[src/trunk]: src/sys/kern Suspend file system after VFS_MOUNT() and before ta...

[hannken <hannken%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/1588e49325be
branches:  trunk
changeset: 368377:1588e49325be
user:      hannken <hannken%NetBSD.org@localhost>
date:      Fri Jul 08 07:43:19 2022 +0000

description:
Suspend file system after VFS_MOUNT() and before taking mnt_updating.
Prevents deadlock against concurrent unmounts of layered file systems.

diffstat:

 sys/kern/vfs_mount.c |  30 ++++++++++++++++--------------
 1 files changed, 16 insertions(+), 14 deletions(-)

diffs (72 lines):

diff -r 6039c5020890 -r 1588e49325be sys/kern/vfs_mount.c
--- a/sys/kern/vfs_mount.c      Fri Jul 08 07:42:47 2022 +0000
+++ b/sys/kern/vfs_mount.c      Fri Jul 08 07:43:19 2022 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: vfs_mount.c,v 1.93 2022/04/09 23:38:33 riastradh Exp $ */
+/*     $NetBSD: vfs_mount.c,v 1.94 2022/07/08 07:43:19 hannken Exp $   */
 
 /*-
  * Copyright (c) 1997-2020 The NetBSD Foundation, Inc.
@@ -67,7 +67,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: vfs_mount.c,v 1.93 2022/04/09 23:38:33 riastradh Exp $");
+__KERNEL_RCSID(0, "$NetBSD: vfs_mount.c,v 1.94 2022/07/08 07:43:19 hannken Exp $");
 
 #include <sys/param.h>
 #include <sys/kernel.h>
@@ -773,12 +773,20 @@
         */
        mp->mnt_flag = flags & (MNT_BASIC_FLAGS | MNT_FORCE | MNT_IGNORE);
 
-       mutex_enter(mp->mnt_updating);
        error = VFS_MOUNT(mp, path, data, data_len);
        mp->mnt_flag &= ~MNT_OP_FLAGS;
 
-       if (error != 0)
-               goto err_unmounted;
+       if (error != 0) {
+               vfs_rele(mp);
+               return error;
+       }
+
+       /* Suspend new file system before taking mnt_updating. */
+       do {
+               error2 = vfs_suspend(mp, 0);
+       } while (error2 == EINTR || error2 == ERESTART);
+       KASSERT(error2 == 0 || error2 == EOPNOTSUPP);
+       mutex_enter(mp->mnt_updating);
 
        /*
         * Validate and prepare the mount point.
@@ -823,6 +831,8 @@
 
        mount_checkdirs(vp);
        mutex_exit(mp->mnt_updating);
+       if (error2 == 0)
+               vfs_resume(mp);
 
        /* Hold an additional reference to the mount across VFS_START(). */
        vfs_ref(mp);
@@ -840,19 +850,11 @@
        return error;
 
 err_mounted:
-       do {
-               error2 = vfs_suspend(mp, 0);
-       } while (error2 == EINTR || error2 == ERESTART);
-       KASSERT(error2 == 0 || error2 == EOPNOTSUPP);
-
        if (VFS_UNMOUNT(mp, MNT_FORCE) != 0)
                panic("Unmounting fresh file system failed");
-
+       mutex_exit(mp->mnt_updating);
        if (error2 == 0)
                vfs_resume(mp);
-
-err_unmounted:
-       mutex_exit(mp->mnt_updating);
        vfs_rele(mp);
 
        return error;