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Re: [Qemu-devel] [PATCH v2] monitor: temporary fix for dead-lock on even
From: |
Markus Armbruster |
Subject: |
Re: [Qemu-devel] [PATCH v2] monitor: temporary fix for dead-lock on event recursion |
Date: |
Tue, 31 Jul 2018 17:25:05 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/26.1 (gnu/linux) |
Marc-André Lureau <address@hidden> writes:
> With a Spice port chardev, it is possible to reenter
> monitor_qapi_event_queue() (when the client disconnects for
> example). This will dead-lock on monitor_lock.
>
> Instead, use some TLS variables to check for recursion and queue the
> events.
>
> Fixes:
> (gdb) bt
> #0 0x00007fa69e7217fd in __lll_lock_wait () at /lib64/libpthread.so.0
> #1 0x00007fa69e71acf4 in pthread_mutex_lock () at /lib64/libpthread.so.0
> #2 0x0000563303567619 in qemu_mutex_lock_impl (mutex=0x563303d3e220
> <monitor_lock>, file=0x5633036589a8 "/home/elmarco/src/qq/monitor.c",
> line=645) at /home/elmarco/src/qq/util/qemu-thread-posix.c:66
> #3 0x0000563302fa6c25 in monitor_qapi_event_queue
> (event=QAPI_EVENT_SPICE_DISCONNECTED, qdict=0x56330602bde0,
> errp=0x7ffc6ab5e728) at /home/elmarco/src/qq/monitor.c:645
> #4 0x0000563303549aca in qapi_event_send_spice_disconnected
> (server=0x563305afd630, client=0x563305745360, errp=0x563303d8d0f0
> <error_abort>) at qapi/qapi-events-ui.c:149
> #5 0x00005633033e600f in channel_event (event=3, info=0x5633061b0050) at
> /home/elmarco/src/qq/ui/spice-core.c:235
> #6 0x00007fa69f6c86bb in reds_handle_channel_event (reds=<optimized out>,
> event=3, info=0x5633061b0050) at reds.c:316
> #7 0x00007fa69f6b193b in main_dispatcher_self_handle_channel_event
> (info=0x5633061b0050, event=3, self=0x563304e088c0) at main-dispatcher.c:197
> #8 0x00007fa69f6b193b in main_dispatcher_channel_event
> (self=0x563304e088c0, address@hidden, info=0x5633061b0050) at
> main-dispatcher.c:197
> #9 0x00007fa69f6d0833 in red_stream_push_channel_event (address@hidden,
> address@hidden) at red-stream.c:414
> #10 0x00007fa69f6d086b in red_stream_free (s=0x563305ad8f50) at
> red-stream.c:388
> #11 0x00007fa69f6b7ddc in red_channel_client_finalize
> (object=0x563304df2360) at red-channel-client.c:347
> #12 0x00007fa6a56b7fb9 in g_object_unref () at /lib64/libgobject-2.0.so.0
> #13 0x00007fa69f6ba212 in red_channel_client_push (rcc=0x563304df2360) at
> red-channel-client.c:1341
> #14 0x00007fa69f68b259 in red_char_device_send_msg_to_client
> (client=<optimized out>, msg=0x5633059b6310, dev=0x563304e08bc0) at
> char-device.c:305
> #15 0x00007fa69f68b259 in red_char_device_send_msg_to_clients
> (msg=0x5633059b6310, dev=0x563304e08bc0) at char-device.c:305
> #16 0x00007fa69f68b259 in red_char_device_read_from_device
> (dev=0x563304e08bc0) at char-device.c:353
> #17 0x000056330317d01d in spice_chr_write (chr=0x563304cafe20,
> buf=0x563304cc50b0 "{\"timestamp\": {\"seconds\": 1532944763,
> \"microseconds\": 326636}, \"event\": \"SHUTDOWN\", \"data\": {\"guest\":
> false}}\r\n", len=111) at /home/elmarco/src/qq/chardev/spice.c:199
> #18 0x00005633034deee7 in qemu_chr_write_buffer (s=0x563304cafe20,
> buf=0x563304cc50b0 "{\"timestamp\": {\"seconds\": 1532944763,
> \"microseconds\": 326636}, \"event\": \"SHUTDOWN\", \"data\": {\"guest\":
> false}}\r\n", len=111, offset=0x7ffc6ab5ea70, write_all=false) at
> /home/elmarco/src/qq/chardev/char.c:112
> #19 0x00005633034df054 in qemu_chr_write (s=0x563304cafe20,
> buf=0x563304cc50b0 "{\"timestamp\": {\"seconds\": 1532944763,
> \"microseconds\": 326636}, \"event\": \"SHUTDOWN\", \"data\": {\"guest\":
> false}}\r\n", len=111, write_all=false) at
> /home/elmarco/src/qq/chardev/char.c:147
> #20 0x00005633034e1e13 in qemu_chr_fe_write (be=0x563304dbb800,
> buf=0x563304cc50b0 "{\"timestamp\": {\"seconds\": 1532944763,
> \"microseconds\": 326636}, \"event\": \"SHUTDOWN\", \"data\": {\"guest\":
> false}}\r\n", len=111) at /home/elmarco/src/qq/chardev/char-fe.c:42
> #21 0x0000563302fa6334 in monitor_flush_locked (mon=0x563304dbb800) at
> /home/elmarco/src/qq/monitor.c:425
> #22 0x0000563302fa6520 in monitor_puts (mon=0x563304dbb800,
> str=0x563305de7e9e "") at /home/elmarco/src/qq/monitor.c:468
> #23 0x0000563302fa680c in qmp_send_response (mon=0x563304dbb800,
> rsp=0x563304df5730) at /home/elmarco/src/qq/monitor.c:517
> #24 0x0000563302fa6905 in qmp_queue_response (mon=0x563304dbb800,
> rsp=0x563304df5730) at /home/elmarco/src/qq/monitor.c:538
> #25 0x0000563302fa6b5b in monitor_qapi_event_emit
> (event=QAPI_EVENT_SHUTDOWN, qdict=0x563304df5730) at
> /home/elmarco/src/qq/monitor.c:624
> #26 0x0000563302fa6c4b in monitor_qapi_event_queue
> (event=QAPI_EVENT_SHUTDOWN, qdict=0x563304df5730, errp=0x7ffc6ab5ed00) at
> /home/elmarco/src/qq/monitor.c:649
> #27 0x0000563303548cce in qapi_event_send_shutdown (guest=false,
> errp=0x563303d8d0f0 <error_abort>) at qapi/qapi-events-run-state.c:58
> #28 0x000056330313bcd7 in main_loop_should_exit () at
> /home/elmarco/src/qq/vl.c:1822
> #29 0x000056330313bde3 in main_loop () at /home/elmarco/src/qq/vl.c:1862
> #30 0x0000563303143781 in main (argc=3, argv=0x7ffc6ab5f068,
> envp=0x7ffc6ab5f088) at /home/elmarco/src/qq/vl.c:4644
>
> Note that error report is now moved to the first caller, which may
> receive an error for a recursed event. This is probably fine (95% of
> callers use &error_abort, the rest have NULL error and ignore it)
>
> Signed-off-by: Marc-André Lureau <address@hidden>
> ---
> monitor.c | 44 +++++++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 43 insertions(+), 1 deletion(-)
>
> diff --git a/monitor.c b/monitor.c
> index d8d8211ae4..4d9c1873bc 100644
> --- a/monitor.c
> +++ b/monitor.c
> @@ -633,7 +633,7 @@ static void monitor_qapi_event_handler(void *opaque);
> * applying any rate limiting if required.
> */
> static void
> -monitor_qapi_event_queue(QAPIEvent event, QDict *qdict, Error **errp)
> +monitor_qapi_event_queue_no_recurse(QAPIEvent event, QDict *qdict)
Let's name it _no_reenter. Can do in my tree.
> {
> MonitorQAPIEventConf *evconf;
> MonitorQAPIEventState *evstate;
> @@ -688,6 +688,48 @@ monitor_qapi_event_queue(QAPIEvent event, QDict *qdict,
> Error **errp)
> qemu_mutex_unlock(&monitor_lock);
> }
>
> +static void
> +monitor_qapi_event_queue(QAPIEvent event, QDict *qdict, Error **errp)
> +{
> + /*
> + * monitor_qapi_event_queue_no_recurse() is not reentrant: it
> + * would deadlock on monitor_lock. Work around by queueing
> + * events in thread-local storage.
> + * TODO: remove this, make it re-enter safe.
> + */
> + static __thread bool reentered;
> + typedef struct MonitorQapiEvent {
> + QAPIEvent event;
> + QDict *qdict;
> + QSIMPLEQ_ENTRY(MonitorQapiEvent) entry;
> + } MonitorQapiEvent;
> + MonitorQapiEvent *ev;
> + static __thread QSIMPLEQ_HEAD(, MonitorQapiEvent) event_queue;
I'd prefer this order:
typedef struct MonitorQapiEvent {
QAPIEvent event;
QDict *qdict;
QSIMPLEQ_ENTRY(MonitorQapiEvent) entry;
} MonitorQapiEvent;
static __thread QSIMPLEQ_HEAD(, MonitorQapiEvent) event_queue;
static __thread bool reentered;
MonitorQapiEvent *ev;
Can do in my tree.
> +
> + if (!reentered) {
> + QSIMPLEQ_INIT(&event_queue);
> + }
> +
> + ev = g_new(MonitorQapiEvent, 1);
> + ev->qdict = qobject_ref(qdict);
> + ev->event = event;
> + QSIMPLEQ_INSERT_TAIL(&event_queue, ev, entry);
> + if (reentered) {
> + return;
> + }
> +
> + reentered = true;
> +
> + while ((ev = QSIMPLEQ_FIRST(&event_queue)) != NULL) {
> + QSIMPLEQ_REMOVE_HEAD(&event_queue, entry);
> + monitor_qapi_event_queue_no_recurse(ev->event, ev->qdict);
> + qobject_unref(ev->qdict);
> + g_free(ev);
> + }
> +
> + reentered = false;
> +}
> +
> /*
> * This function runs evconf->rate ns after sending a throttled
> * event.
Preferably with these touch-ups.
Reviewed-by: Markus Armbruster <address@hidden>