Re: [Qemu-devel] [RFC 01/15] char-io: fix possible race on IOWatchPoll

[Daniel P. Berrange]

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

Regards,
Daniel
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