[Qemu-devel] Re: [PATCH 2/2] vnc: don't mess up with iohandlers in the vnc thread

[Paolo Bonzini]

On 03/10/2011 02:45 PM, Anthony Liguori wrote:
> On 03/10/2011 07:06 AM, Paolo Bonzini wrote:
> > On 03/10/2011 01:59 PM, Corentin Chary wrote:
> > > Instead, we now store the data in a temporary buffer, and use a socket
> > > pair to notify the main thread that new data is available.
> >
> > You can use a bottom half for this instead of a special socket.
> > Signaling a bottom half is async-signal- and thread-safe.
>
> Bottom halves are thread safe?
>
> I don't think so.
>
> They probably should be but they aren't today.

Creating a new bottom half is not thread-safe, but scheduling one is. 
Assuming that you never use qemu_bh_schedule_idle, qemu_bh_schedule 
boils down to:

    if (bh->scheduled)
        return;
    bh->scheduled = 1;
    /* stop the currently executing CPU to execute the BH ASAP */
    qemu_notify_event();

You may have a spurious wakeup if two threads race on the same bottom 
half (including the signaling thread racing with the IO thread), but 
overall you can safely treat them as thread-safe.

Paolo