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

[Anthony Liguori <anthony@codemonkey.ws>]

andrzej zaborowski wrote:
> 2009/2/27 Anthony Liguori <aliguori@us.ibm.com>:
>   
>> 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.
>>     
>
> Maybe a bottom half would work?  The scheduling of a bh shouldn't
> constitute "real work".
>   

I think it still suffers from the same race condition so today it 
wouldn't work.  You could fix the bottom half scheduling though so that 
you could safely schedule a bottom half from a signal handler (using 
roughly the same trick).

Regards,

Anthony Liguori

> Cheers
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