Re: [Qemu-devel] Re: [PATCH] Revert "posix-aio-compat: avoid signal race when spawning a thread"

[Anthony Liguori <anthony@codemonkey.ws>]

Michael S. Tsirkin wrote:
> BTW, tried looking at the code.
> I saw some (unrelated) issues:
>
> static void aio_signal_handler(int signum)
> {
>     if (posix_aio_state) {
>         char byte = 0;
>
>         write(posix_aio_state->wfd, &byte, sizeof(byte));
>     }
>
>     qemu_service_io();
> }
>
> And qemu_service_io does a *ton* of things.
>   

It just does a cpu_exit().

> Questions:
> -  Do we need the call to qemu_service_io? Seems to
>    behave the same with and without it.
>   

Where you run into trouble is when you have a guest that has no periodic 
timer  and needs a mechanism to break out of the tcg loop whenever an IO 
operation completes.  See


Author: aliguori <aliguori@c046a42c-6fe2-441c-8c8c-71466251a162>
Date:   Wed Oct 8 19:50:24 2008 +0000

    Fix IO performance regression in sparc

> -  Are all of the data structures touched by qemu_service_io
>    protected by blocking signals before access?
>   

cpu_exit is pretty careful to not do anything that could be racey.

> Also:
> 	- let's use signalfd on linux, if available?
>   

signalfd doesn't help.  In fact, we originally used signalfd and it 
caused a regression.

> 	- for SIGALARM, maybe timerfd?
>   

Or just get rid of SIGALARM...

Regards,

Anthony Liguori