From d9232a652080ac260c78ab647608860269b45d03 Mon Sep 17 00:00:00 2001
From: NeilBrown <neil@brown.name>
Date: Wed, 8 Nov 2017 19:29:32 +1100
Subject: [PATCH] umount: always use MNT_FORCE in umount_all() (#7213)

The linux umount2() systemcall accepts a MNT_FORCE flags
which some filesystems honor, particularly FUSE and various
network filesystems such as NFS.
These filesystems can sometimes wait for an indefinite period
for a response from an external service, and the wait if
sometimes "uninterruptible" meaning that the process cannot be
killed.
Using MNT_FORCE causes any such request that are outstanding to
be aborted.  This normally allows the waiting process to
be killed.  It will then realease and reference it has to the
filesytem, this allowing the filesystem to be unmounted.

If there remain active references to the filesystem, MNT_FORCE
is *not* forcefull enough to unmount the filesystem anyway.

By the time that umount_all() is run by systemd-shutdown, all
filesystems *should* be unmounted, and sync() will have been
called.  Anything that remains cannot be unmounted in a
completely clean manner and just nees to be dealt with as firmly
as possible.  So use MNT_FORCE and try to explain why in the
comment.

Also enhance an earlier comment to explain why umount2() is
safe even though mount(MNT_REMOUNT) isn't.

(cherry picked from commit c44cac7c6c43407d28bd8daebff39f6145a2a33e)

Resolves: #1571098
---
 src/core/umount.c | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/src/core/umount.c b/src/core/umount.c
index 3eec0d459..91d67c06c 100644
--- a/src/core/umount.c
+++ b/src/core/umount.c
@@ -377,7 +377,9 @@ static int mount_points_list_umount(MountPoint **head, bool *changed, bool log_e
                    the superblock here, not the bind mount.
                    If the filesystem is a network fs, also skip the
                    remount.  It brings no value (we cannot leave
-                   a "dirty fs") and could hang if the network is down.  */
+                   a "dirty fs") and could hang if the network is down.
+                   Note that umount2() is more careful and will not
+                   hang because of the network being down. */
                 if (detect_container(NULL) <= 0 &&
                     !fstype_is_network(m->type)) {
                         _cleanup_free_ char *options = NULL;
@@ -418,11 +420,15 @@ static int mount_points_list_umount(MountPoint **head, bool *changed, bool log_e
                 )
                         continue;
 
-                /* Trying to umount. We don't force here since we rely
-                 * on busy NFS and FUSE file systems to return EBUSY
-                 * until we closed everything on top of them. */
+                /* Trying to umount. Using MNT_FORCE causes some
+                 * filesystems (e.g. FUSE and NFS and other network
+                 * filesystems) to abort any pending requests and
+                 * return -EIO rather than blocking indefinitely.
+                 * If the filesysten is "busy", this may allow processes
+                 * to die, thus making the filesystem less busy so
+                 * the unmount might succeed (rather then return EBUSY).*/
                 log_info("Unmounting %s.", m->path);
-                if (umount2(m->path, 0) == 0) {
+                if (umount2(m->path, MNT_FORCE) == 0) {
                         if (changed)
                                 *changed = true;