migration: Take BQL slightly longer in postcopy_start()

This paves way for some follow up patch to modify migration states at the
end of postcopy_start(), which should better be with the BQL so that
there's no way of concurrent cancellation.

So we'll do something slightly more with BQL but they're really trivial,
hopefully nothing will really chance with this.

A side benefit is we can drop another explicit lock() in failure path.

Signed-off-by: Peter Xu <peterx@redhat.com>
Tested-by: Jiri Denemark <jdenemar@redhat.com>
Reviewed-by: Juraj Marcin <jmarcin@redhat.com>
Link: https://lore.kernel.org/r/20250114230746.3268797-11-peterx@redhat.com
Signed-off-by: Fabiano Rosas <farosas@suse.de>

diff --git a/migration/migration.c b/migration/migration.c
index 5a3d0750ec20ad0a1a8ad62a2d979c7958c5a2bd..4ba6c8912ad199c5e2a1bd2388c6d75a4cda5b19 100644 (file)
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -2753,8 +2753,6 @@ static int postcopy_start(MigrationState *ms, Error **errp)
 
     migration_downtime_end(ms);
 
-    bql_unlock();
-
     if (migrate_postcopy_ram()) {
         /*
          * Although this ping is just for debug, it could potentially be
@@ -2770,7 +2768,6 @@ static int postcopy_start(MigrationState *ms, Error **errp)
     ret = qemu_file_get_error(ms->to_dst_file);
     if (ret) {
         error_setg_errno(errp, -ret, "postcopy_start: Migration stream error");
-        bql_lock();
         goto fail;
     }
     trace_postcopy_preempt_enabled(migrate_postcopy_preempt());
@@ -2781,6 +2778,8 @@ static int postcopy_start(MigrationState *ms, Error **errp)
      */
     migration_rate_set(migrate_max_postcopy_bandwidth());
 
+    bql_unlock();
+
     return ret;
 
 fail_closefb: