[pve-devel] [PATCH v2 qemu 2/2] ui/vnc.c: Fixed a deadlock bug.

Fabian Ebner f.ebner at proxmox.com
Fri May 6 14:38:36 CEST 2022

From: Rao Lei <lei.rao at intel.com>

The GDB statck is as follows:
(gdb) bt
0  __lll_lock_wait (futex=futex at 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();

Fixes: 0bf41cab93e ("ui/vnc: clipboard support")
Signed-off-by: Lei Rao <lei.rao at intel.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau at redhat.com>
Message-Id: <20220105020808.597325-1-lei.rao at intel.com>
Signed-off-by: Gerd Hoffmann <kraxel at redhat.com>
(cherry-picked from commit 1dbbe6f172810026c51dc84ed927a3cc23017949)
[FE: trivial backport for 6.2]
Signed-off-by: Fabian Ebner <f.ebner at proxmox.com>

New in v2.

Reported in the community forum:

 ui/vnc.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/ui/vnc.c b/ui/vnc.c
index af02522e84..b253e85c65 100644
--- a/ui/vnc.c
+++ b/ui/vnc.c
@@ -1354,12 +1354,12 @@ void vnc_disconnect_finish(VncState *vs)
         /* last client gone */
+    vnc_unlock_output(vs);
     if (vs->cbpeer.update.notify) {
-    vnc_unlock_output(vs);
     if (vs->bh != NULL) {

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