[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Qemu-devel] [RFC v2 3/8] char-io: fix possible risk on IOWatchPoll
From: |
Marc-André Lureau |
Subject: |
Re: [Qemu-devel] [RFC v2 3/8] char-io: fix possible risk on IOWatchPoll |
Date: |
Fri, 25 Aug 2017 14:44:51 +0000 |
On Wed, Aug 23, 2017 at 8:54 AM Peter Xu <address@hidden> 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 <address@hidden>
>
Reviewed-by: Marc-André Lureau <address@hidden>
> ---
> 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
- Re: [Qemu-devel] [RFC v2 2/8] monitor: allow monitor to create thread to poll, (continued)
- Re: [Qemu-devel] [RFC v2 2/8] monitor: allow monitor to create thread to poll, Marc-André Lureau, 2017/08/25
- Re: [Qemu-devel] [RFC v2 2/8] monitor: allow monitor to create thread to poll, Dr. David Alan Gilbert, 2017/08/25
- Re: [Qemu-devel] [RFC v2 2/8] monitor: allow monitor to create thread to poll, Marc-André Lureau, 2017/08/25
- Re: [Qemu-devel] [RFC v2 2/8] monitor: allow monitor to create thread to poll, Dr. David Alan Gilbert, 2017/08/25
- Re: [Qemu-devel] [RFC v2 2/8] monitor: allow monitor to create thread to poll, Marc-André Lureau, 2017/08/25
- Re: [Qemu-devel] [RFC v2 2/8] monitor: allow monitor to create thread to poll, Dr. David Alan Gilbert, 2017/08/25
- Re: [Qemu-devel] [RFC v2 2/8] monitor: allow monitor to create thread to poll, Marc-André Lureau, 2017/08/26
- Re: [Qemu-devel] [RFC v2 2/8] monitor: allow monitor to create thread to poll, Peter Xu, 2017/08/27
[Qemu-devel] [RFC v2 3/8] char-io: fix possible risk on IOWatchPoll, Peter Xu, 2017/08/23
[Qemu-devel] [RFC v2 5/8] hmp: support "without_bql", Peter Xu, 2017/08/23
[Qemu-devel] [RFC v2 4/8] QAPI: new QMP command option "without-bql", Peter Xu, 2017/08/23