but I'm not sure. Actually it > should not be very hard to even migrate this one just like other > GSources, however the async one below should be a bit harder. > > > > > > The async connection is carried out in a standalone thread that calls > > > connect(). IMHO we'd better not update the gcontext bound to the > > > async task since otherwise there'll be a race (IIRC I proposed > > > something before using a mutex to update the gcontext, but Dan would > > > prefer not to, and I followed with the suggestion which makes sense to > > > me). > > > > > > Could we just postpone these machine done tasks into > > > qemu_chr_fe_set_handlers() (or say, chr_update_read_handler() hook, > > > just like what I mentioned in the other thread)? Though we'll be sure > > > qemu_chr_fe_set_handlers() will be called for all chardev backends > > > hence I asked question [1] above. > > > > I would rather not to, if possible. unless we take the risk of > > breaking current behaviour and review chardev usage in qemu. > > Yeah, I'd be glad to know any of the behavior breakage if there is, > but I can't figure any out. AFAIU there should be none since we > should always be pairing a backend with a frontend.
Even if we check all usage of chardev in internal qemu, there might be external users that expect that creating a chardev will attempt the connection immediately. > > I fully agree that current way is not ideal since basically the > backend should not depend on the frontend, but now we have the > gcontext as an exception then the backend will somehow depend on the > frontend. If you don't like the way I proposed, another thing I am > thinking is that whether we can assign the gcontext for the chardev > backend before initialization of it (or by parsing the backend & > frontend relationships before init of backends), then we assure that > we never change the gcontext of any chardev backends. Though that Yes, I think that's a cleaner solution. I suggested to use an iothread argument in the cover letter. Paolo, Daniel, any opinion? > will require that we need to setup all possible gcontexts before hand > (e.g., the monitor gcontext). Then we can drop all these dynamic > binding magics (but just to hope we will never need the flexibility in > the future). > > Regards, > > -- > Peter Xu -- Marc-André Lureau
