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. > > 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 :| -- Peter Xu
