Re: [Qemu-devel] [PATCH] event_notifier: move to top-level directory

[Paolo Bonzini]

On 09/27/2011 11:23 PM, Anthony Liguori wrote:
> On 09/27/2011 11:39 AM, Paolo Bonzini wrote:
> > On 09/27/2011 06:05 PM, Anthony Liguori wrote:
> > > Actually, for posix-aio, we can just switch to using g_idle_add().
> > > g_idle_add() uses g_source_attach which is thread safe. g_idle_add()
> > > gives you a thread safe mechanism to defer a piece of work to the
> > > main loop which is really what we want here.
> >
> > For that, a bottom half would also do (apart that I am not sure it is
> > async-safe with TCG). In fact, that would make sense since all of
> > posix_aio_process_queue could become a bottom half.
>
> Bottom halves are signal safe, not thread safe.
>
> To make bottom halves thread safe, you would (in the very least) have to
> add some barriers when reading/writing the scheduling flag.

You can probably assume that qemu_notify_event (and dually the read in 
the main loop) are resp. write/read memory barriers.  Or even full.

If we switch entirely to GSources, it would be nice to use them.  But 
since we aren't, and our main loop functionality is quite different from 
glib's (it doesn't rely on abstractions for file descriptors, for 
example), it is just a painful incomplete transition to use glib's idle 
sources to do the exact same thing that is done by bottom halves (which 
are already in our toolbox).

Paolo