Re: [RFC PATCH] ui/vnc.c: Fixed a deadlock bug.

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

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Hi

On Wed, Dec 29, 2021 at 6:18 AM Rao Lei <lei.rao@intel.com> wrote:

> The GDB statck is as follows:
> (gdb) bt
> 0  __lll_lock_wait (futex=futex@entry=0x56211df20360, private=0) at
> lowlevellock.c:52
> 1  0x00007f263caf20a3 in __GI___pthread_mutex_lock (mutex=0x56211df20360)
> at ../nptl/pthread_mutex_lock.c:80
> 2  0x000056211a757364 in qemu_mutex_lock_impl (mutex=0x56211df20360,
> file=0x56211a804857 "../ui/vnc-jobs.h", line=60)
>     at ../util/qemu-thread-posix.c:80
> 3  0x000056211a0ef8c7 in vnc_lock_output (vs=0x56211df14200) at
> ../ui/vnc-jobs.h:60
> 4  0x000056211a0efcb7 in vnc_clipboard_send (vs=0x56211df14200, count=1,
> dwords=0x7ffdf1701338) at ../ui/vnc-clipboard.c:138
> 5  0x000056211a0f0129 in vnc_clipboard_notify (notifier=0x56211df244c8,
> data=0x56211dd1bbf0) at ../ui/vnc-clipboard.c:209
> 6  0x000056211a75dde8 in notifier_list_notify (list=0x56211afa17d0
> <clipboard_notifiers>, data=0x56211dd1bbf0) at ../util/notify.c:39
> 7  0x000056211a0bf0e6 in qemu_clipboard_update (info=0x56211dd1bbf0) at
> ../ui/clipboard.c:50
> 8  0x000056211a0bf05d in qemu_clipboard_peer_release (peer=0x56211df244c0,
> selection=QEMU_CLIPBOARD_SELECTION_CLIPBOARD)
>     at ../ui/clipboard.c:41
> 9  0x000056211a0bef9b in qemu_clipboard_peer_unregister
> (peer=0x56211df244c0) at ../ui/clipboard.c:19
> 10 0x000056211a0d45f3 in vnc_disconnect_finish (vs=0x56211df14200) at
> ../ui/vnc.c:1358
> 11 0x000056211a0d4c9d in vnc_client_read (vs=0x56211df14200) at
> ../ui/vnc.c:1611
> 12 0x000056211a0d4df8 in vnc_client_io (ioc=0x56211ce70690,
> condition=G_IO_IN, opaque=0x56211df14200) at ../ui/vnc.c:1649
> 13 0x000056211a5b976c in qio_channel_fd_source_dispatch
>     (source=0x56211ce50a00, callback=0x56211a0d4d71 <vnc_client_io>,
> user_data=0x56211df14200) at ../io/channel-watch.c:84
> 14 0x00007f263ccede8e in g_main_context_dispatch () at
> /lib/x86_64-linux-gnu/libglib-2.0.so.0
> 15 0x000056211a77d4a1 in glib_pollfds_poll () at ../util/main-loop.c:232
> 16 0x000056211a77d51f in os_host_main_loop_wait (timeout=958545) at
> ../util/main-loop.c:255
> 17 0x000056211a77d630 in main_loop_wait (nonblocking=0) at
> ../util/main-loop.c:531
> 18 0x000056211a45bc8e in qemu_main_loop () at ../softmmu/runstate.c:726
> 19 0x000056211a0b45fa in main (argc=69, argv=0x7ffdf1701778,
> envp=0x7ffdf17019a8) at ../softmmu/main.c:50
>
> From the call trace, we can see it is a deadlock bug.
> vnc_disconnect_finish will acquire the output_mutex.
> But, the output_mutex will be acquired again in vnc_clipboard_send.
> Repeated locking will cause deadlock. So, I move
> qemu_clipboard_peer_unregister() behind vnc_unlock_output();
>
> Signed-off-by: Lei Rao <lei.rao@intel.com>
>

Looks good to me. Please add to the commit message

Fixes: 0bf41cab93e ("ui/vnc: clipboard support")

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

---
>  ui/vnc.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/ui/vnc.c b/ui/vnc.c
> index 1ed1c7efc6..3ccd33dedc 100644
> --- a/ui/vnc.c
> +++ b/ui/vnc.c
> @@ -1354,12 +1354,12 @@ void vnc_disconnect_finish(VncState *vs)
>          /* last client gone */
>          vnc_update_server_surface(vs->vd);
>      }
> +    vnc_unlock_output(vs);
> +
>      if (vs->cbpeer.notifier.notify) {
>          qemu_clipboard_peer_unregister(&vs->cbpeer);
>      }
>
> -    vnc_unlock_output(vs);
> -
>      qemu_mutex_destroy(&vs->output_mutex);
>      if (vs->bh != NULL) {
>          qemu_bh_delete(vs->bh);
> --
> 2.32.0
>
>
>

-- 
Marc-André Lureau

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