[src/trunk]: src/sys/kern Use fstrans_start()/fstrans_done() to cross the mou...

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

details:   https://anonhg.NetBSD.org/src/rev/da23e37f748b
branches:  trunk
changeset: 369583:da23e37f748b
user:      hannken <hannken%NetBSD.org@localhost>
date:      Mon Aug 22 09:14:59 2022 +0000

description:
Use fstrans_start()/fstrans_done() to cross the mount in lookup_crossmount().
It is sufficient here as it prevents the file system from unmount and
makes it safe to use VFS_ROOT() here.

Removes a rare deadlock where one thread has "foundobj" locked and waits
for "foundobj->v_mountedhere" to resume while the thread holding the file
system suspended tries to lookup a node and needs a lock on "foundobj".

diffstat:

 sys/kern/vfs_lookup.c |  40 +++++++++++++---------------------------
 1 files changed, 13 insertions(+), 27 deletions(-)

diffs (76 lines):

diff -r f5b92c8829ed -r da23e37f748b sys/kern/vfs_lookup.c
--- a/sys/kern/vfs_lookup.c     Mon Aug 22 09:14:24 2022 +0000
+++ b/sys/kern/vfs_lookup.c     Mon Aug 22 09:14:59 2022 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: vfs_lookup.c,v 1.231 2022/02/10 10:59:12 hannken Exp $ */
+/*     $NetBSD: vfs_lookup.c,v 1.232 2022/08/22 09:14:59 hannken Exp $ */
 
 /*
  * Copyright (c) 1982, 1986, 1989, 1993
@@ -37,7 +37,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: vfs_lookup.c,v 1.231 2022/02/10 10:59:12 hannken Exp $");
+__KERNEL_RCSID(0, "$NetBSD: vfs_lookup.c,v 1.232 2022/08/22 09:14:59 hannken Exp $");
 
 #ifdef _KERNEL_OPT
 #include "opt_magiclinks.h"
@@ -51,6 +51,7 @@
 #include <sys/namei.h>
 #include <sys/vnode.h>
 #include <sys/vnode_impl.h>
+#include <sys/fstrans.h>
 #include <sys/mount.h>
 #include <sys/errno.h>
 #include <sys/filedesc.h>
@@ -948,31 +949,16 @@
                        vrele(foundobj);
                        foundobj = vp;
                } else {
-                       /* First get the vnode stable. */
-                       error = vn_lock(foundobj, LK_SHARED);
-                       if (error != 0) {
-                               vrele(foundobj);
-                               foundobj = NULL;
-                               break;
+                       /* First get the vnodes mount stable. */
+                       while ((mp = foundobj->v_mountedhere) != NULL) {
+                               fstrans_start(mp);
+                               if (fstrans_held(mp) &&
+                                   mp == foundobj->v_mountedhere) {
+                                       break;
+                               }
+                               fstrans_done(mp);
                        }
-
-                       /*
-                        * Check to see if something is still mounted on it.
-                        */
-                       if ((mp = foundobj->v_mountedhere) == NULL) {
-                               VOP_UNLOCK(foundobj);
-                               break;
-                       }
-
-                       /*
-                        * Get a reference to the mountpoint, and unlock
-                        * foundobj.
-                        */
-                       error = vfs_busy(mp);
-                       VOP_UNLOCK(foundobj);
-                       if (error != 0) {
-                               vrele(foundobj);
-                               foundobj = NULL;
+                       if (mp == NULL) {
                                break;
                        }
 
@@ -992,7 +978,7 @@
 
                        /* Finally, drop references to foundobj & mountpoint. */
                        vrele(foundobj);
-                       vfs_unbusy(mp);
+                       fstrans_done(mp);
                        if (error) {
                                foundobj = NULL;
                                break;