Merge pull request #27700 from poettering/switch-root-sync

switch-root: add sync() before getting rid of auxiliary mounts of old root

src/basic/async.c

@@ -72,7 +72,7 @@ int asynchronous_sync(pid_t *ret_pid) {
                 return r;
         if (r == 0) {
                 /* Child process */
-                (void) sync();
+                sync();
                 _exit(EXIT_SUCCESS);
         }
 

src/shared/switch-root.c

@@ -63,6 +63,13 @@ int switch_root(const char *new_root,
                         (void) mkdir_p_label(resolved_old_root_after, 0755);
         }
 
+        /* We are about to unmount various file systems with MNT_DETACH (either explicitly via umount() or
+         * indirectly via pivot_root()), and thus do not synchronously wait for them to be fully sync'ed —
+         * all while making them invisible/inaccessible in the file system tree for later code. That makes
+         * sync'ing them then difficult. Let's hence issue a manual sync() here, so that we at least can
+         * guarantee all file systems are an a good state before entering this state. */
+        sync();
+
         /* Work-around for kernel design: the kernel refuses MS_MOVE if any file systems are mounted
          * MS_SHARED. Hence remount them MS_PRIVATE here as a work-around.
          *

src/systemctl/systemctl-util.c

@@ -921,7 +921,7 @@ int halt_now(enum action a) {
         /* The kernel will automatically flush ATA disks and suchlike on reboot(), but the file systems need
          * to be synced explicitly in advance. */
         if (!arg_no_sync && !arg_dry_run)
-                (void) sync();
+                sync();
 
         /* Make sure C-A-D is handled by the kernel from this point on... */
         if (!arg_dry_run)