Re: [PATCH v3] vfs: introduce UMOUNT_WAIT to wait for delayed_fput/mntput completion

[Jaegeuk Kim]

This patch introduces UMOUNT_WAIT flag for umount(2) which let user wait
for the completion of delayed fput()/mntput() explicitly.

Espcially, it can be used for Android reboot procedure, and resolve the below
issue where a kernel panic happens when a living filesystem tries to access
dead block device after device_shutdown done by kernel_restart.

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@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
---
 fs/file_table.c      |  6 ++++++
 fs/namespace.c       | 26 +++++++++++++++++++++++++-
 include/linux/file.h |  1 +
 include/linux/fs.h   |  1 +
 4 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/fs/file_table.c b/fs/file_table.c
index 72e861a35a7f..35b32ffdb934 100644
--- a/fs/file_table.c
+++ b/fs/file_table.c
@@ -263,6 +263,12 @@ void flush_delayed_fput(void)
 
 static DECLARE_DELAYED_WORK(delayed_fput_work, delayed_fput);
 
+void flush_delayed_fput_wait(void)
+{
+	delayed_fput(NULL);
+	flush_delayed_work(&delayed_fput_work);
+}
+
 void fput(struct file *file)
 {
 	if (atomic_long_dec_and_test(&file->f_count)) {
diff --git a/fs/namespace.c b/fs/namespace.c
index f8893dc6a989..e2586a38c83c 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>
@@ -1133,6 +1134,12 @@ static void delayed_mntput(struct work_struct *unused)
 }
 static DECLARE_DELAYED_WORK(delayed_mntput_work, delayed_mntput);
 
+void flush_delayed_mntput_wait(void)
+{
+	delayed_mntput(NULL);
+	flush_delayed_work(&delayed_mntput_work);
+}
+
 static void mntput_no_expire(struct mount *mnt)
 {
 	rcu_read_lock();
@@ -1629,7 +1636,8 @@ 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 +1661,27 @@ 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_wait();
+
 	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);
+
+	if (!retval && (flags & UMOUNT_WAIT)) {
+		/*
+		 * If the last delayed_fput() is called during do_umount()
+		 * and makes mnt_count zero, we need to guarantee to register
+		 * delayed_mntput by waiting for delayed_fput work again.
+		 */
+		flush_delayed_fput_wait();
+
+		/* flush delayed_mntput_work to put sb->s_active */
+		flush_delayed_mntput_wait();
+	}
 out:
 	return retval;
 }
diff --git a/include/linux/file.h b/include/linux/file.h
index 61eb82cbafba..ffb4236cde39 100644
--- a/include/linux/file.h
+++ b/include/linux/file.h
@@ -84,6 +84,7 @@ extern void put_unused_fd(unsigned int fd);
 extern void fd_install(unsigned int fd, struct file *file);
 
 extern void flush_delayed_fput(void);
+extern void flush_delayed_fput_wait(void);
 extern void __fput_sync(struct file *);
 
 #endif /* __LINUX_FILE_H */
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