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sd-event: Make sure iterations of defer and exit sources are updated

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Defer and exit event sources are marked pending once when they are added
and never again afterwards. This means their pending_iteration is never
incremented after they are initially added, which breaks fairness among
event sources with equal priority which depend on the pending_iteration
variable getting updated in source_set_pending(). To fix this, let's assign
iterations for defer and exit sources in source_dispatch() instead so that
those get their pending_iteration updated as well.
This commit is contained in:
Daan De Meyer
2025-11-13 22:15:01 +01:00
parent 5a5cb6ba50
commit a3dd54c097
2 changed files with 43 additions and 1 deletions
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8
src/libsystemd/sd-event/sd-event.c
View File

@@ -4120,7 +4120,13 @@ static int source_dispatch(sd_event_source *s) {
return 1;
}
if (!IN_SET(s->type, SOURCE_DEFER, SOURCE_EXIT)) {
if (IN_SET(s->type, SOURCE_DEFER, SOURCE_EXIT)) {
/* Make sure this event source is moved to the end of the priority list now. We do this here
* because defer and exit event sources are always pending from the moment they're added so
* the same logic in source_set_pending() is never triggered. */
s->pending_iteration = s->event->iteration;
event_source_pp_prioq_reshuffle(s);
} else {
r = source_set_pending(s, false);
if (r < 0)
return r;

36
src/libsystemd/sd-event/test-event.c
View File

@@ -1131,4 +1131,40 @@ TEST(exit_on_idle_no_sources) {
ASSERT_OK(sd_event_loop(e));
}
static int defer_fair_handler(sd_event_source *s, void *userdata) {
unsigned *counter = ASSERT_PTR(userdata);
/* If we're about to increment above 5, exit the event loop */
if (*counter >= 5)
return sd_event_exit(sd_event_source_get_event(s), 0);
(*counter)++;
return 0;
}
TEST(defer_fair_scheduling) {
_cleanup_(sd_event_unrefp) sd_event *e = NULL;
sd_event_source *sources[5] = {};
unsigned counters[5] = {};
ASSERT_OK(sd_event_new(&e));
ASSERT_OK(sd_event_set_exit_on_idle(e, true));
/* Create 5 defer sources with equal priority */
for (unsigned i = 0; i < 5; i++) {
ASSERT_OK(sd_event_add_defer(e, &sources[i], defer_fair_handler, &counters[i]));
ASSERT_OK(sd_event_source_set_enabled(sources[i], SD_EVENT_ON));
}
/* Run the event loop until one of the handlers exits */
ASSERT_OK(sd_event_loop(e));
/* All counters should be equal to 5, demonstrating fair scheduling */
for (unsigned i = 0; i < 5; i++) {
ASSERT_EQ(counters[i], 5u);
sd_event_source_unref(sources[i]);
}
}
DEFINE_TEST_MAIN(LOG_DEBUG);