Re: [RFC] Patch for VNC SPU getting "stuck"

Niraj Tolia wrote:
> This report is against CVS HEAD. With the last round of changes, the 
> Async IO code path will block everytime there is no update to send. The 
> relevant stack trace is
> 
> #3  0x00a63d75 in crWaitSemaphore (s=0xca46b8) at threads.c:259
> #4  0x00c8786c in vncspuWaitDirtyRects (region=0xa246884, 
> frame_num=0xa2469e8)
>     at vncspu.c:335
> #5  0x00c8b13e in rf_client_updatereq () at client_io.c:516
> #6  0x00c8989d in aio_process_input (slot=0xa246728) at async_io.c:562
> #7  0x00c89486 in aio_mainloop () at async_io.c:390
> 
> We never ran into this issue earlier as the VNC SPU had a fixed frame 
> update rate and we would always have something to send out. However, now 
> we can get stuck because of the above. Things that the user runs into 
> includes attempts at new connections "hanging". This happens because we 
> never return to the aio_mainloop unless an update was received from the 
> remote application running under crappfaker.

The CVS HEAD code should work fine for a single VNC viewer, but I can 
see how it would hang with multiple viewers.  I hadn't gotten around 
to fixing that yet.


> As we already record that an update was requested on the current client 
> slot, one option is to return to the aio_mainloop and defer sending the 
> update till something is actually available. The deferred update can be 
> sent by leveraging the s_idle_func used in the AIO code. An example 
> patch of how this could be done is attached. One disadvantage is that we 
> need to reduce the timeout value of the select() call in aio_mainloop(). 
> Testing did not indicate any increased CPU usage though as the idle 
> function is cheap.
> 
> I have a decent handle on the VNC SPU code and so, if you feel this 
> should be done differently, just let me know and I should be able to 
> code up another patch.

Even with your patch, when I attach two VNC viewers, things aren't 
running as expected.  One viewer or the other will freeze up for long 
periods.

I'm looking into it.  I may not have a fix until next week though.

-Brian


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