commit 6ab553e46cf3cb78d8a6fb1752235feadc9b3750
Author: Tong Zhang <ztong0001@gmail.com>
Date:   Sun Nov 22 18:50:16 2020 -0500

    vfs_domount_first: do not crash on mount failure
    
    vfs_write_suspend_umnt() expect a non-zero vfs_write_suspend_umnt,
    otherwise it will panic
    
     panic: vn_finished_write: neg cnt
     cpuid = 7
     time = 1606004193
     KDB: stack backtrace:
     #0 0xffffffff80c0a8f5 at kdb_backtrace+0x65
     #1 0xffffffff80bbeb1b at vpanic+0x17b
     #2 0xffffffff80bbe993 at panic+0x43
     #3 0xffffffff80c9da05 at vn_finished_write+0xc5
     #4 0xffffffff80c9f726 at vfs_write_suspend_umnt+0x16
     #5 0xffffffff80ecdd31 at ffs_unmount+0x71
     #6 0xffffffff80c844dd at vfs_domount+0xc0d
     #7 0xffffffff80c83228 at vfs_donmount+0x988
     #8 0xffffffff80c82871 at sys_nmount+0x71
     #9 0xffffffff810904c7 at amd64_syscall+0x387
     #10 0xffffffff8106785e at fast_syscall_common+0xf8
     Uptime: 2h10m11s

diff --git a/sys/kern/vfs_mount.c b/sys/kern/vfs_mount.c
index afbb3e313d7..03e4394cf75 100644
--- a/sys/kern/vfs_mount.c
+++ b/sys/kern/vfs_mount.c
@@ -895,6 +895,15 @@ vfs_domount_first(
 	    (error1 = VFS_ROOT(mp, LK_EXCLUSIVE, &newdp)) != 0) {
 		if (error1 != 0) {
 			error = error1;
+			/*
+			 * if mounted as rw - we should increase mnt_writeopcount here
+			 * since VFS_UNMOUNT will call vfs_write_suspend_umnt() and
+			 * it expects a non-zero vfs_write_suspend_umnt
+			 */
+			if ((mp->mnt_flag & MNT_RDONLY) == 0) {
+				if ((error = vn_start_write(NULL, &mp, V_WAIT)) != 0)
+					return (error);
+			}
 			if ((error1 = VFS_UNMOUNT(mp, 0)) != 0)
 				printf("VFS_UNMOUNT returned %d\n", error1);
 		}