On Wed, Sep 20, 2017 at 07:18:49PM +0800, Peter Xu wrote: > On Wed, Sep 20, 2017 at 12:03:09PM +0100, Daniel P. Berrange wrote: > > On Wed, Sep 20, 2017 at 06:49:58PM +0800, Peter Xu wrote: > > > On Wed, Sep 20, 2017 at 10:14:38AM +0100, Daniel P. Berrange wrote: > > > > On Wed, Sep 20, 2017 at 05:09:26PM +0800, Peter Xu wrote: > > > > > On Wed, Sep 20, 2017 at 08:57:03AM +0100, Daniel P. Berrange wrote: > > > > > > On Thu, Sep 14, 2017 at 03:50:22PM +0800, Peter Xu wrote: > > > > > > > This is not a problem if we are only having one single loop > > > > > > > thread like > > > > > > > before. However, after per-monitor thread is introduced, this is > > > > > > > not > > > > > > > true any more, and the race can happen. > > > > > > > > > > > > > > The race can be triggered with "make check -j8" sometimes: > > > > > > > > > > > > > > qemu-system-x86_64: /root/git/qemu/chardev/char-io.c:91: > > > > > > > io_watch_poll_finalize: Assertion `iwp->src == NULL' failed. > > > > > > > > > > > > > > This patch keeps the reference for the watch object when creating > > > > > > > in > > > > > > > io_add_watch_poll(), so that the object will never be released in > > > > > > > the > > > > > > > context main loop, especially when the context loop is running in > > > > > > > another standalone thread. Meanwhile, when we want to remove the > > > > > > > watch > > > > > > > object, we always first detach the watch object from its owner > > > > > > > context, > > > > > > > then we continue with the cleanup. > > > > > > > > > > > > > > Without this patch, calling io_remove_watch_poll() in main loop > > > > > > > thread > > > > > > > is not thread-safe, since the other per-monitor thread may be > > > > > > > modifying > > > > > > > the watch object at the same time. > > > > > > > > > > > > This doesn't feel right to me. Why is the main loop thread doing > > > > > > anything > > > > > > at all with the Chardev, if there is a per-monitor thread ? The > > > > > > Chardev > > > > > > code isn't thread safe so it isn't safe to have two separate threads > > > > > > accessing the same Chardev. IOW, if we want a per-monitor thread, > > > > > > then > > > > > > we must make sure the main thread never touches that monitor's > > > > > > chardev > > > > > > at all. While your patch here might have avoided the assertion you > > > > > > mention above, I fear this is just papering over a fundamental > > > > > > problem > > > > > > that still exists, that can only be solved by not letting the > > > > > > mainloop > > > > > > touch the chardev at all. > > > > > > > > > > The stack I encountered: > > > > > > > > > > #0 0x00007f658234c765 in __GI_raise (sig=sig@entry=6) at > > > > > ../sysdeps/unix/sysv/linux/raise.c:54 > > > > > #1 0x00007f658234e36a in __GI_abort () at abort.c:89 > > > > > #2 0x00007f6582344f97 in __assert_fail_base (fmt=<optimized out>, > > > > > assertion=assertion@entry=0x55c76345fce1 "iwp->src == NULL", > > > > > file=file@entry=0x55c76345fcc0 "/root/git/qemu/chardev/char-io.c", > > > > > line=line@entry=91, function=function@entry=0x55c76345fd10 > > > > > <__PRETTY_FUNCTION__.21863> "io_watch_poll_finalize") at assert.c:92 > > > > > #3 0x00007f6582345042 in __GI___assert_fail > > > > > (assertion=0x55c76345fce1 "iwp->src == NULL", file=0x55c76345fcc0 > > > > > "/root/git/qemu/chardev/char-io.c", line=91, function=0x55c76345fd10 > > > > > <__PRETTY_FUNCTION__.21863> "io_watch_poll_finalize") at assert.c:101 > > > > > #4 0x000055c7632c2be5 in io_watch_poll_finalize > > > > > (source=0x55c7651cd450) at /root/git/qemu/chardev/char-io.c:91 > > > > > #5 0x00007f65847bb859 in g_source_unref_internal () at > > > > > /lib64/libglib-2.0.so.0 > > > > > #6 0x00007f65847bca29 in g_source_destroy_internal () at > > > > > /lib64/libglib-2.0.so.0 > > > > > #7 0x000055c7632c2d30 in io_remove_watch_poll > > > > > (source=0x55c7651cd450) at /root/git/qemu/chardev/char-io.c:139 > > > > > #8 0x000055c7632c2d5c in remove_fd_in_watch (chr=0x55c7651ccdf0) at > > > > > /root/git/qemu/chardev/char-io.c:145 > > > > > #9 0x000055c7632c2368 in qemu_chr_fe_set_handlers (b=0x55c7651f6410, > > > > > fd_can_read=0x0, fd_read=0x0, fd_event=0x0, be_change=0x0, > > > > > opaque=0x0, context=0x0, set_open=true) > > > > > at /root/git/qemu/chardev/char-fe.c:267 > > > > > #10 0x000055c7632c2221 in qemu_chr_fe_deinit (b=0x55c7651f6410, > > > > > del=false) at /root/git/qemu/chardev/char-fe.c:231 > > > > > #11 0x000055c762e2b15c in monitor_data_destroy (mon=0x55c7651f6410) > > > > > at /root/git/qemu/monitor.c:600 > > > > > #12 0x000055c762e340ec in monitor_cleanup () at > > > > > /root/git/qemu/monitor.c:4346 > > > > > #13 0x000055c762f9445d in main (argc=19, argv=0x7ffc6846d0e8, > > > > > envp=0x7ffc6846d188) at /root/git/qemu/vl.c:4889 > > > > > > > > > > So it's destroying the CharBackend, but it'll then call > > > > > qemu_chr_fe_set_handlers() which finally tries to remove the watch > > > > > poll. > > > > > > > > Ok that code is broken - it must not call monitor_cleanup from the main > > > > thread - it needs to be called from the monitor thread, unless it can > > > > guarantee that the monitor thread has already exited, which seems > > > > unlikely > > > > > > The problem is that not all monitors are parsed in the IO thread, but > > > only those with use_io_thr=true set. > > > > > > How about I move the calls of monitor_data_destroy() into that monitor > > > IO thread when use_io_thr=true? And for the rest, I think they still > > > need to be destroyed in the main thread. > > > > I think having the monitor sometimes run in the main thread and sometimes > > run in a background thread is a recipe for ongoing trouble, of which this > > problem is just the first example that will hurt us. People will test > > behaviour of a feature with one setup and then users will later run it in > > a different setup and potentially experiance obscure bugs as a result. > > IOW, use_io_thr flag should not exist, and every monitor should be run > > unconditionally in the background thread from the point at which your > > patch series merges. > > I agree with you that this may bring trouble in some aspect. I just > don't know whether it'll bring more trouble if we move all the > monitor-related chardev IO into monitor thread. > > The key is the muxed typed chardev. > > If we don't have muxed typed chardev, I'll surely consider to use IO > thread for all the monitors. > > However, the muxed chardevs can support e.g. one monitor plus a serial > port. Can we just run the IO stuff in monitor thread even part of its > frontend is a serial port? And also I'm not sure what would happen if > it's a monitor plus something else I even don't aware of.
Urgh, I forgot about the horrible mux chardev concept, that does rather complicate life - moving the guest device interaction to the monitor thread would be dubious. So yeah, given that, it probably is simplest to change monitor_cleanup to skip destroy of monitors which have a background thread. Regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
