From 5bace483dedc9098da8191f39c823649948a7a3c 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 3eec0d4592..91d67c06ca 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;