1 files changed, 206 insertions, 211 deletions

diff --git a/fs/namespace.c b/fs/namespace.c
index 134d494158d9..3dc283fd4716 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -42,6 +42,8 @@ __cacheline_aligned_in_smp DEFINE_SPINLOCK(vfsmount_lock);
 static int event;
 static DEFINE_IDA(mnt_id_ida);
 static DEFINE_IDA(mnt_group_ida);
+static int mnt_id_start = 0;
+static int mnt_group_start = 1;
 
 static struct list_head *mount_hashtable __read_mostly;
 static struct kmem_cache *mnt_cache __read_mostly;
@@ -69,7 +71,9 @@ static int mnt_alloc_id(struct vfsmount *mnt)
 retry:
         ida_pre_get(&mnt_id_ida, GFP_KERNEL);
         spin_lock(&vfsmount_lock);
-        res = ida_get_new(&mnt_id_ida, &mnt->mnt_id);
+        res = ida_get_new_above(&mnt_id_ida, mnt_id_start, &mnt->mnt_id);
+        if (!res)
+                mnt_id_start = mnt->mnt_id + 1;
         spin_unlock(&vfsmount_lock);
         if (res == -EAGAIN)
                 goto retry;
@@ -79,8 +83,11 @@ retry:
 
 static void mnt_free_id(struct vfsmount *mnt)
 {
+        int id = mnt->mnt_id;
         spin_lock(&vfsmount_lock);
-        ida_remove(&mnt_id_ida, mnt->mnt_id);
+        ida_remove(&mnt_id_ida, id);
+        if (mnt_id_start > id)
+                mnt_id_start = id;
         spin_unlock(&vfsmount_lock);
 }
 
@@ -91,10 +98,18 @@ static void mnt_free_id(struct vfsmount *mnt)
  */
 static int mnt_alloc_group_id(struct vfsmount *mnt)
 {
+        int res;
+
         if (!ida_pre_get(&mnt_group_ida, GFP_KERNEL))
                 return -ENOMEM;
 
-        return ida_get_new_above(&mnt_group_ida, 1, &mnt->mnt_group_id);
+        res = ida_get_new_above(&mnt_group_ida,
+                                mnt_group_start,
+                                &mnt->mnt_group_id);
+        if (!res)
+                mnt_group_start = mnt->mnt_group_id + 1;
+
+        return res;
 }
 
 /*
@@ -102,7 +117,10 @@ static int mnt_alloc_group_id(struct vfsmount *mnt)
  */
 void mnt_release_group_id(struct vfsmount *mnt)
 {
-        ida_remove(&mnt_group_ida, mnt->mnt_group_id);
+        int id = mnt->mnt_group_id;
+        ida_remove(&mnt_group_ida, id);
+        if (mnt_group_start > id)
+                mnt_group_start = id;
         mnt->mnt_group_id = 0;
 }
 
@@ -131,10 +149,20 @@ struct vfsmount *alloc_vfsmnt(const char *name)
                 INIT_LIST_HEAD(&mnt->mnt_share);
                 INIT_LIST_HEAD(&mnt->mnt_slave_list);
                 INIT_LIST_HEAD(&mnt->mnt_slave);
-                atomic_set(&mnt->__mnt_writers, 0);
+#ifdef CONFIG_SMP
+                mnt->mnt_writers = alloc_percpu(int);
+                if (!mnt->mnt_writers)
+                        goto out_free_devname;
+#else
+                mnt->mnt_writers = 0;
+#endif
         }
         return mnt;
 
+#ifdef CONFIG_SMP
+out_free_devname:
+        kfree(mnt->mnt_devname);
+#endif
 out_free_id:
         mnt_free_id(mnt);
 out_free_cache:
@@ -171,65 +199,38 @@ int __mnt_is_readonly(struct vfsmount *mnt)
 }
 EXPORT_SYMBOL_GPL(__mnt_is_readonly);
 
-struct mnt_writer {
-        /*
-         * If holding multiple instances of this lock, they
-         * must be ordered by cpu number.
-         */
-        spinlock_t lock;
-        struct lock_class_key lock_class; /* compiles out with !lockdep */
-        unsigned long count;
-        struct vfsmount *mnt;
-} ____cacheline_aligned_in_smp;
-static DEFINE_PER_CPU(struct mnt_writer, mnt_writers);
+static inline void inc_mnt_writers(struct vfsmount *mnt)
+{
+#ifdef CONFIG_SMP
+        (*per_cpu_ptr(mnt->mnt_writers, smp_processor_id()))++;
+#else
+        mnt->mnt_writers++;
+#endif
+}
 
-static int __init init_mnt_writers(void)
+static inline void dec_mnt_writers(struct vfsmount *mnt)
 {
-        int cpu;
-        for_each_possible_cpu(cpu) {
-                struct mnt_writer *writer = &per_cpu(mnt_writers, cpu);
-                spin_lock_init(&writer->lock);
-                lockdep_set_class(&writer->lock, &writer->lock_class);
-                writer->count = 0;
-        }
-        return 0;
+#ifdef CONFIG_SMP
+        (*per_cpu_ptr(mnt->mnt_writers, smp_processor_id()))--;
+#else
+        mnt->mnt_writers--;
+#endif
 }
-fs_initcall(init_mnt_writers);
 
-static void unlock_mnt_writers(void)
+static unsigned int count_mnt_writers(struct vfsmount *mnt)
 {
+#ifdef CONFIG_SMP
+        unsigned int count = 0;
         int cpu;
-        struct mnt_writer *cpu_writer;
 
         for_each_possible_cpu(cpu) {
-                cpu_writer = &per_cpu(mnt_writers, cpu);
-                spin_unlock(&cpu_writer->lock);
+                count += *per_cpu_ptr(mnt->mnt_writers, cpu);
         }
-}
 
-static inline void __clear_mnt_count(struct mnt_writer *cpu_writer)
-{
-        if (!cpu_writer->mnt)
-                return;
-        /*
-         * This is in case anyone ever leaves an invalid,
-         * old ->mnt and a count of 0.
-         */
-        if (!cpu_writer->count)
-                return;
-        atomic_add(cpu_writer->count, &cpu_writer->mnt->__mnt_writers);
-        cpu_writer->count = 0;
-}
- /*
- * must hold cpu_writer->lock
- */
-static inline void use_cpu_writer_for_mount(struct mnt_writer *cpu_writer,
-                                          struct vfsmount *mnt)
-{
-        if (cpu_writer->mnt == mnt)
-                return;
-        __clear_mnt_count(cpu_writer);
-        cpu_writer->mnt = mnt;
+        return count;
+#else
+        return mnt->mnt_writers;
+#endif
 }
 
 /*
@@ -253,74 +254,73 @@ static inline void use_cpu_writer_for_mount(struct mnt_writer *cpu_writer,
 int mnt_want_write(struct vfsmount *mnt)
 {
         int ret = 0;
-        struct mnt_writer *cpu_writer;
 
-        cpu_writer = &get_cpu_var(mnt_writers);
-        spin_lock(&cpu_writer->lock);
+        preempt_disable();
+        inc_mnt_writers(mnt);
+        /*
+         * The store to inc_mnt_writers must be visible before we pass
+         * MNT_WRITE_HOLD loop below, so that the slowpath can see our
+         * incremented count after it has set MNT_WRITE_HOLD.
+         */
+        smp_mb();
+        while (mnt->mnt_flags & MNT_WRITE_HOLD)
+                cpu_relax();
+        /*
+         * After the slowpath clears MNT_WRITE_HOLD, mnt_is_readonly will
+         * be set to match its requirements. So we must not load that until
+         * MNT_WRITE_HOLD is cleared.
+         */
+        smp_rmb();
         if (__mnt_is_readonly(mnt)) {
+                dec_mnt_writers(mnt);
                 ret = -EROFS;
                 goto out;
         }
-        use_cpu_writer_for_mount(cpu_writer, mnt);
-        cpu_writer->count++;
 out:
-        spin_unlock(&cpu_writer->lock);
-        put_cpu_var(mnt_writers);
+        preempt_enable();
         return ret;
 }
 EXPORT_SYMBOL_GPL(mnt_want_write);
 
-static void lock_mnt_writers(void)
-{
-        int cpu;
-        struct mnt_writer *cpu_writer;
-
-        for_each_possible_cpu(cpu) {
-                cpu_writer = &per_cpu(mnt_writers, cpu);
-                spin_lock(&cpu_writer->lock);
-                __clear_mnt_count(cpu_writer);
-                cpu_writer->mnt = NULL;
-        }
+/**
+ * mnt_clone_write - get write access to a mount
+ * @mnt: the mount on which to take a write
+ *
+ * This is effectively like mnt_want_write, except
+ * it must only be used to take an extra write reference
+ * on a mountpoint that we already know has a write reference
+ * on it. This allows some optimisation.
+ *
+ * After finished, mnt_drop_write must be called as usual to
+ * drop the reference.
+ */
+int mnt_clone_write(struct vfsmount *mnt)
+{
+        /* superblock may be r/o */
+        if (__mnt_is_readonly(mnt))
+                return -EROFS;
+        preempt_disable();
+        inc_mnt_writers(mnt);
+        preempt_enable();
+        return 0;
 }
+EXPORT_SYMBOL_GPL(mnt_clone_write);
 
-/*
- * These per-cpu write counts are not guaranteed to have
- * matched increments and decrements on any given cpu.
- * A file open()ed for write on one cpu and close()d on
- * another cpu will imbalance this count.  Make sure it
- * does not get too far out of whack.
+/**
+ * mnt_want_write_file - get write access to a file's mount
+ * @file: the file who's mount on which to take a write
+ *
+ * This is like mnt_want_write, but it takes a file and can
+ * do some optimisations if the file is open for write already
  */
-static void handle_write_count_underflow(struct vfsmount *mnt)
+int mnt_want_write_file(struct file *file)
 {
-        if (atomic_read(&mnt->__mnt_writers) >=
-            MNT_WRITER_UNDERFLOW_LIMIT)
-                return;
-        /*
-         * It isn't necessary to hold all of the locks
-         * at the same time, but doing it this way makes
-         * us share a lot more code.
-         */
-        lock_mnt_writers();
-        /*
-         * vfsmount_lock is for mnt_flags.
-         */
-        spin_lock(&vfsmount_lock);
-        /*
-         * If coalescing the per-cpu writer counts did not
-         * get us back to a positive writer count, we have
-         * a bug.
-         */
-        if ((atomic_read(&mnt->__mnt_writers) < 0) &&
-            !(mnt->mnt_flags & MNT_IMBALANCED_WRITE_COUNT)) {
-                WARN(1, KERN_DEBUG "leak detected on mount(%p) writers "
-                                "count: %d\n",
-                        mnt, atomic_read(&mnt->__mnt_writers));
-                /* use the flag to keep the dmesg spam down */
-                mnt->mnt_flags |= MNT_IMBALANCED_WRITE_COUNT;
-        }
-        spin_unlock(&vfsmount_lock);
-        unlock_mnt_writers();
+        if (!(file->f_mode & FMODE_WRITE))
+                return mnt_want_write(file->f_path.mnt);
+        else
+                return mnt_clone_write(file->f_path.mnt);
 }
+EXPORT_SYMBOL_GPL(mnt_want_write_file);
 
 /**
  * mnt_drop_write - give up write access to a mount
@@ -332,37 +332,9 @@ static void handle_write_count_underflow(struct vfsmount *mnt)
  */
 void mnt_drop_write(struct vfsmount *mnt)
 {
-        int must_check_underflow = 0;
-        struct mnt_writer *cpu_writer;
-
-        cpu_writer = &get_cpu_var(mnt_writers);
-        spin_lock(&cpu_writer->lock);
-
-        use_cpu_writer_for_mount(cpu_writer, mnt);
-        if (cpu_writer->count > 0) {
-                cpu_writer->count--;
-        } else {
-                must_check_underflow = 1;
-                atomic_dec(&mnt->__mnt_writers);
-        }
-
-        spin_unlock(&cpu_writer->lock);
-        /*
-         * Logically, we could call this each time,
-         * but the __mnt_writers cacheline tends to
-         * be cold, and makes this expensive.
-         */
-        if (must_check_underflow)
-                handle_write_count_underflow(mnt);
-        /*
-         * This could be done right after the spinlock
-         * is taken because the spinlock keeps us on
-         * the cpu, and disables preemption.  However,
-         * putting it here bounds the amount that
-         * __mnt_writers can underflow.  Without it,
-         * we could theoretically wrap __mnt_writers.
-         */
-        put_cpu_var(mnt_writers);
+        preempt_disable();
+        dec_mnt_writers(mnt);
+        preempt_enable();
 }
 EXPORT_SYMBOL_GPL(mnt_drop_write);
 
@@ -370,24 +342,41 @@ static int mnt_make_readonly(struct vfsmount *mnt)
 {
         int ret = 0;
 
-        lock_mnt_writers();
+        spin_lock(&vfsmount_lock);
+        mnt->mnt_flags |= MNT_WRITE_HOLD;
         /*
-         * With all the locks held, this value is stable
+         * After storing MNT_WRITE_HOLD, we'll read the counters. This store
+         * should be visible before we do.
          */
-        if (atomic_read(&mnt->__mnt_writers) > 0) {
-                ret = -EBUSY;
-                goto out;
-        }
+        smp_mb();
+
         /*
-         * nobody can do a successful mnt_want_write() with all
-         * of the counts in MNT_DENIED_WRITE and the locks held.
+         * With writers on hold, if this value is zero, then there are
+         * definitely no active writers (although held writers may subsequently
+         * increment the count, they'll have to wait, and decrement it after
+         * seeing MNT_READONLY).
+         *
+         * It is OK to have counter incremented on one CPU and decremented on
+         * another: the sum will add up correctly. The danger would be when we
+         * sum up each counter, if we read a counter before it is incremented,
+         * but then read another CPU's count which it has been subsequently
+         * decremented from -- we would see more decrements than we should.
+         * MNT_WRITE_HOLD protects against this scenario, because
+         * mnt_want_write first increments count, then smp_mb, then spins on
+         * MNT_WRITE_HOLD, so it can't be decremented by another CPU while
+         * we're counting up here.
          */
-        spin_lock(&vfsmount_lock);
-        if (!ret)
+        if (count_mnt_writers(mnt) > 0)
+                ret = -EBUSY;
+        else
                 mnt->mnt_flags |= MNT_READONLY;
+        /*
+         * MNT_READONLY must become visible before ~MNT_WRITE_HOLD, so writers
+         * that become unheld will see MNT_READONLY.
+         */
+        smp_wmb();
+        mnt->mnt_flags &= ~MNT_WRITE_HOLD;
         spin_unlock(&vfsmount_lock);
-out:
-        unlock_mnt_writers();
         return ret;
 }
 
@@ -410,6 +399,9 @@ void free_vfsmnt(struct vfsmount *mnt)
 {
         kfree(mnt->mnt_devname);
         mnt_free_id(mnt);
+#ifdef CONFIG_SMP
+        free_percpu(mnt->mnt_writers);
+#endif
         kmem_cache_free(mnt_cache, mnt);
 }
 
@@ -442,11 +434,11 @@ struct vfsmount *__lookup_mnt(struct vfsmount *mnt, struct dentry *dentry,
  * lookup_mnt increments the ref count before returning
  * the vfsmount struct.
  */
-struct vfsmount *lookup_mnt(struct vfsmount *mnt, struct dentry *dentry)
+struct vfsmount *lookup_mnt(struct path *path)
 {
         struct vfsmount *child_mnt;
         spin_lock(&vfsmount_lock);
-        if ((child_mnt = __lookup_mnt(mnt, dentry, 1)))
+        if ((child_mnt = __lookup_mnt(path->mnt, path->dentry, 1)))
                 mntget(child_mnt);
         spin_unlock(&vfsmount_lock);
         return child_mnt;
@@ -604,38 +596,18 @@ static struct vfsmount *clone_mnt(struct vfsmount *old, struct dentry *root,
 
 static inline void __mntput(struct vfsmount *mnt)
 {
-        int cpu;
         struct super_block *sb = mnt->mnt_sb;
         /*
-         * We don't have to hold all of the locks at the
-         * same time here because we know that we're the
-         * last reference to mnt and that no new writers
-         * can come in.
-         */
-        for_each_possible_cpu(cpu) {
-                struct mnt_writer *cpu_writer = &per_cpu(mnt_writers, cpu);
-                spin_lock(&cpu_writer->lock);
-                if (cpu_writer->mnt != mnt) {
-                        spin_unlock(&cpu_writer->lock);
-                        continue;
-                }
-                atomic_add(cpu_writer->count, &mnt->__mnt_writers);
-                cpu_writer->count = 0;
-                /*
-                 * Might as well do this so that no one
-                 * ever sees the pointer and expects
-                 * it to be valid.
-                 */
-                cpu_writer->mnt = NULL;
-                spin_unlock(&cpu_writer->lock);
-        }
-        /*
          * This probably indicates that somebody messed
          * up a mnt_want/drop_write() pair.  If this
          * happens, the filesystem was probably unable
          * to make r/w->r/o transitions.
          */
-        WARN_ON(atomic_read(&mnt->__mnt_writers));
+        /*
+         * atomic_dec_and_lock() used to deal with ->mnt_count decrements
+         * provides barriers, so count_mnt_writers() below is safe.  AV
+         */
+        WARN_ON(count_mnt_writers(mnt));
         dput(mnt->mnt_root);
         free_vfsmnt(mnt);
         deactivate_super(sb);
@@ -1106,11 +1078,8 @@ static int do_umount(struct vfsmount *mnt, int flags)
                  * we just try to remount it readonly.
                  */
                 down_write(&sb->s_umount);
-                if (!(sb->s_flags & MS_RDONLY)) {
-                        lock_kernel();
+                if (!(sb->s_flags & MS_RDONLY))
                         retval = do_remount_sb(sb, MS_RDONLY, NULL, 0);
-                        unlock_kernel();
-                }
                 up_write(&sb->s_umount);
                 return retval;
         }
@@ -1253,11 +1222,11 @@ Enomem:
         return NULL;
 }
 
-struct vfsmount *collect_mounts(struct vfsmount *mnt, struct dentry *dentry)
+struct vfsmount *collect_mounts(struct path *path)
 {
         struct vfsmount *tree;
         down_write(&namespace_sem);
-        tree = copy_tree(mnt, dentry, CL_COPY_ALL | CL_PRIVATE);
+        tree = copy_tree(path->mnt, path->dentry, CL_COPY_ALL | CL_PRIVATE);
         up_write(&namespace_sem);
         return tree;
 }
@@ -1430,7 +1399,7 @@ static int graft_tree(struct vfsmount *mnt, struct path *path)
                 goto out_unlock;
 
         err = -ENOENT;
-        if (IS_ROOT(path->dentry) || !d_unhashed(path->dentry))
+        if (!d_unlinked(path->dentry))
                 err = attach_recursive_mnt(mnt, path, NULL);
 out_unlock:
         mutex_unlock(&path->dentry->d_inode->i_mutex);
@@ -1601,7 +1570,7 @@ static int do_move_mount(struct path *path, char *old_name)
 
         down_write(&namespace_sem);
         while (d_mountpoint(path->dentry) &&
-               follow_down(&path->mnt, &path->dentry))
+               follow_down(path))
                 ;
         err = -EINVAL;
         if (!check_mnt(path->mnt) || !check_mnt(old_path.mnt))
@@ -1612,7 +1581,7 @@ static int do_move_mount(struct path *path, char *old_name)
         if (IS_DEADDIR(path->dentry->d_inode))
                 goto out1;
 
-        if (!IS_ROOT(path->dentry) && d_unhashed(path->dentry))
+        if (d_unlinked(path->dentry))
                 goto out1;
 
         err = -EINVAL;
@@ -1676,7 +1645,9 @@ static int do_new_mount(struct path *path, char *type, int flags,
         if (!capable(CAP_SYS_ADMIN))
                 return -EPERM;
 
+        lock_kernel();
         mnt = do_kern_mount(type, flags, name, data);
+        unlock_kernel();
         if (IS_ERR(mnt))
                 return PTR_ERR(mnt);
 
@@ -1695,10 +1666,10 @@ int do_add_mount(struct vfsmount *newmnt, struct path *path,
         down_write(&namespace_sem);
         /* Something was mounted here while we slept */
         while (d_mountpoint(path->dentry) &&
-               follow_down(&path->mnt, &path->dentry))
+               follow_down(path))
                 ;
         err = -EINVAL;
-        if (!check_mnt(path->mnt))
+        if (!(mnt_flags & MNT_SHRINKABLE) && !check_mnt(path->mnt))
                 goto unlock;
 
         /* Refuse the same filesystem on the same mount point */
@@ -1984,6 +1955,21 @@ dput_out:
         return retval;
 }
 
+static struct mnt_namespace *alloc_mnt_ns(void)
+{
+        struct mnt_namespace *new_ns;
+
+        new_ns = kmalloc(sizeof(struct mnt_namespace), GFP_KERNEL);
+        if (!new_ns)
+                return ERR_PTR(-ENOMEM);
+        atomic_set(&new_ns->count, 1);
+        new_ns->root = NULL;
+        INIT_LIST_HEAD(&new_ns->list);
+        init_waitqueue_head(&new_ns->poll);
+        new_ns->event = 0;
+        return new_ns;
+}
+
 /*
  * Allocate a new namespace structure and populate it with contents
  * copied from the namespace of the passed in task structure.
@@ -1995,14 +1981,9 @@ static struct mnt_namespace *dup_mnt_ns(struct mnt_namespace *mnt_ns,
         struct vfsmount *rootmnt = NULL, *pwdmnt = NULL;
         struct vfsmount *p, *q;
 
-        new_ns = kmalloc(sizeof(struct mnt_namespace), GFP_KERNEL);
-        if (!new_ns)
-                return ERR_PTR(-ENOMEM);
-
-        atomic_set(&new_ns->count, 1);
-        INIT_LIST_HEAD(&new_ns->list);
-        init_waitqueue_head(&new_ns->poll);
-        new_ns->event = 0;
+        new_ns = alloc_mnt_ns();
+        if (IS_ERR(new_ns))
+                return new_ns;
 
         down_write(&namespace_sem);
         /* First pass: copy the tree topology */
@@ -2066,6 +2047,24 @@ struct mnt_namespace *copy_mnt_ns(unsigned long flags, struct mnt_namespace *ns,
         return new_ns;
 }
 
+/**
+ * create_mnt_ns - creates a private namespace and adds a root filesystem
+ * @mnt: pointer to the new root filesystem mountpoint
+ */
+struct mnt_namespace *create_mnt_ns(struct vfsmount *mnt)
+{
+        struct mnt_namespace *new_ns;
+
+        new_ns = alloc_mnt_ns();
+        if (!IS_ERR(new_ns)) {
+                mnt->mnt_ns = new_ns;
+                new_ns->root = mnt;
+                list_add(&new_ns->list, &new_ns->root->mnt_list);
+        }
+        return new_ns;
+}
+EXPORT_SYMBOL(create_mnt_ns);
+
 SYSCALL_DEFINE5(mount, char __user *, dev_name, char __user *, dir_name,
                 char __user *, type, unsigned long, flags, void __user *, data)
 {
@@ -2092,10 +2091,8 @@ SYSCALL_DEFINE5(mount, char __user *, dev_name, char __user *, dir_name,
         if (retval < 0)
                 goto out3;
 
-        lock_kernel();
         retval = do_mount((char *)dev_page, dir_page, (char *)type_page,
                           flags, (void *)data_page);
-        unlock_kernel();
         free_page(data_page);
 
 out3:
@@ -2175,9 +2172,9 @@ SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
         error = -ENOENT;
         if (IS_DEADDIR(new.dentry->d_inode))
                 goto out2;
-        if (d_unhashed(new.dentry) && !IS_ROOT(new.dentry))
+        if (d_unlinked(new.dentry))
                 goto out2;
-        if (d_unhashed(old.dentry) && !IS_ROOT(old.dentry))
+        if (d_unlinked(old.dentry))
                 goto out2;
         error = -EBUSY;
         if (new.mnt == root.mnt ||
@@ -2243,16 +2240,9 @@ static void __init init_mount_tree(void)
         mnt = do_kern_mount("rootfs", 0, "rootfs", NULL);
         if (IS_ERR(mnt))
                 panic("Can't create rootfs");
-        ns = kmalloc(sizeof(*ns), GFP_KERNEL);
-        if (!ns)
+        ns = create_mnt_ns(mnt);
+        if (IS_ERR(ns))
                 panic("Can't allocate initial namespace");
-        atomic_set(&ns->count, 1);
-        INIT_LIST_HEAD(&ns->list);
-        init_waitqueue_head(&ns->poll);
-        ns->event = 0;
-        list_add(&mnt->mnt_list, &ns->list);
-        ns->root = mnt;
-        mnt->mnt_ns = ns;
 
         init_task.nsproxy->mnt_ns = ns;
         get_mnt_ns(ns);
@@ -2295,10 +2285,14 @@ void __init mnt_init(void)
         init_mount_tree();
 }
 
-void __put_mnt_ns(struct mnt_namespace *ns)
+void put_mnt_ns(struct mnt_namespace *ns)
 {
-        struct vfsmount *root = ns->root;
+        struct vfsmount *root;
         LIST_HEAD(umount_list);
+
+        if (!atomic_dec_and_lock(&ns->count, &vfsmount_lock))
+                return;
+        root = ns->root;
         ns->root = NULL;
         spin_unlock(&vfsmount_lock);
         down_write(&namespace_sem);
@@ -2309,3 +2303,4 @@ void __put_mnt_ns(struct mnt_namespace *ns)
         release_mounts(&umount_list);
         kfree(ns);
 }
+EXPORT_SYMBOL(put_mnt_ns);