[systemd-devel] Unexpected D-Bus timeout (https://github.com/systemd/systemd/issues/32381)

[Michal Sekletar]

Hi everyone,

I am currently struggling to understand why I am running into D-Bus related
timeout on PID 1 API bus, which in turn causes systemd to transition the
bus to BUS_CLOSING state and stop all Type=dbus services. Here is a
reproducer that was put together by Frantisek.

#/bin/bash
set -eux
set -o pipefail

NMOUNTS=800
MPOINT="$(findmnt --mountpoint / --noheading -o SOURCE)"
TS="$(date +"%s.%N")"

systemctl mask --now --runtime systemd-oomd.{socket,service}
id testuser || useradd testuser
systemctl stop "tmp-mnt*.mount" || :
systemctl log-level debug

for i in {0..100}; do
        echo "=== $i ==="
        for i in $(seq 0 $NMOUNTS); do
                mkdir -p "/tmp/mnt$i"
                systemd-mount "$MPOINT" "/tmp/mnt$i" &
        done
        wait
        for i in $(seq 0 $NMOUNTS); do
                systemd-umount "/tmp/mnt$i" &
        done
        wait

        busctl status --user --machine=testuser@ --no-pager
        systemctl stop user at 4711.service
        (! journalctl --since="@$TS" --grep "Connection terminated")
done

The timeout is detected in process_timeout() in sd-bus, however, the
reply_callback which timed out has a message with matching reply_cookie
already in the bus read queue (for a long time). How is it possible it
wasn't processed in the previous event loop iteration? Clearly in
reproducer we force systemd to do a bunch of work, i.e. even loop is
getting dispatched all the time? Btw, in order to verify that reply is
already sitting in the queue I've compiled systemd with the following patch.

diff --git a/src/libsystemd/sd-bus/bus-message.h
b/src/libsystemd/sd-bus/bus-message.h
index 76f0d853d3..16d25e11a7 100644
--- a/src/libsystemd/sd-bus/bus-message.h
+++ b/src/libsystemd/sd-bus/bus-message.h
@@ -119,6 +119,8 @@ struct sd_bus_message {
         unsigned n_header_offsets;

         uint64_t read_counter;
+
+        time_t rq_in;
 };

 static inline bool BUS_MESSAGE_NEED_BSWAP(sd_bus_message *m) {
diff --git a/src/libsystemd/sd-bus/bus-socket.c
b/src/libsystemd/sd-bus/bus-socket.c
index 07179e0705..6c1bc3e13b 100644
--- a/src/libsystemd/sd-bus/bus-socket.c
+++ b/src/libsystemd/sd-bus/bus-socket.c
@@ -1337,6 +1337,7 @@ static int bus_socket_make_message(sd_bus *bus,
size_t size) {
         if (t) {
                 t->read_counter = ++bus->read_counter;
                 bus->rqueue[bus->rqueue_size++] =
bus_message_ref_queued(t, bus);
+                t->rq_in = time(NULL);
                 sd_bus_message_unref(t);
         }

diff --git a/src/libsystemd/sd-bus/sd-bus.c b/src/libsystemd/sd-bus/sd-bus.c
index 22784e8f66..9cc7ce212f 100644
--- a/src/libsystemd/sd-bus/sd-bus.c
+++ b/src/libsystemd/sd-bus/sd-bus.c
@@ -511,6 +511,7 @@ static int synthesize_connected_signal(sd_bus *bus) {
         memmove(bus->rqueue + 1, bus->rqueue, sizeof(sd_bus_message*) *
bus->rqueue_size);
         bus->rqueue[0] = bus_message_ref_queued(m, bus);
         bus->rqueue_size++;
+        m->rq_in = time(NULL);

         return 0;
 }
@@ -2681,6 +2682,7 @@ static int process_timeout(sd_bus *bus) {
         struct reply_callback *c;
         sd_bus_slot *slot;
         bool is_hello;
+        unsigned i;
         usec_t n;
         int r;

@@ -2695,6 +2697,15 @@ static int process_timeout(sd_bus *bus) {
         if (c->timeout_usec > n)
                 return 0;

+        for (i = 0; i < bus->rqueue_size; i++) {
+                sd_bus_message *q = bus->rqueue[i];
+
+                if (c->cookie == q->reply_cookie) {
+                        log_error("Message was put to read queue %"
PRIiMAX " seconds ago.", time(NULL) - q->rq_in);
+                        assert_not_reached();
+                }
+        }
+
         r = bus_message_new_synthetic_error(
                         bus,
                         c->cookie,
@@ -4464,5 +4475,6 @@ _public_ int sd_bus_enqueue_for_read(sd_bus *bus,
sd_bus_message *m) {
                 return r;

         bus->rqueue[bus->rqueue_size++] = bus_message_ref_queued(m, bus);
+        m->rq_in = time(NULL);
         return 0;
 }

Once compiled with above patch and running reproducer for some time I am
hitting assert_not_reached() and log says that matching reply was in read
queue for more than 82 seconds! Any ideas or debugging tips would be highly
appreciated.

Cheers,
Michal
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