[Qemu-devel] Re: [PATCH] Fix freezing bug in curses console

[Anthony Liguori]

Matthew Bloch wrote:
> Hi there,
>
> We are running lots of kvm processes in screen and found that about 1 in
> 5 froze shortly after startup startup with a backtrace like this one:
>
> #0  0xf7c7fcd9 in pthread_exit () from /lib/tls/libc.so.6
> #1  0xf7cfbe62 in wresize () from /lib/libncurses.so.5
> #2  0xf7cfb7ab in is_term_resized () from /lib/libncurses.so.5
> #3  0xf7cfb877 in is_term_resized () from /lib/libncurses.so.5
> #4  0xf7cfba31 in resize_term () from /lib/libncurses.so.5
> #5  0x080d3dd9 in vga_init ()
> #6  <signal handler called>
> #7  0xf7c0da5b in free () from /lib/tls/libc.so.6
> #8  0xf7c0effe in calloc () from /lib/tls/libc.so.6
> #9  0xf7cf222e in newpad () from /lib/libncurses.so.5
> #10 0x080d3549 in vga_init ()
>
> We're just using the lenny version of kvm from 2008-12-16.
>
> On casual inspection, the SIGWINCH signal handling looked ropey to me -
> grandpa always told me not to do any real work in a signal handler, and
> the backtrace suggested re-entrancy problems in curses, so I changed the
> behaviour to set a flag and do the work in the main loop instead.  Maybe
> I'm reading the backtrace wrong.
>
> So far that means that when you resize the window, the display is
> corrupt until the VM outputs some text, or the user hits a key.  But I
> think it has solved the freezing / crashing bug too - would appreciate
> any comments on my analysis or proposed solution.
>   

It's racy with select().  A better fix would be to create a pipe and 
write to that pipe in the SIGWINCH handler.  You should then register an 
io callback using qemu_set_fd_handler2() that does the actions for SIGWINCH.

Regards,

Anthony Liguori