[PATCH 1/2] introduce for_each_thread() to replace the buggy while_each_thread()

[Oleg Nesterov <oleg@redhat.com>]

while_each_thread() and next_thread() should die, almost every
lockless usage is wrong.

1. Unless g == current, the lockless while_each_thread() is not safe.

   while_each_thread(g, t) can loop forever if g exits, next_thread()
   can't reach the unhashed thread in this case. Note that this can
   happen even if g is the group leader, it can exec.

2. Even if while_each_thread() itself was correct, people often use
   it wrongly.

   It was never safe to just take rcu_read_lock() and loop unless
   you verify that pid_alive(g) == T, even the first next_thread()
   can point to the already freed/reused memory.

This patch adds signal_struct->thread_head and task->thread_node
to create the normal rcu-safe list with the stable head. The new
for_each_thread(g, t) helper is always safe under rcu_read_lock()
as long as this task_struct can't go away.

Note: of course it is ugly to have both task_struct->thread_node
and the old task_struct->thread_group, we will kill it later, after
we change the users of while_each_thread() to use for_each_thread().

Perhaps we can kill it even before we convert all users, we can
reimplement next_thread(t) using the new thread_head/thread_node.
But we can't do this right now because this will lead to subtle
behavioural changes. For example, do/while_each_thread() always
sees at least one task, while for_each_thread() can do nothing if
the whole thread group has died. Or thread_group_empty(), currently
its semantics is not clear unless thread_group_leader(p) and we
need to audit the callers before we can change it.

So this patch adds the new interface which has to coexist with the
old one for some time, hopefully the next changes will be more or
less straightforward and the old one will go away soon.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 include/linux/init_task.h |    2 ++
 include/linux/sched.h     |   12 ++++++++++++
 kernel/exit.c             |    1 +
 kernel/fork.c             |    7 +++++++
 4 files changed, 22 insertions(+), 0 deletions(-)

diff --git a/include/linux/init_task.h b/include/linux/init_task.h
index b0ed422..668aae0 100644
--- a/include/linux/init_task.h
+++ b/include/linux/init_task.h
@@ -40,6 +40,7 @@ extern struct fs_struct init_fs;
 
 #define INIT_SIGNALS(sig) {						\
 	.nr_threads	= 1,						\
+	.thread_head	= LIST_HEAD_INIT(init_task.thread_node),	\
 	.wait_chldexit	= __WAIT_QUEUE_HEAD_INITIALIZER(sig.wait_chldexit),\
 	.shared_pending	= { 						\
 		.list = LIST_HEAD_INIT(sig.shared_pending.list),	\
@@ -213,6 +214,7 @@ extern struct task_group root_task_group;
 		[PIDTYPE_SID]  = INIT_PID_LINK(PIDTYPE_SID),		\
 	},								\
 	.thread_group	= LIST_HEAD_INIT(tsk.thread_group),		\
+	.thread_node	= LIST_HEAD_INIT(init_signals.thread_head),	\
 	INIT_IDS							\
 	INIT_PERF_EVENTS(tsk)						\
 	INIT_TRACE_IRQFLAGS						\
diff --git a/include/linux/sched.h b/include/linux/sched.h
index dc48056..0550083 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -487,6 +487,7 @@ struct signal_struct {
 	atomic_t		sigcnt;
 	atomic_t		live;
 	int			nr_threads;
+	struct list_head	thread_head;
 
 	wait_queue_head_t	wait_chldexit;	/* for wait4() */
 
@@ -1162,6 +1163,7 @@ struct task_struct {
 	/* PID/PID hash table linkage. */
 	struct pid_link pids[PIDTYPE_MAX];
 	struct list_head thread_group;
+	struct list_head thread_node;
 
 	struct completion *vfork_done;		/* for vfork() */
 	int __user *set_child_tid;		/* CLONE_CHILD_SETTID */
@@ -2225,6 +2227,16 @@ extern bool current_is_single_threaded(void);
 #define while_each_thread(g, t) \
 	while ((t = next_thread(t)) != g)
 
+#define __for_each_thread(signal, t)	\
+	list_for_each_entry_rcu(t, &(signal)->thread_head, thread_node)
+
+#define for_each_thread(p, t)		\
+	__for_each_thread((p)->signal, t)
+
+/* Careful: this is a double loop, 'break' won't work as expected. */
+#define for_each_process_thread(p, t)	\
+	for_each_process(p) for_each_thread(p, t)
+
 static inline int get_nr_threads(struct task_struct *tsk)
 {
 	return tsk->signal->nr_threads;
diff --git a/kernel/exit.c b/kernel/exit.c
index a949819..1e77fc6 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -74,6 +74,7 @@ static void __unhash_process(struct task_struct *p, bool group_dead)
 		__this_cpu_dec(process_counts);
 	}
 	list_del_rcu(&p->thread_group);
+	list_del_rcu(&p->thread_node);
 }
 
 /*
diff --git a/kernel/fork.c b/kernel/fork.c
index 9be5154..b84bef7 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1034,6 +1034,11 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
 	sig->nr_threads = 1;
 	atomic_set(&sig->live, 1);
 	atomic_set(&sig->sigcnt, 1);
+
+	/* list_add(thread_node, thread_head) without INIT_LIST_HEAD() */
+	sig->thread_head = (struct list_head)LIST_HEAD_INIT(tsk->thread_node);
+	tsk->thread_node = (struct list_head)LIST_HEAD_INIT(sig->thread_head);
+
 	init_waitqueue_head(&sig->wait_chldexit);
 	sig->curr_target = tsk;
 	init_sigpending(&sig->shared_pending);
@@ -1470,6 +1475,8 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 			atomic_inc(&current->signal->sigcnt);
 			list_add_tail_rcu(&p->thread_group,
 					  &p->group_leader->thread_group);
+			list_add_tail_rcu(&p->thread_node,
+					  &p->signal->thread_head);
 		}
 		attach_pid(p, PIDTYPE_PID);
 		nr_threads++;
-- 
1.5.5.1