[PATCH] Make access to task's nsproxy liter

[Pavel Emelyanov <xemul@openvz.org>]

When someone wants to deal with some other taks's namespaces
it has to lock the task and then to get the desired namespace
if the one exists. This is slow on read-only paths and may be
impossible in some cases.

E.g. Oleg recently noticed a race between unshare() and the
(sent for review in containers) pid namespaces - when the task 
notifies the parent it has to know the parent's namespace, but 
taking the task_lock() is impossible there - the code is under 
write locked tasklist lock.

On the other hand switching the namespace on task (daemonize) 
and releasing the namespace (after the last task exit) is rather
rare operation and we can sacrifice its speed to solve the
issues above.

The access to other task namespaces is proposed to be performed
like this:

     rcu_read_lock();
     nsproxy = task_nsproxy(tsk);
     if (nsproxy != NULL) {
             / *
               * work with the namespaces here
               * e.g. get the reference on one of them
               * /
     } / *
         * NULL task_nsproxy() means that this task is
         * almost dead (zombie)
         * /
     rcu_read_unlock();

This patch has passed the review by Eric and Oleg :) and,
of course, tested.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Serge Hallyn <serue@us.ibm.com>

---

 fs/proc/base.c          |   27 +++++++++++++++++----------
 include/linux/nsproxy.h |   44 ++++++++++++++++++++++++++++++++------------
 kernel/exit.c           |    4 +---
 kernel/fork.c           |   11 +++++------
 kernel/nsproxy.c        |   39 +++++++++++++++++++++++++++++++--------
 5 files changed, 86 insertions(+), 39 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index ed2b224..614851a 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -371,18 +371,21 @@ struct proc_mounts {
 static int mounts_open(struct inode *inode, struct file *file)
 {
 	struct task_struct *task = get_proc_task(inode);
+	struct nsproxy *nsp;
 	struct mnt_namespace *ns = NULL;
 	struct proc_mounts *p;
 	int ret = -EINVAL;
 
 	if (task) {
-		task_lock(task);
-		if (task->nsproxy) {
-			ns = task->nsproxy->mnt_ns;
+		rcu_read_lock();
+		nsp = task_nsproxy(task);
+		if (nsp) {
+			ns = nsp->mnt_ns;
 			if (ns)
 				get_mnt_ns(ns);
 		}
-		task_unlock(task);
+		rcu_read_unlock();
+
 		put_task_struct(task);
 	}
 
@@ -445,16 +448,20 @@ static int mountstats_open(struct inode 
 
 	if (!ret) {
 		struct seq_file *m = file->private_data;
+		struct nsproxy *nsp;
 		struct mnt_namespace *mnt_ns = NULL;
 		struct task_struct *task = get_proc_task(inode);
 
 		if (task) {
-			task_lock(task);
-			if (task->nsproxy)
-				mnt_ns = task->nsproxy->mnt_ns;
-			if (mnt_ns)
-				get_mnt_ns(mnt_ns);
-			task_unlock(task);
+			rcu_read_lock();
+			nsp = task_nsproxy(task);
+			if (nsp) {
+				mnt_ns = nsp->mnt_ns;
+				if (mnt_ns)
+					get_mnt_ns(mnt_ns);
+			}
+			rcu_read_unlock();
+
 			put_task_struct(task);
 		}
 
diff --git a/include/linux/nsproxy.h b/include/linux/nsproxy.h
index 525d8fc..ef4e863 100644
--- a/include/linux/nsproxy.h
+++ b/include/linux/nsproxy.h
@@ -32,8 +32,39 @@ struct nsproxy {
 };
 extern struct nsproxy init_nsproxy;
 
+/*
+ * the namespaces access rules are:
+ *
+ *  1. only current task is allowed to change tsk->nsproxy pointer or
+ *     any pointer on the nsproxy itself
+ *
+ *  2. when accessing (i.e. reading) current task's namespaces - no
+ *     precautions should be taken - just dereference the pointers
+ *
+ *  3. the access to other task namespaces is performed like this
+ *     rcu_read_lock();
+ *     nsproxy = task_nsproxy(tsk);
+ *     if (nsproxy != NULL) {
+ *             / *
+ *               * work with the namespaces here
+ *               * e.g. get the reference on one of them
+ *               * /
+ *     } / *
+ *         * NULL task_nsproxy() means that this task is
+ *         * almost dead (zombie)
+ *         * /
+ *     rcu_read_unlock();
+ *
+ */
+
+static inline struct nsproxy *task_nsproxy(struct task_struct *tsk)
+{
+	return rcu_dereference(tsk->nsproxy);
+}
+
 int copy_namespaces(unsigned long flags, struct task_struct *tsk);
-void get_task_namespaces(struct task_struct *tsk);
+void exit_task_namespaces(struct task_struct *tsk);
+void switch_task_namespaces(struct task_struct *tsk, struct nsproxy *new);
 void free_nsproxy(struct nsproxy *ns);
 int unshare_nsproxy_namespaces(unsigned long, struct nsproxy **,
 	struct fs_struct *);
@@ -45,17 +76,6 @@ static inline void put_nsproxy(struct ns
 	}
 }
 
-static inline void exit_task_namespaces(struct task_struct *p)
-{
-	struct nsproxy *ns = p->nsproxy;
-	if (ns) {
-		task_lock(p);
-		p->nsproxy = NULL;
-		task_unlock(p);
-		put_nsproxy(ns);
-	}
-}
-
 #ifdef CONFIG_CONTAINER_NS
 int ns_container_clone(struct task_struct *tsk);
 #else
diff --git a/kernel/exit.c b/kernel/exit.c
index 2f4f35e..a2aef1f 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -409,9 +409,7 @@ void daemonize(const char *name, ...)
 	current->fs = fs;
 	atomic_inc(&fs->count);
 
-	exit_task_namespaces(current);
-	current->nsproxy = init_task.nsproxy;
-	get_task_namespaces(current);
+	switch_task_namespaces(current, init_task.nsproxy);
 
 	exit_files(current);
 	current->files = init_task.files;
diff --git a/kernel/fork.c b/kernel/fork.c
index bc48e0f..0c6dd67 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1618,7 +1618,7 @@ asmlinkage long sys_unshare(unsigned lon
 	struct mm_struct *mm, *new_mm = NULL, *active_mm = NULL;
 	struct files_struct *fd, *new_fd = NULL;
 	struct sem_undo_list *new_ulist = NULL;
-	struct nsproxy *new_nsproxy = NULL, *old_nsproxy = NULL;
+	struct nsproxy *new_nsproxy = NULL;
 
 	check_unshare_flags(&unshare_flags);
 
@@ -1647,14 +1647,13 @@ asmlinkage long sys_unshare(unsigned lon
 
 	if (new_fs ||  new_mm || new_fd || new_ulist || new_nsproxy) {
 
-		task_lock(current);
-
 		if (new_nsproxy) {
-			old_nsproxy = current->nsproxy;
-			current->nsproxy = new_nsproxy;
-			new_nsproxy = old_nsproxy;
+			switch_task_namespaces(current, new_nsproxy);
+			new_nsproxy = NULL;
 		}
 
+		task_lock(current);
+
 		if (new_fs) {
 			fs = current->fs;
 			current->fs = new_fs;
diff --git a/kernel/nsproxy.c b/kernel/nsproxy.c
index 58843ae..48bc070 100644
--- a/kernel/nsproxy.c
+++ b/kernel/nsproxy.c
@@ -30,14 +30,6 @@ static inline void get_nsproxy(struct ns
 	atomic_inc(&ns->count);
 }
 
-void get_task_namespaces(struct task_struct *tsk)
-{
-	struct nsproxy *ns = tsk->nsproxy;
-	if (ns) {
-		get_nsproxy(ns);
-	}
-}
-
 /*
  * creates a copy of "orig" with refcount 1.
  */
@@ -205,6 +197,37 @@ out:
 	return err;
 }
 
+void switch_task_namespaces(struct task_struct *p, struct nsproxy *new)
+{
+	struct nsproxy *ns;
+
+	might_sleep();
+
+	ns = p->nsproxy;
+	if (ns == new)
+		return;
+
+	if (new)
+		get_nsproxy(new);
+	rcu_assign_pointer(p->nsproxy, new);
+
+	if (ns && atomic_dec_and_test(&ns->count)) {
+		/*
+		 * wait for others to get what they want from this nsproxy.
+		 *
+		 * cannot release this nsproxy via the call_rcu() since
+		 * put_mnt_ns() will want to sleep
+		 */
+		synchronize_rcu();
+		free_nsproxy(ns);
+	}
+}
+
+void exit_task_namespaces(struct task_struct *p)
+{
+	switch_task_namespaces(p, NULL);
+}
+
 static int __init nsproxy_cache_init(void)
 {
 	nsproxy_cachep = kmem_cache_create("nsproxy", sizeof(struct nsproxy),