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

[Avi Kivity]

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....
>
> 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.
>   

pselect():

1. run tcg code
2. block signals
3. pselect (enabling signals)
4. process pselect results (signals disabled again)
5. unblock signals
6. goto 1

Prior to Linux 2.6.16, pselect() was racy since it was implemented in 
userspace rather than the kernel.  I don't think it's a problem now.

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.