diff mbox series

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

Message ID 20211229021404.100550-1-lei.rao@intel.com (mailing list archive)
State New, archived
Headers show
Series [RFC] ui/vnc.c: Fixed a deadlock bug. | expand

Commit Message

Rao, Lei Dec. 29, 2021, 2:14 a.m. UTC 
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>
---
 ui/vnc.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

Comments

Marc-André Lureau Jan. 3, 2022, 8:14 a.m. UTC | #1 
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
>
>
>
diff mbox series

Patch

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);