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) > > [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? > 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. -- Marc-André Lureau
