[PATCH RFC 03/13] pid_ns: Implement rwlock_t pid_ns::cr_lock for locking child_reaper

[Kirill Tkhai <ktkhai@odin.com>]

Protects child_reaper modifitations.

Signed-off-by: Kirill Tkhai <ktkhai@odin.com>
---
 include/linux/pid_namespace.h |    1 +
 kernel/exit.c                 |   15 ++++++++++++---
 kernel/fork.c                 |    1 +
 kernel/pid.c                  |   10 +++++++++-
 kernel/pid_namespace.c        |    5 +++--
 5 files changed, 26 insertions(+), 6 deletions(-)

diff --git a/include/linux/pid_namespace.h b/include/linux/pid_namespace.h
index 918b117..3e59d2a 100644
--- a/include/linux/pid_namespace.h
+++ b/include/linux/pid_namespace.h
@@ -28,6 +28,7 @@ struct pid_namespace {
 	int last_pid;
 	unsigned int nr_hashed;
 	struct task_struct *child_reaper;
+	rwlock_t cr_lock;
 	struct kmem_cache *pid_cachep;
 	unsigned int level;
 	struct pid_namespace *parent;
diff --git a/kernel/exit.c b/kernel/exit.c
index a29c35d..a1b2bf7 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -455,16 +455,23 @@ static void check_pid_ns_reaper_exit(struct task_struct *father)
 	__acquires(&tasklist_lock)
 {
 	struct pid_namespace *pid_ns = task_active_pid_ns(father);
-	struct task_struct *reaper = pid_ns->child_reaper;
+	struct task_struct *reaper;
+
+	read_lock(&pid_ns->cr_lock);
+	reaper = pid_ns->child_reaper;
+	read_unlock(&pid_ns->cr_lock);
 
 	if (likely(reaper != father))
 		return;
 
+	write_lock(&pid_ns->cr_lock);
 	reaper = find_alive_thread(father);
-	if (reaper) {
+	if (reaper)
 		pid_ns->child_reaper = reaper;
+	write_unlock(&pid_ns->cr_lock);
+
+	if (reaper)
 		return;
-	}
 
 	write_unlock_irq(&tasklist_lock);
 	if (unlikely(pid_ns == &init_pid_ns)) {
@@ -560,6 +567,7 @@ static void forget_original_parent(struct task_struct *father,
 	if (list_empty(&father->children))
 		return;
 
+	read_lock(&task_active_pid_ns(father)->cr_lock);
 	reaper = find_new_reaper(father);
 	list_for_each_entry(p, &father->children, sibling) {
 		for_each_thread(p, t) {
@@ -579,6 +587,7 @@ static void forget_original_parent(struct task_struct *father,
 			reparent_leader(father, p, dead);
 	}
 	list_splice_tail_init(&father->children, &reaper->children);
+	read_unlock(&task_active_pid_ns(father)->cr_lock);
 }
 
 /*
diff --git a/kernel/fork.c b/kernel/fork.c
index 0bb88b5..66e31eb 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1563,6 +1563,7 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 			init_task_pid(p, PIDTYPE_SID, task_session(current));
 
 			if (is_child_reaper(pid)) {
+				/* Lockless, as we're the only process in ns */
 				ns_of_pid(pid)->child_reaper = p;
 				p->signal->flags |= SIGNAL_UNKILLABLE;
 			}
diff --git a/kernel/pid.c b/kernel/pid.c
index 4fd07d5..39a8b0a 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -78,6 +78,7 @@ struct pid_namespace init_pid_ns = {
 	.nr_hashed = PIDNS_HASH_ADDING,
 	.level = 0,
 	.child_reaper = &init_task,
+	.cr_lock = __RW_LOCK_UNLOCKED(&init_pid_ns.cr_lock),
 	.user_ns = &init_user_ns,
 	.ns.inum = PROC_PID_INIT_INO,
 #ifdef CONFIG_PID_NS
@@ -259,6 +260,7 @@ static void delayed_put_pid(struct rcu_head *rhp)
 void free_pid(struct pid *pid)
 {
 	/* We can be called with write_lock_irq(&tasklist_lock) held */
+	struct task_struct *child_reaper = NULL;
 	int i;
 	unsigned long flags;
 
@@ -274,7 +276,8 @@ void free_pid(struct pid *pid)
 			 * is the reaper wake up the reaper.  The reaper
 			 * may be sleeping in zap_pid_ns_processes().
 			 */
-			wake_up_process(ns->child_reaper);
+			child_reaper = ns->child_reaper;
+			get_task_struct(child_reaper);
 			break;
 		case PIDNS_HASH_ADDING:
 			/* Handle a fork failure of the first process */
@@ -288,6 +291,11 @@ void free_pid(struct pid *pid)
 	}
 	spin_unlock_irqrestore(&pidmap_lock, flags);
 
+	if (child_reaper) {
+		wake_up_process(child_reaper);
+		put_task_struct(child_reaper);
+	}
+
 	for (i = 0; i <= pid->level; i++)
 		free_pidmap(pid->numbers + i);
 
diff --git a/kernel/pid_namespace.c b/kernel/pid_namespace.c
index a65ba13..bbaa072 100644
--- a/kernel/pid_namespace.c
+++ b/kernel/pid_namespace.c
@@ -115,6 +115,7 @@ static struct pid_namespace *create_pid_namespace(struct user_namespace *user_ns
 	ns->parent = get_pid_ns(parent_pid_ns);
 	ns->user_ns = get_user_ns(user_ns);
 	ns->nr_hashed = PIDNS_HASH_ADDING;
+	rwlock_init(&ns->cr_lock);
 	INIT_WORK(&ns->proc_work, proc_cleanup_work);
 
 	set_bit(0, ns->pidmap[0].page);
@@ -324,9 +325,9 @@ int reboot_pid_ns(struct pid_namespace *pid_ns, int cmd)
 		return -EINVAL;
 	}
 
-	read_lock(&tasklist_lock);
+	read_lock(&pid_ns->cr_lock);
 	force_sig(SIGKILL, pid_ns->child_reaper);
-	read_unlock(&tasklist_lock);
+	read_unlock(&pid_ns->cr_lock);
 
 	do_exit(0);