Re: [PATCH] tests/iothread: Always connect iothread GSource to a GMainContext

[Marc-André Lureau]

Hi

On Mon, Jan 6, 2020 at 7:03 PM Peter Maydell <address@hidden> wrote:
>
> On older versions of glib (anything prior to glib commit 0f056ebe
> from May 2019), the implementation of g_source_ref() and
> g_source_unref() is not threadsafe for a GSource which is not
> attached to a GMainContext.
>
> QEMU's real iothread.c implementation always attaches its
> iothread->ctx's GSource to a GMainContext created for that iothread,
> so it is OK, but the simple test framework implementation in
> tests/iothread.c was not doing this.  This was causing intermittent
> assertion failures in the test-aio-multithread subtest
> "/aio/multi/mutex/contended" test on the BSD hosts.  (It's unclear
> why only BSD seems to have been affected -- perhaps a combination of
> the specific glib version being used in the VMs and their happening
> to run on a host with a lot of CPUs).
>
> Borrow the iothread_init_gcontext() from the real iothread.c
> and add the corresponding cleanup code and the calls to
> g_main_context_push/pop_thread_default() so we actually use
> the GMainContext we create.
>
> Signed-off-by: Peter Maydell <address@hidden>
> ---
> I don't really have a good understanding of the glib APIs here,
> so I'm mostly just cribbing code from the real iothread.c;
> review by people who do know the glib/iothread stuff better
> welcomed. It does seem to fix the intermittent test failure
> on NetBSD, at least, where we were running into an assertion
> failure because a g_source_unref() incorrectly thought it
> had decremented the refcount to 0 and should delete a context
> that was actually still in use.
>
>  tests/iothread.c | 30 ++++++++++++++++++++++++++++++
>  1 file changed, 30 insertions(+)
>
> diff --git a/tests/iothread.c b/tests/iothread.c
> index 13c9fdcd8df..d3a2ee9a014 100644
> --- a/tests/iothread.c
> +++ b/tests/iothread.c
> @@ -21,6 +21,8 @@
>
>  struct IOThread {
>      AioContext *ctx;
> +    GMainContext *worker_context;
> +    GMainLoop *main_loop;
>
>      QemuThread thread;
>      QemuMutex init_done_lock;
> @@ -35,6 +37,17 @@ AioContext *qemu_get_current_aio_context(void)
>      return my_iothread ? my_iothread->ctx : qemu_get_aio_context();
>  }
>
> +static void iothread_init_gcontext(IOThread *iothread)
> +{
> +    GSource *source;
> +
> +    iothread->worker_context = g_main_context_new();
> +    source = aio_get_g_source(iothread_get_aio_context(iothread));
> +    g_source_attach(source, iothread->worker_context);
> +    g_source_unref(source);
> +    iothread->main_loop = g_main_loop_new(iothread->worker_context, TRUE);
> +}
> +
>  static void *iothread_run(void *opaque)
>  {
>      IOThread *iothread = opaque;
> @@ -44,6 +57,20 @@ static void *iothread_run(void *opaque)
>      my_iothread = iothread;
>      qemu_mutex_lock(&iothread->init_done_lock);
>      iothread->ctx = aio_context_new(&error_abort);
> +
> +    /*
> +     * We must connect the ctx to a GMainContext, because in older versions
> +     * of glib the g_source_ref()/unref() functions are not threadsafe
> +     * on sources without a context.
> +     */
> +    iothread_init_gcontext(iothread);
> +
> +    /*
> +     * g_main_context_push_thread_default() must be called before anything
> +     * in this new thread uses glib.

in/if, I suppose

> +     */
> +    g_main_context_push_thread_default(iothread->worker_context);
> +
>      qemu_cond_signal(&iothread->init_done_cond);
>      qemu_mutex_unlock(&iothread->init_done_lock);
>
> @@ -51,6 +78,7 @@ static void *iothread_run(void *opaque)
>          aio_poll(iothread->ctx, true);
>      }
>
> +    g_main_context_pop_thread_default(iothread->worker_context);
>      rcu_unregister_thread();
>      return NULL;
>  }
> @@ -66,6 +94,8 @@ void iothread_join(IOThread *iothread)
>  {
>      aio_bh_schedule_oneshot(iothread->ctx, iothread_stop_bh, iothread);
>      qemu_thread_join(&iothread->thread);
> +    g_main_context_unref(iothread->worker_context);
> +    g_main_loop_unref(iothread->main_loop);
>      qemu_cond_destroy(&iothread->init_done_cond);
>      qemu_mutex_destroy(&iothread->init_done_lock);
>      aio_context_unref(iothread->ctx);
> --
> 2.20.1
>
>

Reviewed-by: Marc-André Lureau <address@hidden>


-- 
Marc-André Lureau