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Re: [Qemu-devel] [PATCH v2 6/6] monitor: avoid potential dead-lock when
From: |
Markus Armbruster |
Subject: |
Re: [Qemu-devel] [PATCH v2 6/6] monitor: avoid potential dead-lock when cleaning up |
Date: |
Mon, 03 Dec 2018 10:26:05 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) |
Marc-André Lureau <address@hidden> writes:
> When a monitor is connected to a Spice chardev, the monitor cleanup
> can dead-lock:
>
> #0 0x00007f43446637fd in __lll_lock_wait () at /lib64/libpthread.so.0
> #1 0x00007f434465ccf4 in pthread_mutex_lock () at /lib64/libpthread.so.0
> #2 0x0000556dd79f22ba in qemu_mutex_lock_impl (mutex=0x556dd81c9220
> <monitor_lock>, file=0x556dd7ae3648 "/home/elmarco/src/qq/monitor.c",
> line=645) at /home/elmarco/src/qq/util/qemu-thread-posix.c:66
> #3 0x0000556dd7431bd5 in monitor_qapi_event_queue
> (event=QAPI_EVENT_SPICE_DISCONNECTED, qdict=0x556dd9abc850,
> errp=0x7fffb7bbddd8) at /home/elmarco/src/qq/monitor.c:645
> #4 0x0000556dd79d476b in qapi_event_send_spice_disconnected
> (server=0x556dd98ee760, client=0x556ddaaa8560, errp=0x556dd82180d0
> <error_abort>) at qapi/qapi-events-ui.c:149
> #5 0x0000556dd7870fc1 in channel_event (event=3, info=0x556ddad1b590) at
> /home/elmarco/src/qq/ui/spice-core.c:235
> #6 0x00007f434560a6bb in reds_handle_channel_event (reds=<optimized out>,
> event=3, info=0x556ddad1b590) at reds.c:316
> #7 0x00007f43455f393b in main_dispatcher_self_handle_channel_event
> (info=0x556ddad1b590, event=3, self=0x556dd9a7d8c0) at main-dispatcher.c:197
> #8 0x00007f43455f393b in main_dispatcher_channel_event
> (self=0x556dd9a7d8c0, address@hidden, info=0x556ddad1b590) at
> main-dispatcher.c:197
> #9 0x00007f4345612833 in red_stream_push_channel_event (address@hidden,
> address@hidden) at red-stream.c:414
> #10 0x00007f434561286b in red_stream_free (s=0x556ddae2ef40) at
> red-stream.c:388
> #11 0x00007f43455f9ddc in red_channel_client_finalize
> (object=0x556dd9bb21a0) at red-channel-client.c:347
> #12 0x00007f434b5f9fb9 in g_object_unref () at /lib64/libgobject-2.0.so.0
> #13 0x00007f43455fc212 in red_channel_client_push (rcc=0x556dd9bb21a0) at
> red-channel-client.c:1341
> #14 0x0000556dd76081ba in spice_port_set_fe_open (chr=0x556dd9925e20,
> fe_open=0) at /home/elmarco/src/qq/chardev/spice.c:241
> #15 0x0000556dd796d74a in qemu_chr_fe_set_open (be=0x556dd9a37c00,
> fe_open=0) at /home/elmarco/src/qq/chardev/char-fe.c:340
> #16 0x0000556dd796d4d9 in qemu_chr_fe_set_handlers (b=0x556dd9a37c00,
> fd_can_read=0x0, fd_read=0x0, fd_event=0x0, be_change=0x0, opaque=0x0,
> context=0x0, set_open=true) at /home/elmarco/src/qq/chardev/char-fe.c:280
> #17 0x0000556dd796d359 in qemu_chr_fe_deinit (b=0x556dd9a37c00, del=false)
> at /home/elmarco/src/qq/chardev/char-fe.c:233
> #18 0x0000556dd7432240 in monitor_data_destroy (mon=0x556dd9a37c00) at
> /home/elmarco/src/qq/monitor.c:786
> #19 0x0000556dd743b968 in monitor_cleanup () at
> /home/elmarco/src/qq/monitor.c:4683
> #20 0x0000556dd75ce776 in main (argc=3, argv=0x7fffb7bbe458,
> envp=0x7fffb7bbe478) at /home/elmarco/src/qq/vl.c:4660
>
> Because spice code tries to emit a "disconnected" signal on the
> monitors. Fix this dead-lock by releasing the monitor lock for
> flush/destroy.
>
> Signed-off-by: Marc-André Lureau <address@hidden>
> ---
> monitor.c | 2 ++
> 1 file changed, 2 insertions(+)
>
> diff --git a/monitor.c b/monitor.c
> index 7fe89daa87..b55e604a98 100644
> --- a/monitor.c
> +++ b/monitor.c
> @@ -4643,8 +4643,10 @@ void monitor_cleanup(void)
> monitor_destroyed = true;
> QTAILQ_FOREACH_SAFE(mon, &mon_list, entry, next) {
> QTAILQ_REMOVE(&mon_list, mon, entry);
> + qemu_mutex_unlock(&monitor_lock);
> monitor_flush(mon);
> monitor_data_destroy(mon);
> + qemu_mutex_lock(&monitor_lock);
> g_free(mon);
> }
> qemu_mutex_unlock(&monitor_lock);
I think a comment hinting at the reason for relinquishing the lock would
be in order. Perhaps
/* Permit QAPI event emission from character frontend release */
qemu_mutex_unlock(&monitor_lock);
We need to demonstrate calling monitor_flush() and
monitor_data_destroy() without holding @monitor_lock is safe.
@monitor_lock's comment states it "protects mon_list,
monitor_qapi_event_state." Looks plausible from how it's used.
As far as I can tell, monitor_flush() and monitor_data_destroy() don't
access mon_list and monitor_qapi_event_state.
monitor_cleanup()'s loop itself is safe because it uses
QTAILQ_FOREACH_SAFE(), unlike similar loops elsewhere.
I think we're good. I'd like you to work this argument into the commit
message.
- Re: [Qemu-devel] [PATCH v2 6/6] monitor: avoid potential dead-lock when cleaning up,
Markus Armbruster <=