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Re: [Qemu-devel] [RFC 01/15] char-io: fix possible race on IOWatchPoll
From: |
Daniel P. Berrange |
Subject: |
Re: [Qemu-devel] [RFC 01/15] char-io: fix possible race on IOWatchPoll |
Date: |
Wed, 20 Sep 2017 12:29:21 +0100 |
User-agent: |
Mutt/1.8.3 (2017-05-23) |
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 (address@hidden) 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>,
> > > > > address@hidden "iwp->src == NULL", address@hidden
> > > > > "/root/git/qemu/chardev/char-io.c", address@hidden, address@hidden
> > > > > <__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
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- [Qemu-devel] [RFC 00/15] QMP: out-of-band (OOB) execution support, Peter Xu, 2017/09/14
- [Qemu-devel] [RFC 01/15] char-io: fix possible race on IOWatchPoll, Peter Xu, 2017/09/14
- Re: [Qemu-devel] [RFC 01/15] char-io: fix possible race on IOWatchPoll, Eric Blake, 2017/09/19
- Re: [Qemu-devel] [RFC 01/15] char-io: fix possible race on IOWatchPoll, Daniel P. Berrange, 2017/09/20
- Re: [Qemu-devel] [RFC 01/15] char-io: fix possible race on IOWatchPoll, Peter Xu, 2017/09/20
- Re: [Qemu-devel] [RFC 01/15] char-io: fix possible race on IOWatchPoll, Daniel P. Berrange, 2017/09/20
- Re: [Qemu-devel] [RFC 01/15] char-io: fix possible race on IOWatchPoll, Peter Xu, 2017/09/20
- Re: [Qemu-devel] [RFC 01/15] char-io: fix possible race on IOWatchPoll, Daniel P. Berrange, 2017/09/20
- Re: [Qemu-devel] [RFC 01/15] char-io: fix possible race on IOWatchPoll, Peter Xu, 2017/09/20
- Re: [Qemu-devel] [RFC 01/15] char-io: fix possible race on IOWatchPoll,
Daniel P. Berrange <=
- Re: [Qemu-devel] [RFC 01/15] char-io: fix possible race on IOWatchPoll, Peter Xu, 2017/09/20
[Qemu-devel] [RFC 02/15] qobject: allow NULL for qstring_get_str(), Peter Xu, 2017/09/14
[Qemu-devel] [RFC 03/15] qobject: introduce qobject_to_str(), Peter Xu, 2017/09/14
[Qemu-devel] [RFC 04/15] monitor: move skip_flush into monitor_data_init, Peter Xu, 2017/09/14
[Qemu-devel] [RFC 05/15] qjson: add "opaque" field to JSONMessageParser, Peter Xu, 2017/09/14
[Qemu-devel] [RFC 06/15] monitor: move the cur_mon hack deeper for QMP, Peter Xu, 2017/09/14