migration: Replace QemuSemaphore with QemuEvent

pause_event can utilize qemu_event_reset() to discard events.

Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Link: https://lore.kernel.org/r/20250529-event-v5-7-53b285203794@daynix.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

2 files changed, 10 insertions, 13 deletions

diff --git a/migration/migration.c b/migration/migration.c
index 4697732..4098870 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -1630,7 +1630,7 @@ void migration_cancel(void)
         }
         /* If the migration is paused, kick it out of the pause */
         if (old_state == MIGRATION_STATUS_PRE_SWITCHOVER) {
-            qemu_sem_post(&s->pause_sem);
+            qemu_event_set(&s->pause_event);
         }
         migrate_set_state(&s->state, old_state, MIGRATION_STATUS_CANCELLING);
     } while (s->state != MIGRATION_STATUS_CANCELLING);
@@ -2342,7 +2342,7 @@ void qmp_migrate_continue(MigrationStatus state, Error **errp)
                    MigrationStatus_str(s->state));
         return;
     }
-    qemu_sem_post(&s->pause_sem);
+    qemu_event_set(&s->pause_event);
 }
 
 int migration_rp_wait(MigrationState *s)
@@ -2911,21 +2911,18 @@ static bool migration_switchover_prepare(MigrationState *s)
         return true;
     }
 
-    /* Since leaving this state is not atomic with posting the semaphore
+    /*
+     * Since leaving this state is not atomic with setting the event
      * it's possible that someone could have issued multiple migrate_continue
-     * and the semaphore is incorrectly positive at this point;
-     * the docs say it's undefined to reinit a semaphore that's already
-     * init'd, so use timedwait to eat up any existing posts.
+     * and the event is incorrectly set at this point so reset it.
      */
-    while (qemu_sem_timedwait(&s->pause_sem, 1) == 0) {
-        /* This block intentionally left blank */
-    }
+    qemu_event_reset(&s->pause_event);
 
     /* Update [POSTCOPY_]ACTIVE to PRE_SWITCHOVER */
     migrate_set_state(&s->state, s->state, MIGRATION_STATUS_PRE_SWITCHOVER);
     bql_unlock();
 
-    qemu_sem_wait(&s->pause_sem);
+    qemu_event_wait(&s->pause_event);
 
     bql_lock();
     /*
@@ -4057,7 +4054,7 @@ static void migration_instance_finalize(Object *obj)
     qemu_mutex_destroy(&ms->qemu_file_lock);
     qemu_sem_destroy(&ms->wait_unplug_sem);
     qemu_sem_destroy(&ms->rate_limit_sem);
-    qemu_sem_destroy(&ms->pause_sem);
+    qemu_event_destroy(&ms->pause_event);
     qemu_sem_destroy(&ms->postcopy_pause_sem);
     qemu_sem_destroy(&ms->rp_state.rp_sem);
     qemu_sem_destroy(&ms->rp_state.rp_pong_acks);
@@ -4072,7 +4069,7 @@ static void migration_instance_init(Object *obj)
     ms->state = MIGRATION_STATUS_NONE;
     ms->mbps = -1;
     ms->pages_per_second = -1;
-    qemu_sem_init(&ms->pause_sem, 0);
+    qemu_event_init(&ms->pause_event, false);
     qemu_mutex_init(&ms->error_mutex);
 
     migrate_params_init(&ms->parameters);
diff --git a/migration/migration.h b/migration/migration.h
index d53f7ca..21aa6a3 100644
--- a/migration/migration.h
+++ b/migration/migration.h
@@ -379,7 +379,7 @@ struct MigrationState {
     QemuSemaphore wait_unplug_sem;
 
     /* Migration is paused due to pause-before-switchover */
-    QemuSemaphore pause_sem;
+    QemuEvent pause_event;
 
     /* The semaphore is used to notify COLO thread that failover is finished */
     QemuSemaphore colo_exit_sem;