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diff --git a/sys/ufs/ufs/ufs_vnops.c b/sys/ufs/ufs/ufs_vnops.c
--- a/sys/ufs/ufs/ufs_vnops.c
+++ b/sys/ufs/ufs/ufs_vnops.c
@@ -205,16 +205,17 @@
static int
ufs_sync_nlink1(struct mount *mp)
{
- int error;
- error = vfs_busy(mp, 0);
- if (error == 0) {
- VFS_SYNC(mp, MNT_WAIT);
- vfs_unbusy(mp);
- error = ERELOOKUP;
- }
+ /*
+ * Do not busy the mount point. The caller is between
+ * vn_start_write() and vn_finished_write(), which already keeps
+ * an unmount from getting past the write suspension in
+ * VFS_UNMOUNT(), while vfs_busy() would wait for the unmount to
+ * finish and deadlock with it.
+ */
+ VFS_SYNC(mp, MNT_WAIT);
vfs_rel(mp);
- return (error);
+ return (ERELOOKUP);
}
static int
diff --git a/tools/test/stress2/misc/nlink6.sh b/tools/test/stress2/misc/nlink6.sh
new file mode 100755
--- /dev/null
+++ b/tools/test/stress2/misc/nlink6.sh
@@ -0,0 +1,191 @@
+#!/bin/sh
+
+#
+# SPDX-License-Identifier: BSD-2-Clause
+#
+# Copyright (c) 2026 Maksym Sobolyev <sobomax@FreeBSD.org>
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions
+# are met:
+# 1. Redistributions of source code must retain the above copyright
+# notice, this list of conditions and the following disclaimer.
+# 2. Redistributions in binary form must reproduce the above copyright
+# notice, this list of conditions and the following disclaimer in the
+# documentation and/or other materials provided with the distribution.
+#
+# THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
+# ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+# IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+# ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
+# FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+# DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
+# OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
+# HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+# LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
+# OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
+# SUCH DAMAGE.
+#
+
+# Forced unmount racing mkdir(2) in a directory at UFS_LINK_MAX on SU.
+#
+# With soft updates, rmdir(2) lowers the parent's i_effnlink at once and
+# its i_nlink only when the removal is on disk. A mkdir(2) in between
+# finds i_nlink at UFS_LINK_MAX and goes through ufs_sync_nlink1(), which
+# syncs the file system and restarts. ufs_sync_nlink1() used to sleep in
+# vfs_busy() while an unmount was in progress, with the mkdir(2) still
+# counted as a write by vn_start_write(), while the unmount waited in
+# vfs_write_suspend() for that write to end. The unmount and every later
+# vfs_busy() of the mount point hung.
+#
+# Set dtrace=1 to count, while an unmount is in progress, the vfs_busy()
+# calls by caller, and the mkdir(2) calls that synced and restarted with
+# ERELOOKUP. Without the fix, a vfs_busy() from ufs_sync_nlink() here is
+# the deadlock; with it, the mkdir(2) calls show the race was hit.
+
+[ `id -u` -ne 0 ] && echo "Must be root!" && exit 1
+
+. ../default.cfg
+
+UFS_LINK_MAX=`grep UFS_LINK_MAX /usr/include/ufs/ufs/dinode.h 2>/dev/null`
+[ -z "$UFS_LINK_MAX" ] && exit 0
+UFS_LINK_MAX=`echo $UFS_LINK_MAX | awk '{print $3}'`
+nworkers=8
+runtime=${runtime:-300}
+log=/tmp/nlink6.log
+
+cat > /tmp/nlink6.c <<EOF
+#include <sys/stat.h>
+#include <ufs/ufs/dinode.h>
+#include <err.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <time.h>
+#include <unistd.h>
+
+/*
+ * "setup": fill directory d with subdirectories up to UFS_LINK_MAX, the
+ * first nworkers of them named w<id>.
+ * "work <d> <id> <seconds>": remove and recreate <d>/w<id> in a loop.
+ * Work relative to <d>: a lookup through the mount point would wait for
+ * the unmount on the covered vnode lock and never race with it.
+ */
+int
+main(int argc, char *argv[])
+{
+ char path[1024];
+ time_t start;
+ int i, nworkers, secs;
+
+ if (argc == 3 && strcmp(argv[1], "setup") == 0) {
+ nworkers = atoi(argv[2]);
+ if (mkdir("d", 0755) == -1)
+ err(1, "mkdir(d)");
+ for (i = 0; i < UFS_LINK_MAX - 2; i++) {
+ if (i < nworkers)
+ snprintf(path, sizeof(path), "d/w%d", i);
+ else
+ snprintf(path, sizeof(path), "d/%d", i);
+ if (mkdir(path, 0755) == -1)
+ err(1, "mkdir(%s)", path);
+ }
+ return (0);
+ }
+ if (argc != 5 || strcmp(argv[1], "work") != 0)
+ errx(1, "usage: nlink6 setup <n> | work <dir> <id> <seconds>");
+ if (chdir(argv[2]) == -1)
+ err(1, "chdir(%s)", argv[2]);
+ snprintf(path, sizeof(path), "w%s", argv[3]);
+ secs = atoi(argv[4]);
+ start = time(NULL);
+ while (time(NULL) - start < secs) {
+ /* Errors are expected once the file system is gone. */
+ if (rmdir(path) == -1 || mkdir(path, 0755) == -1)
+ usleep(1000);
+ }
+ return (0);
+}
+EOF
+mycc -o /tmp/nlink6 -Wall -Wextra -O2 /tmp/nlink6.c || exit 1
+rm /tmp/nlink6.c
+
+set -e
+mount | grep -q "on $mntpoint " && umount -f $mntpoint
+mdconfig -l | grep -q "md$mdstart" && mdconfig -d -u $mdstart
+mdconfig -a -t swap -s 1g -u $mdstart
+newfs -Un /dev/md$mdstart > /dev/null
+mount /dev/md$mdstart $mntpoint
+(cd $mntpoint; /tmp/nlink6 setup $nworkers)
+umount $mntpoint
+set +e
+
+if [ "$dtrace" = 1 ]; then
+ # The unmount-in-progress flag (MNTK_UNMOUNT) is 0x01000000.
+ dtrace -q -n 'fbt::vfs_busy:entry
+ /(args[0]->mnt_kern_flag & 0x01000000) != 0/
+ { @b[stack(2)] = count(); }
+ fbt::ufs_mkdir:entry { self->mp = args[0]->a_dvp->v_mount; }
+ fbt::ufs_mkdir:return
+ /self->mp != NULL && (int)arg1 == -5 &&
+ (self->mp->mnt_kern_flag & 0x01000000) != 0/
+ { @m = count(); }
+ fbt::ufs_mkdir:return { self->mp = NULL; }
+ END {
+ printf("vfs_busy() during unmount, by caller:");
+ printa(@b);
+ printa("mkdir(2) synced and restarted during unmount: %@d\n",
+ @m);
+ }' > $log.dtrace &
+ dpid=$!
+ sleep 5
+fi
+
+s=0
+start=`date +%s`
+n=0
+while [ $((`date +%s` - start)) -lt $runtime ]; do
+ mount /dev/md$mdstart $mntpoint || { s=1; break; }
+ pids=
+ for i in `jot $nworkers 0`; do
+ /tmp/nlink6 work $mntpoint/d $i 10 &
+ pids="$pids $!"
+ done
+ sleep `jot -r 1 1 5`
+ umount -f $mntpoint &
+ upid=$!
+ t=0
+ while kill -0 $upid 2>/dev/null; do
+ sleep 1
+ t=$((t + 1))
+ if [ $t -eq 120 ]; then
+ echo "FAIL: umount -f $mntpoint is stuck"
+ procstat -kk $upid `pgrep -x nlink6`
+ if [ -n "$dpid" ]; then
+ kill -TERM $dpid
+ wait $dpid
+ cat $log.dtrace
+ fi
+ # The mount point stays busy, leave everything as is.
+ exit 1
+ fi
+ done
+ wait $upid $pids
+ n=$((n + 1))
+ if ! fsck_ffs -fy /dev/md$mdstart > $log 2>&1; then
+ echo "FAIL: fsck_ffs exit $? after cycle $n"
+ tail -20 $log
+ s=1
+ break
+ fi
+done
+echo "$n forced unmounts"
+
+if [ -n "$dpid" ]; then
+ kill -TERM $dpid
+ wait $dpid
+ cat $log.dtrace
+fi
+mdconfig -d -u $mdstart
+rm -f /tmp/nlink6 $log $log.dtrace
+exit $s

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