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Re: [Qemu-devel] [PATCH] monitor: avoid potential dead-lock when cleanin
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From: |
Marc-André Lureau |
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Subject: |
Re: [Qemu-devel] [PATCH] monitor: avoid potential dead-lock when cleaning up |
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Date: |
Fri, 17 Aug 2018 19:50:59 +0200 |
Hi
On Wed, Aug 1, 2018 at 5:09 PM Markus Armbruster <address@hidden> wrote:
>
> Marc-André Lureau <address@hidden> writes:
>
> > Hi
> >
> > On Wed, Aug 1, 2018 at 3:19 PM, Markus Armbruster <address@hidden> wrote:
> >> 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 situation by tightening the monitor lock time to
> >>> the monitor list removal.
> >>>
> >>> Signed-off-by: Marc-André Lureau <address@hidden>
> >>
> >> Do you think this should go into 3.0?
> >>
> >>> ---
> >>> monitor.c | 22 +++++++++++++++-------
> >>> 1 file changed, 15 insertions(+), 7 deletions(-)
> >>>
> >>> diff --git a/monitor.c b/monitor.c
> >>> index 0fa0910a2a..a16a6c5311 100644
> >>> --- a/monitor.c
> >>> +++ b/monitor.c
> >>> @@ -4702,8 +4702,6 @@ void monitor_init(Chardev *chr, int flags)
> >>>
> >>> void monitor_cleanup(void)
> >>> {
> >>> - Monitor *mon, *next;
> >>> -
> >>> /*
> >>> * We need to explicitly stop the I/O thread (but not destroy it),
> >>> * clean up the monitor resources, then destroy the I/O thread since
> >>> @@ -4719,14 +4717,24 @@ void monitor_cleanup(void)
> >>> monitor_qmp_bh_responder(NULL);
> >>>
> >>> /* Flush output buffers and destroy monitors */
> >>> - qemu_mutex_lock(&monitor_lock);
> >>> - QTAILQ_FOREACH_SAFE(mon, &mon_list, entry, next) {
> >>> - QTAILQ_REMOVE(&mon_list, mon, entry);
> >>> + do {
> >>
> >> for (;;), please.
> >>
> >>> + Monitor *mon;
> >>> +
> >>> + qemu_mutex_lock(&monitor_lock);
> >>> + mon = QTAILQ_FIRST(&mon_list);
> >>> + if (mon) {
> >>> + QTAILQ_REMOVE(&mon_list, mon, entry);
> >>> + }
> >>> + qemu_mutex_unlock(&monitor_lock);
> >>> +
> >>> + if (!mon) {
> >>> + break;
> >>> + }
> >>> +
> >>> monitor_flush(mon);
> >>> monitor_data_destroy(mon);
> >>> g_free(mon);
> >>> - }
> >>> - qemu_mutex_unlock(&monitor_lock);
> >>> + } while (true);
> >>>
> >>> /* QEMUBHs needs to be deleted before destroying the I/O thread */
> >>> qemu_bh_delete(qmp_dispatcher_bh);
> >>
> >> Iterating safely over a list protected by a lock should be easier than
> >> that. Sad.
> >>
> >> Hmm, what about this:
> >>
> >> diff --git a/monitor.c b/monitor.c
> >> index 77861e96af..4a23f6c7bc 100644
> >> --- a/monitor.c
> >> +++ b/monitor.c
> >> @@ -4721,9 +4721,11 @@ void monitor_cleanup(void)
> >> qemu_mutex_lock(&monitor_lock);
> >> 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);
> >> g_free(mon);
> >> + qemu_mutex_lock(&monitor_lock);
> >
> > Although unlikely, there is a chance the monitor list could be
> > modified while flushing/cleaning up, I suppose, in this case we could
> > miss the new monitors (if next is NULL).
>
> Your loop prevents that from happening while it runs, but does nothing
> to stop it from happening afterwards. If we want to lock out new
> monitors, we need to make monitor_init() fail or impossible to call.
Not so trivial. Is there other threads capable of calling
monitor_init() by the time monitor_cleanup() is called? It looks like
monitor_init() may only be called from the main thread.
>
> >
> >> }
> >> qemu_mutex_unlock(&monitor_lock);
> >>
> >>
--
Marc-André Lureau