> E.g. having a separate task count, which is incremented/decremented
> only by clone/exit. If the task count goes to zero, umount_tree is
> called on root, but namespace is not freed.
>
> And each mnt_namespace holds a proper reference to the namespace, so
> it's safe to dereference it anytime. When truly no more references
> remain, the namespace can go away.
>
> Hmm?
Here's a patch (compile and boot tested).
Now there are two solutions to one problem, the previous one is ugly,
this one is buggy. Which should it be?
Thanks,
Miklos
Index: linux/include/linux/namespace.h
===================================================================
--- linux.orig/include/linux/namespace.h 2005-05-18 18:02:24.000000000 +0200
+++ linux/include/linux/namespace.h 2005-05-18 18:28:48.000000000 +0200
@@ -7,18 +7,26 @@
struct namespace {
atomic_t count;
+ atomic_t task_count; /* how many tasks use this */
struct vfsmount * root;
struct list_head list;
struct rw_semaphore sem;
};
extern int copy_namespace(int, struct task_struct *);
-extern void __put_namespace(struct namespace *namespace);
+extern void __exit_namespace(struct namespace *namespace);
static inline void put_namespace(struct namespace *namespace)
{
- if (atomic_dec_and_test(&namespace->count))
- __put_namespace(namespace);
+ if (namespace && atomic_dec_and_test(&namespace->count))
+ kfree(namespace);
+}
+
+static inline struct namespace *get_namespace(struct namespace *namespace)
+{
+ if (namespace)
+ atomic_inc(&namespace->count);
+ return namespace;
}
static inline void exit_namespace(struct task_struct *p)
@@ -28,14 +36,10 @@ static inline void exit_namespace(struct
task_lock(p);
p->namespace = NULL;
task_unlock(p);
- put_namespace(namespace);
+ if (atomic_dec_and_test(&p->namespace->task_count))
+ __exit_namespace(p->namespace);
}
}
-static inline void get_namespace(struct namespace *namespace)
-{
- atomic_inc(&namespace->count);
-}
-
#endif
#endif
Index: linux/fs/namespace.c
===================================================================
--- linux.orig/fs/namespace.c 2005-05-18 17:42:46.000000000 +0200
+++ linux/fs/namespace.c 2005-05-18 18:32:29.000000000 +0200
@@ -160,7 +160,7 @@ clone_mnt(struct vfsmount *old, struct d
mnt->mnt_root = dget(root);
mnt->mnt_mountpoint = mnt->mnt_root;
mnt->mnt_parent = mnt;
- mnt->mnt_namespace = current->namespace;
+ mnt->mnt_namespace = get_namespace(current->namespace);
/* stick the duplicate mount on the same expiry list
* as the original if that was on one */
@@ -345,13 +345,15 @@ static void umount_tree(struct vfsmount
for (p = mnt; p; p = next_mnt(p, mnt)) {
list_del(&p->mnt_list);
list_add(&p->mnt_list, &kill);
- p->mnt_namespace = NULL;
}
while (!list_empty(&kill)) {
+ struct namespace *n;
mnt = list_entry(kill.next, struct vfsmount, mnt_list);
list_del_init(&mnt->mnt_list);
list_del_init(&mnt->mnt_fslink);
+ n = mnt->mnt_namespace;
+ mnt->mnt_namespace = NULL;
if (mnt->mnt_parent == mnt) {
spin_unlock(&vfsmount_lock);
} else {
@@ -361,6 +363,7 @@ static void umount_tree(struct vfsmount
path_release(&old_nd);
}
mntput(mnt);
+ put_namespace(n);
spin_lock(&vfsmount_lock);
}
}
@@ -866,12 +869,9 @@ void mark_mounts_for_expiry(struct list_
mnt = list_entry(graveyard.next, struct vfsmount, mnt_fslink);
list_del_init(&mnt->mnt_fslink);
- /* don't do anything if the namespace is dead - all the
- * vfsmounts from it are going away anyway */
- namespace = mnt->mnt_namespace;
- if (!namespace || atomic_read(&namespace->count) <= 0)
+ namespace = get_namespace(mnt->mnt_namespace);
+ if (!namespace)
continue;
- get_namespace(namespace);
spin_unlock(&vfsmount_lock);
down_write(&namespace->sem);
@@ -1073,8 +1073,10 @@ int copy_namespace(int flags, struct tas
get_namespace(namespace);
- if (!(flags & CLONE_NEWNS))
+ if (!(flags & CLONE_NEWNS)) {
+ atomic_inc(&namespace->task_count);
return 0;
+ }
if (!capable(CAP_SYS_ADMIN)) {
put_namespace(namespace);
@@ -1086,6 +1088,7 @@ int copy_namespace(int flags, struct tas
goto out;
atomic_set(&new_ns->count, 1);
+ atomic_set(&new_ns->task_count, 1);
init_rwsem(&new_ns->sem);
INIT_LIST_HEAD(&new_ns->list);
@@ -1109,7 +1112,9 @@ int copy_namespace(int flags, struct tas
p = namespace->root;
q = new_ns->root;
while (p) {
- q->mnt_namespace = new_ns;
+ struct namespace *oldns = q->mnt_namespace;
+ q->mnt_namespace = get_namespace(new_ns);
+ put_namespace(oldns);
if (fs) {
if (p == fs->rootmnt) {
rootmnt = p;
@@ -1381,16 +1386,17 @@ static void __init init_mount_tree(void)
if (!namespace)
panic("Can't allocate initial namespace");
atomic_set(&namespace->count, 1);
+ atomic_set(&namespace->task_count, 0);
INIT_LIST_HEAD(&namespace->list);
init_rwsem(&namespace->sem);
list_add(&mnt->mnt_list, &namespace->list);
namespace->root = mnt;
- mnt->mnt_namespace = namespace;
+ mnt->mnt_namespace = get_namespace(namespace);
init_task.namespace = namespace;
read_lock(&tasklist_lock);
do_each_thread(g, p) {
- get_namespace(namespace);
+ atomic_inc(&namespace->task_count);
p->namespace = namespace;
} while_each_thread(g, p);
read_unlock(&tasklist_lock);
@@ -1448,12 +1454,13 @@ void __init mnt_init(unsigned long mempa
init_mount_tree();
}
-void __put_namespace(struct namespace *namespace)
+void __exit_namespace(struct namespace *namespace)
{
down_write(&namespace->sem);
spin_lock(&vfsmount_lock);
umount_tree(namespace->root);
+ namespace->root = NULL;
spin_unlock(&vfsmount_lock);
up_write(&namespace->sem);
- kfree(namespace);
+ put_namespace(namespace);
}
Index: linux/fs/super.c
===================================================================
--- linux.orig/fs/super.c 2005-05-17 13:46:56.000000000 +0200
+++ linux/fs/super.c 2005-05-18 18:44:18.000000000 +0200
@@ -37,6 +37,7 @@
#include <linux/writeback.h> /* for the emergency remount stuff */
#include <linux/idr.h>
#include <linux/kobject.h>
+#include <linux/namespace.h>
#include <asm/uaccess.h>
@@ -842,7 +843,7 @@ do_kern_mount(const char *fstype, int fl
mnt->mnt_root = dget(sb->s_root);
mnt->mnt_mountpoint = sb->s_root;
mnt->mnt_parent = mnt;
- mnt->mnt_namespace = current->namespace;
+ mnt->mnt_namespace = get_namespace(current->namespace);
up_write(&sb->s_umount);
put_filesystem(type);
return mnt;
-
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