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Move the "activate linked connections" logic to a postloop event. 

A linked connection_t is one that gets its I/O, not from the
network, but from another connection_t.  When such a connection has
something to write, we want the corresponding connection to run its
read callback ... but not immediately, to avoid infinite recursion
and/or event loop starvation.

Previously we handled this case by activating the read events
outside the event loop.  Now we use the "postloop event" logic.
This lets us simplify do_main_loop_once() a little.
This commit is contained in:
Nick Mathewson 2018-04-02 15:08:41 -04:00
parent c5a3e2ca44
commit 5719dfb48f
1 changed files with 29 additions and 10 deletions
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39
src/or/main.c
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@ -410,6 +410,27 @@ connection_unlink(connection_t *conn)
connection_free(conn);
}
/**
* Callback: used to activate read events for all linked connections, so
* libevent knows to call their read callbacks. This callback run as a
* postloop event, so that the events _it_ activates don't happen until
* Libevent has a chance to check for other events.
*/
static void
schedule_active_linked_connections_cb(mainloop_event_t *event, void *arg)
{
(void)event;
(void)arg;
/* All active linked conns should get their read events activated,
* so that libevent knows to run their callbacks. */
SMARTLIST_FOREACH(active_linked_connection_lst, connection_t *, conn,
event_active(conn->read_event, EV_READ, 1));
}
/** Event that invokes schedule_active_linked_connections_cb. */
static mainloop_event_t *schedule_active_linked_connections_event = NULL;
/** Initialize the global connection list, closeable connection list,
* and active connection list. */
STATIC void
@ -803,10 +824,7 @@ connection_start_reading_from_linked_conn(connection_t *conn)
if (!conn->active_on_link) {
conn->active_on_link = 1;
smartlist_add(active_linked_connection_lst, conn);
/* make sure that the event_base_loop() function exits at
* the end of its run through the current connections, so we can
* activate read events for linked connections. */
tell_event_loop_to_run_external_code();
mainloop_event_activate(schedule_active_linked_connections_event);
} else {
tor_assert(smartlist_contains(active_linked_connection_lst, conn));
}
@ -2579,6 +2597,11 @@ do_main_loop(void)
initialize_periodic_events();
}
if (!schedule_active_linked_connections_event) {
schedule_active_linked_connections_event =
mainloop_event_postloop_new(schedule_active_linked_connections_cb, NULL);
}
/* initialize dns resolve map, spawn workers if needed */
if (dns_init() < 0) {
if (get_options()->ServerDNSAllowBrokenConfig)
@ -2783,17 +2806,12 @@ run_main_loop_once(void)
errno = 0;
#endif
/* All active linked conns should get their read events activated,
* so that libevent knows to run their callbacks. */
SMARTLIST_FOREACH(active_linked_connection_lst, connection_t *, conn,
event_active(conn->read_event, EV_READ, 1));
if (get_options()->MainloopStats) {
/* We always enforce that EVLOOP_ONCE is passed to event_base_loop() if we
* are collecting main loop statistics. */
called_loop_once = 1;
} else {
called_loop_once = smartlist_len(active_linked_connection_lst) ? 1 : 0;
called_loop_once = 0;
}
/* Make sure we know (about) what time it is. */
@ -3496,6 +3514,7 @@ tor_free_all(int postfork)
tor_event_free(shutdown_did_not_work_event);
tor_event_free(initialize_periodic_events_event);
mainloop_event_free(directory_all_unreachable_cb_event);
mainloop_event_free(schedule_active_linked_connections_event);
#ifdef HAVE_SYSTEMD_209
periodic_timer_free(systemd_watchdog_timer);