migration/multifd: Split multifd_send_terminate_threads()

Split multifd_send_terminate_threads() into two functions:

  - multifd_send_set_error(): used when an error happened on the sender
    side, set error and quit state only

  - multifd_send_terminate_threads(): used only by the main thread to kick
    all multifd send threads out of sleep, for the last recycling.

Use multifd_send_set_error() in the three old call sites where only the
error will be set.

Use multifd_send_terminate_threads() in the last one where the main thread
will kick the multifd threads at last in multifd_save_cleanup().

Both helpers will need to set quitting=1.

Suggested-by: Fabiano Rosas <farosas@suse.de>
Reviewed-by: Fabiano Rosas <farosas@suse.de>
Link: https://lore.kernel.org/r/20240202102857.110210-16-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>

migration/multifd.c

@@ -536,10 +536,9 @@ int multifd_queue_page(RAMBlock *block, ram_addr_t offset)
     return 1;
 }
 
-static void multifd_send_terminate_threads(Error *err)
+/* Multifd send side hit an error; remember it and prepare to quit */
+static void multifd_send_set_error(Error *err)
 {
-    int i;
-
     /*
      * We don't want to exit each threads twice.  Depending on where
      * we get the error, or if there are two independent errors in two
@@ -550,8 +549,6 @@ static void multifd_send_terminate_threads(Error *err)
         return;
     }
 
-    trace_multifd_send_terminate_threads(err != NULL);
-
     if (err) {
         MigrationState *s = migrate_get_current();
         migrate_set_error(s, err);
@@ -563,7 +560,19 @@ static void multifd_send_terminate_threads(Error *err)
                               MIGRATION_STATUS_FAILED);
         }
     }
+}
+
+static void multifd_send_terminate_threads(void)
+{
+    int i;
+
+    trace_multifd_send_terminate_threads();
 
+    /*
+     * Tell everyone we're quitting.  No xchg() needed here; we simply
+     * always set it.
+     */
+    qatomic_set(&multifd_send_state->exiting, 1);
     for (i = 0; i < migrate_multifd_channels(); i++) {
         MultiFDSendParams *p = &multifd_send_state->params[i];
 
@@ -586,7 +595,7 @@ void multifd_save_cleanup(void)
     if (!migrate_multifd()) {
         return;
     }
-    multifd_send_terminate_threads(NULL);
+    multifd_send_terminate_threads();
     for (i = 0; i < migrate_multifd_channels(); i++) {
         MultiFDSendParams *p = &multifd_send_state->params[i];
 
@@ -780,7 +789,7 @@ static void *multifd_send_thread(void *opaque)
     if (ret) {
         assert(local_err);
         trace_multifd_send_error(p->id);
-        multifd_send_terminate_threads(local_err);
+        multifd_send_set_error(local_err);
         multifd_send_kick_main(p);
         error_free(local_err);
     }
@@ -816,7 +825,7 @@ static void multifd_tls_outgoing_handshake(QIOTask *task,
 
     trace_multifd_tls_outgoing_handshake_error(ioc, error_get_pretty(err));
 
-    multifd_send_terminate_threads(err);
+    multifd_send_set_error(err);
     multifd_send_kick_main(p);
     error_free(err);
 }
@@ -898,7 +907,7 @@ static void multifd_new_send_channel_async(QIOTask *task, gpointer opaque)
     }
 
     trace_multifd_new_send_channel_async_error(p->id, local_err);
-    multifd_send_terminate_threads(local_err);
+    multifd_send_set_error(local_err);
     multifd_send_kick_main(p);
     object_unref(OBJECT(ioc));
     error_free(local_err);

migration/trace-events

@@ -141,7 +141,7 @@ multifd_send_error(uint8_t id) "channel %u"
 multifd_send_sync_main(long packet_num) "packet num %ld"
 multifd_send_sync_main_signal(uint8_t id) "channel %u"
 multifd_send_sync_main_wait(uint8_t id) "channel %u"
-multifd_send_terminate_threads(bool error) "error %d"
+multifd_send_terminate_threads(void) ""
 multifd_send_thread_end(uint8_t id, uint64_t packets, uint64_t normal_pages) "channel %u packets %" PRIu64 " normal pages %"  PRIu64
 multifd_send_thread_start(uint8_t id) "%u"
 multifd_tls_outgoing_handshake_start(void *ioc, void *tioc, const char *hostname) "ioc=%p tioc=%p hostname=%s"