Re: [Qemu-devel] [PATCH v2] qemu-iothread: IOThread supports the GMainContext event loop

[Stefan Hajnoczi]

On Tue, Aug 15, 2017 at 03:28:15PM +0800, Wang yong wrote:
> From: Wang Yong <address@hidden>
> 
> IOThread uses AioContext event loop and does not run a GMainContext.
> Therefore,chardev cannot work in IOThread,such as the chardev is
> used for colo-compare packets reception.
> 
> This patch makes the IOThread run the GMainContext event loop,
> chardev and IOThread can work together.
> 
> Signed-off-by: Wang Yong<address@hidden>
> Signed-off-by: Wang Guang<address@hidden>

There is usually a space between the name and email address.  Tools and
humans searching git log might not find you if you format the S-o-b line
without a space.

> ---
>  include/sysemu/iothread.h | 10 +++++++++
>  iothread.c                | 54 
> +++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 64 insertions(+)
> 
> diff --git a/include/sysemu/iothread.h b/include/sysemu/iothread.h
> index e6da1a4..89e913c 100644
> --- a/include/sysemu/iothread.h
> +++ b/include/sysemu/iothread.h
> @@ -20,10 +20,19 @@
>  #define TYPE_IOTHREAD "iothread"
>  
>  typedef struct {
> +    GMainContext *worker_context;
> +    GMainLoop *main_loop;
> +    GOnce once;
> +
> +    QEMUBH *bh;
> +} GMainOnce;

Please do not use the glib namespace (G*, g_*) because it's easy to
confuse GMainOnce with glib APIs like GOnce.

I suggest inlining these fields in IOThread instead of defining a new
struct.

> +
> +typedef struct {
>      Object parent_obj;
>  
>      QemuThread thread;
>      AioContext *ctx;
> +    GMainOnce thread_gonce;
>      QemuMutex init_done_lock;
>      QemuCond init_done_cond;    /* is thread initialization done? */
>      bool stopping;
> @@ -41,5 +50,6 @@ typedef struct {
>  char *iothread_get_id(IOThread *iothread);
>  AioContext *iothread_get_aio_context(IOThread *iothread);
>  void iothread_stop_all(void);
> +GMainContext *iothread_get_g_main_context(IOThread *iothread);
>  
>  #endif /* IOTHREAD_H */
> diff --git a/iothread.c b/iothread.c
> index beeb870..9995eb0 100644
> --- a/iothread.c
> +++ b/iothread.c
> @@ -72,6 +72,9 @@ static int iothread_stop(Object *object, void *opaque)
>          return 0;
>      }
>      iothread->stopping = true;
> +    if (iothread->thread_gonce.main_loop) {
> +        g_main_loop_quit(iothread->thread_gonce.main_loop);
> +    }
>      aio_notify(iothread->ctx);
>      qemu_thread_join(&iothread->thread);
>      return 0;
> @@ -125,6 +128,7 @@ static void iothread_complete(UserCreatable *obj, Error 
> **errp)
>  
>      qemu_mutex_init(&iothread->init_done_lock);
>      qemu_cond_init(&iothread->init_done_cond);
> +    iothread->thread_gonce.once = (GOnce) G_ONCE_INIT;
>  
>      /* This assumes we are called from a thread with useful CPU affinity for 
> us
>       * to inherit.
> @@ -309,3 +313,53 @@ void iothread_stop_all(void)
>  
>      object_child_foreach(container, iothread_stop, NULL);
>  }
> +
> +static void iothread_g_main_context_bh(void *opaque)
> +{
> +    GMainOnce *g = opaque;
> +
> +    qemu_bh_delete(g->bh);
> +    g->bh = NULL;
> +
> +    g_main_context_push_thread_default(g->worker_context);
> +
> +    g->main_loop = g_main_loop_new(g->worker_context, TRUE);
> +    g_main_loop_run(g->main_loop);

Running the glib main loop from a BH function means deleted BHs can
never be freed in aio-posix.c:aio_bh_poll():

  if (qemu_lockcnt_dec_if_lock(&ctx->list_lock)) {
      bhp = &ctx->first_bh;
      while (*bhp) {
          bh = *bhp;
          if (bh->deleted && !bh->scheduled) {
              *bhp = bh->next;
              g_free(bh);
          } else {
              bhp = &bh->next;
          }
      }
      qemu_lockcnt_inc_and_unlock(&ctx->list_lock);
  }

ctx->list_lock is held by iothread_run() -> aio_poll() so
qemu_lockcnt_dec_if_lock() always returns false.

Instead of running the glib event loop inside aio_poll() I would modify
iothread_run():

    while (!atomic_read(&iothread->stopping)) {
        if (atomic_read(&iothread->use_glib_event_loop)) {
            iothread_glib_event_loop();
            break;
        }

        aio_poll(iothread->ctx, true);
    }

> +
> +    g_main_loop_unref(g->main_loop);
> +    g->main_loop = NULL;
> +
> +    g_main_context_pop_thread_default(g->worker_context);
> +    g_main_context_unref(g->worker_context);
> +    g->worker_context = NULL;
> +}
> +
> +static gpointer iothread_g_main_context_init(gpointer g_data)
> +{
> +    AioContext *ctx;
> +    IOThread *iothread = (IOThread *)g_data;

No cast is necessary from void * to another pointer type in C.  Please
remove the cast.

> +    GMainOnce *g = &iothread->thread_gonce;
> +    GSource *source;
> +
> +    g->worker_context = g_main_context_new();
> +
> +    ctx = iothread_get_aio_context(iothread);
> +    source = aio_get_g_source(ctx);
> +    g_source_attach(source, g->worker_context);
> +    g_source_unref(source);
> +
> +    g->bh = aio_bh_new(ctx,
> +                       iothread_g_main_context_bh, g);
> +    qemu_bh_schedule(g->bh);
> +
> +    return (gpointer) g->worker_context;

No cast is necessary from a pointer type to void * in C.  Please remove
the cast.

> +}
> +
> +GMainContext *iothread_get_g_main_context(IOThread *iothread)
> +{
> +    GMainOnce *g = &iothread->thread_gonce;
> +
> +    g_once(&g->once, iothread_g_main_context_init, iothread);
> +
> +    return (GMainContext *) g->once.retval;

No cast is necessary from a pointer type to void * in C.  Please remove
the cast.

A dangling pointer is returned if this this function is called after
iothread_g_main_context_bh() has terminated.  It would be safer to
return g->worker_context instead of g->once.retval.

signature.asc
Description: PGP signature