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Date: Tue, 16 Apr 2013 11:15:16 +0200
From: Gerd Hoffmann <kraxel@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] qemu-char: another io_add_watch_poll fix
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: amit.shah@redhat.com, aliguori@us.ibm.com, qemu-devel@nongnu.org

On 04/10/13 15:23, Paolo Bonzini wrote:
> After attaching the source, we have to remove the reference we hold
> to it, because we do not hold anymore a pointer to the source.
> 
> If we do not do this, removing the source will not finalize it and
> will not drop the "real" I/O watch source.
> 
> This showed up when backporting the new flow control patches to older
> versions of QEMU that still used select.  The whole select then failed
> with EBADF (poll instead will reporting POLLNVAL on a single pollfd)
> and QEMU froze.

I get freezes now in master, bisecting points to this patch.

Reproducer: "qemu -serial pty".

qemu is pretty much unusable with libvirt now as libvirt uses pty
chardevs by default for serial & monitor ...

(gdb) bt
#0  __lll_lock_wait () at
../nptl/sysdeps/unix/sysv/linux/x86_64/lowlevellock.S:136
#1  0x00007f4141ce7388 in _L_lock_854 () from /lib64/libpthread.so.0
#2  0x00007f4141ce7257 in __pthread_mutex_lock (mutex=0x7f4145639128) at
pthread_mutex_lock.c:61
#3  0x00007f4142f41c37 in ?? () from /lib64/libglib-2.0.so.0
#4  0x00007f41439ff1b1 in io_watch_poll_finalize (source=<value
optimized out>)
    at /home/kraxel/projects/qemu/qemu-char.c:647
#5  0x00007f4142f4182a in ?? () from /lib64/libglib-2.0.so.0
#6  0x00007f4142f41b85 in ?? () from /lib64/libglib-2.0.so.0
#7  0x00007f4142f4416e in g_source_remove () from /lib64/libglib-2.0.so.0
#8  0x00007f4143a02f38 in pty_chr_state (chr=0x7f4145644b70,
connected=<value optimized out>)
    at /home/kraxel/projects/qemu/qemu-char.c:1151
#9  0x00007f4143a0303c in pty_chr_read (chan=<value optimized out>,
cond=<value optimized out>,
    opaque=0x7f4145644b70) at /home/kraxel/projects/qemu/qemu-char.c:1116
#10 0x00007f4142f41f0e in g_main_context_dispatch () from
/lib64/libglib-2.0.so.0
#11 0x00007f41439d8259 in glib_pollfds_poll (nonblocking=<value
optimized out>)
    at /home/kraxel/projects/qemu/main-loop.c:187
#12 os_host_main_loop_wait (nonblocking=<value optimized out>)
    at /home/kraxel/projects/qemu/main-loop.c:232
#13 main_loop_wait (nonblocking=<value optimized out>)
    at /home/kraxel/projects/qemu/main-loop.c:468
#14 0x00007f4143a4f055 in main_loop (argc=<value optimized out>,
argv=<value optimized out>,
    envp=<value optimized out>) at /home/kraxel/projects/qemu/vl.c:2039
#15 main (argc=<value optimized out>, argv=<value optimized out>,
envp=<value optimized out>)
    at /home/kraxel/projects/qemu/vl.c:4432

cheers,
  Gerd