Re: [Qemu-devel] [RFC v3 01/27] char-io: fix possible race on IOWatchPoll

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Mon, Nov 06, 2017 at 05:46:17PM +0800, Peter Xu 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 race can happen.
> 
> The race can be triggered with "make check -j8" sometimes:

Please mention a specific test case that fails.

> 
>   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.
> 
> Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> Signed-off-by: Peter Xu <peterx@redhat.com>
> ---
>  chardev/char-io.c | 16 ++++++++++++++--
>  1 file changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/chardev/char-io.c b/chardev/char-io.c
> index f81052481a..50b5bac704 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 +130,25 @@ 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 race condition
> +     * between current thread and the context loop thread.
> +     *
> +     * Let's blame the glib bug mentioned in commit 2b316774f6
> +     * ("qemu-char: do not operate on sources from finalize
> +     * callbacks") for this extra complexity.

I don't understand how this bug is to blame.  Isn't the problem here a
race condition between two QEMU threads?

Why are two threads accessing the watch at the same time?

> +     */
> +    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)
> -- 
> 2.13.5
> 

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