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SubjectRe: [PATCH v2] vfs: introduce UMOUNT_WAIT which waits for umount completion
This patch introduces UMOUNT_WAIT flag for umount(2) which let user wait for
umount(2) to complete filesystem shutdown. This should fix a kernel panic
triggered when a living filesystem tries to access dead block device after
device_shutdown done by kernel_restart as below.

Term: namespace(mnt_get_count())

1. create_new_namespaces() creates ns1 and ns2,

/data(1) ns1(1) ns2(1)
| | |
---------------------
|
sb->s_active = 3

2. after binder_proc_clear_zombies() for ns2 and ns1 triggers
- delayed_fput()
- delayed_mntput_work(ns2)

/data(1) ns1(1)
| |
----------
|
sb->s_active = 2

3. umount() for /data is successed.

ns1(1)
|
sb->s_active = 1

4. device_shutdown() by init

5. - delayed_mntput_work(ns1)
- put_super(), since sb->s_active = 0
- -EIO

Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
---
fs/namespace.c | 12 +++++++++++-
include/linux/fs.h | 1 +
2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/fs/namespace.c b/fs/namespace.c
index f8893dc6a989..f2c15c4f6e23 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -21,6 +21,7 @@
#include <linux/fs_struct.h> /* get_fs_root et.al. */
#include <linux/fsnotify.h> /* fsnotify_vfsmount_delete */
#include <linux/uaccess.h>
+#include <linux/file.h>
#include <linux/proc_ns.h>
#include <linux/magic.h>
#include <linux/bootmem.h>
@@ -1629,7 +1630,8 @@ SYSCALL_DEFINE2(umount, char __user *, name, int, flags)
int retval;
int lookup_flags = 0;

- if (flags & ~(MNT_FORCE | MNT_DETACH | MNT_EXPIRE | UMOUNT_NOFOLLOW))
+ if (flags & ~(MNT_FORCE | MNT_DETACH | MNT_EXPIRE | UMOUNT_NOFOLLOW |
+ UMOUNT_WAIT))
return -EINVAL;

if (!may_mount())
@@ -1653,11 +1655,19 @@ SYSCALL_DEFINE2(umount, char __user *, name, int, flags)
if (flags & MNT_FORCE && !capable(CAP_SYS_ADMIN))
goto dput_and_out;

+ /* flush delayed_fput to put mnt_count */
+ if (flags & UMOUNT_WAIT)
+ flush_delayed_fput();
+
retval = do_umount(mnt, flags);
dput_and_out:
/* we mustn't call path_put() as that would clear mnt_expiry_mark */
dput(path.dentry);
mntput_no_expire(mnt);
+
+ /* flush delayed_mntput_work to put sb->s_active */
+ if (!retval && (flags & UMOUNT_WAIT))
+ flush_scheduled_work();
out:
return retval;
}
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 6e1fd5d21248..69f0fd53c9c7 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1278,6 +1278,7 @@ struct mm_struct;
#define MNT_DETACH 0x00000002 /* Just detach from the tree */
#define MNT_EXPIRE 0x00000004 /* Mark for expiry */
#define UMOUNT_NOFOLLOW 0x00000008 /* Don't follow symlink on umount */
+#define UMOUNT_WAIT 0x00000010 /* Wait to unmount completely */
#define UMOUNT_UNUSED 0x80000000 /* Flag guaranteed to be unused */

/* sb->s_iflags */
--
2.14.0.rc1.383.gd1ce394fe2-goog
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