Re: [Qemu-devel] failed migration makes monitor stuck

[Michael Tokarev]

Ping?  Anyone know this area of code?  Can we just remove
one monitor_resume() call from migrate_fd_put_buffer() ?

09.07.2011 15:07, Michael Tokarev wrote:
> After some debugging I found a programming error in
> error handling in migration, but I'm not sure how to
> fix it.
> 
> When migration starts, monitor gets suspended, calling
> monitor_suspend() routine which increments assotiated
> suspend_cnt counter.
> 
> At the end of migration, in migrate_fd_cleanup(),
> monitor_resume() gets called, which decrements the
> counter.
> 
> But monitor_resume() gets also called from another
> place, in migrate_fd_put_buffer(), in case we
> encountered a write error.
> 
> So, suppose a tcp endpoint has disconnected, or the
> exec: program terminated due to error or whatnot --
> in all these cases write will fail, and we'll call
> monitor_resume() twice as a result: once in this
> place in migrate_fd_put_buffer(), and once more at
> the end in migrate_fd_cleanup().
> 
> This results in suspend_cnt being decremented twice,
> with the resultant value being -1.
> 
> So monitor_can_read() will return 0 from now on, since
> it compares suspend_cnt with 0.  And hence, monitor will
> stop working.
> 
> To me it looks like monitor_resume() call should be
> removed from migrate_fd_put_buffer(), but I'm not sure
> _why_ it were here in the first place.
> 
> There's more: monitor_suspend() gets called from within
> protocol handlers (using migrate_fd_monitor_suspend()
> routine), -- are we sure that all current and future
> protocol handlers will call this function?
> 
> Thanks!
> 
> /mjt
>