Re: [Qemu-devel] [RFC v2 3/8] char-io: fix possible risk on IOWatchPoll

["Marc-André Lureau" <marcandre.lureau@gmail.com>]

On Wed, Aug 23, 2017 at 8:54 AM Peter Xu <peterx@redhat.com> wrote:

> This is not a problem if we are only having one single loop thread like
> before.  However, after per-monitor thread is introduced, this is not
> true any more, and the risk can happen.
>
> The risk can be triggered with "make check -j8" sometimes:
>
>   qemu-system-x86_64: /root/git/qemu/chardev/char-io.c:91:
>   io_watch_poll_finalize: Assertion `iwp->src == NULL' failed.
>
> This patch keeps the reference for the watch object when creating in
> io_add_watch_poll(), so that the object will never be released in the
> context main loop, especially when the context loop is running in
> another standalone thread.  Meanwhile, when we want to remove the watch
> object, we always first detach the watch object from its owner context,
> then we continue with the cleanup.
>
> Without this patch, calling io_remove_watch_poll() in main loop thread
> is not thread-safe, since the other per-monitor thread may be modifying
> the watch object at the same time.
>
> Signed-off-by: Peter Xu <peterx@redhat.com>
>

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>



> ---
>  chardev/char-io.c | 15 +++++++++++++--
>  1 file changed, 13 insertions(+), 2 deletions(-)
>
> diff --git a/chardev/char-io.c b/chardev/char-io.c
> index f810524..5c52c40 100644
> --- a/chardev/char-io.c
> +++ b/chardev/char-io.c
> @@ -122,7 +122,6 @@ GSource *io_add_watch_poll(Chardev *chr,
>      g_free(name);
>
>      g_source_attach(&iwp->parent, context);
> -    g_source_unref(&iwp->parent);
>      return (GSource *)iwp;
>  }
>
> @@ -131,12 +130,24 @@ static void io_remove_watch_poll(GSource *source)
>      IOWatchPoll *iwp;
>
>      iwp = io_watch_poll_from_source(source);
> +
> +    /*
> +     * Here the order of destruction really matters.  We need to first
> +     * detach the IOWatchPoll object from the context (which may still
> +     * be running in another loop thread), only after that could we
> +     * continue to operate on iwp->src, or there may be risk condition
> +     * between current thread and the context loop thread.
> +     *
> +     * Let's blame the glib bug mentioned in commit 2b3167 (again) for
> +     * this extra complexity.
> +     */
> +    g_source_destroy(&iwp->parent);
>      if (iwp->src) {
>          g_source_destroy(iwp->src);
>          g_source_unref(iwp->src);
>          iwp->src = NULL;
>      }
> -    g_source_destroy(&iwp->parent);
> +    g_source_unref(&iwp->parent);
>  }
>
>  void remove_fd_in_watch(Chardev *chr)
> --
> 2.7.4
>
>
> --
Marc-André Lureau