[Qemu-devel] Re: [5578] Increase default IO timeout from 10ms to 5s

[Anthony Liguori]

Jan Kiszka wrote:
> Jan Kiszka wrote:
>   
>
> There is a race between the alarm_timer firing SIGALRM and
> main_loop_wait reaching the safe harbor of select (with that infamous 5
> second timeout). If the signal comes when already blocked in select, it
> will properly resume the latter immediately. But if the timer fired
> BEFORE that point, host_alarm_handler will only set a flag that the host
> timer has fired, the actual rearming will be done AFTER return from
> select. Ooops....
>   

Ah, so before this was causing the timer to potentially come 10ms later 
than it should have.  I was hoping that this change would shake out this 
stuff :-)

> So, select should actually include the host timer as event. timerfd?
> Unfortunately a recent Linux-only feature :-/. I don't think we can
> rearm the timer from within the signal handler, at least not without
> running all the pending qemu timers. And that is surely not a signal
> handler job (qemu timer handler aren't thread-safe in general).
>
> Anyone any ideas? /me is thinking a bit more about it as well.
>   

host_alarm_handler should write to a file descriptor instead of setting 
a flag.  That file descriptor should then be select()'d on (just like we 
do for SIGUSR2 in block-raw-posix.c).

Regards,

Anthony Liguori

> Jan
>
>