Re: [Qemu-devel] [RFC PATCH V2] qemu-char: Fix context for g_source_attach()

[Fam Zheng <famz@redhat.com>]

On Wed, 06/22 18:49, Zhang Chen wrote:
> We want to poll and handle chardev in another thread
> other than main loop. But qemu_chr_add_handlers() can only
> work for global default context other than thread default context.
> So we use g_source_attach(xx, g_main_context_get_thread_default())
> replace g_source_attach(xx, NULL) to attach g_source.
> Comments from jason.
> 
> Signed-off-by: Zhang Chen <zhangchen.fnst@cn.fujitsu.com>
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
>  io/channel.c | 2 +-
>  qemu-char.c  | 6 +++---
>  2 files changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/io/channel.c b/io/channel.c
> index 692eb17..cd25677 100644
> --- a/io/channel.c
> +++ b/io/channel.c
> @@ -146,7 +146,7 @@ guint qio_channel_add_watch(QIOChannel *ioc,
>  
>      g_source_set_callback(source, (GSourceFunc)func, user_data, notify);
>  
> -    id = g_source_attach(source, NULL);
> +    id = g_source_attach(source, g_main_context_get_thread_default());
>      g_source_unref(source);
>  
>      return id;
> diff --git a/qemu-char.c b/qemu-char.c
> index 84f49ac..4340457 100644
> --- a/qemu-char.c
> +++ b/qemu-char.c
> @@ -859,7 +859,7 @@ static gboolean io_watch_poll_prepare(GSource *source, gint *timeout_)
>          iwp->src = qio_channel_create_watch(
>              iwp->ioc, G_IO_IN | G_IO_ERR | G_IO_HUP | G_IO_NVAL);
>          g_source_set_callback(iwp->src, iwp->fd_read, iwp->opaque, NULL);
> -        g_source_attach(iwp->src, NULL);
> +        g_source_attach(iwp->src, g_main_context_get_thread_default());
>      } else {
>          g_source_destroy(iwp->src);
>          g_source_unref(iwp->src);
> @@ -918,7 +918,7 @@ static guint io_add_watch_poll(QIOChannel *ioc,
>      iwp->fd_read = (GSourceFunc) fd_read;
>      iwp->src = NULL;
>  
> -    tag = g_source_attach(&iwp->parent, NULL);
> +    tag = g_source_attach(&iwp->parent, g_main_context_get_thread_default());
>      g_source_unref(&iwp->parent);
>      return tag;
>  }
> @@ -3982,7 +3982,7 @@ int qemu_chr_fe_add_watch(CharDriverState *s, GIOCondition cond,
>      }
>  
>      g_source_set_callback(src, (GSourceFunc)func, user_data, NULL);
> -    tag = g_source_attach(src, NULL);
> +    tag = g_source_attach(src, g_main_context_get_thread_default());
>      g_source_unref(src);
>  
>      return tag;
> -- 

IIRC this opens a gate for your special thread (COLO compare thread?) to use
QIOChannel.

I think in the long run it is better to think about allowing integrating QIO to
AioContext, to support its usage outside main loop.  Given how opaque GSource
is, I'm not sure how feasible that is, or how useful it will be.  Anyway we
should definitely hear more opinions from Daniel and Paolo.

Fam