Re: [Qemu-devel] [PATCH 1/4] iothread: wait until the glib context is acquired

[Peter Xu <peterx@redhat.com>]

On Wed, Feb 20, 2019 at 05:06:25PM +0100, Marc-André Lureau wrote:
> Another thread may acquire the glib context (temporarily) before
> g_main_context_push_thread_default().
> 
> This can happen with the following qemu_chr_fe_set_handlers()
> modifications.
> 
> Unfortunately, g_main_context_wait() is deprecated in glib
> 2.58 (apparently it was a broken interface). Use a polling loop.
> 
> Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> ---
>  iothread.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/iothread.c b/iothread.c
> index e615b7ae52..93cc3aa875 100644
> --- a/iothread.c
> +++ b/iothread.c
> @@ -70,6 +70,11 @@ static void *iothread_run(void *opaque)
>          if (iothread->running && atomic_read(&iothread->worker_context)) {
>              GMainLoop *loop;
>  
> +            /* we may race with another thread acquiring the context */
> +            while (!g_main_context_acquire(iothread->worker_context)) {
> +                g_usleep(10000);
> +            }

Could you help explain why need this explicitly?  Since AFAIU
g_main_loop_run() below will do context acquire too so IIUC you're
taking it twice (while g_main_context_acquire should allow it to
happen, though)?

> +
>              g_main_context_push_thread_default(iothread->worker_context);
>              iothread->main_loop =
>                  g_main_loop_new(iothread->worker_context, TRUE);
> @@ -80,6 +85,8 @@ static void *iothread_run(void *opaque)
>              g_main_loop_unref(loop);
>  
>              g_main_context_pop_thread_default(iothread->worker_context);
> +
> +            g_main_context_release(iothread->worker_context);
>          }
>      }
>  
> -- 
> 2.21.0.rc1
> 
> 

Regards,

-- 
Peter Xu