[PATCH] fix tasklist + find_pid() with CONFIG_PREEMPT_RCU

[Oleg Nesterov <oleg@tv-sign.ru>]

With CONFIG_PREEMPT_RCU read_lock(tasklist_lock) doesn't imply rcu_read_lock(),
but find_pid_ns()->hlist_for_each_entry_rcu() should be safe under tasklist.

Usually it is, detach_pid() is always called under write_lock(tasklist_lock),
but copy_process() calls free_pid() lockless.

"#ifdef CONFIG_PREEMPT_RCU" is added mostly as documentation, perhaps it is
too ugly and should be removed.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

--- MM/kernel/fork.c~PR_RCU	2008-01-27 17:09:47.000000000 +0300
+++ MM/kernel/fork.c	2008-01-29 19:23:44.000000000 +0300
@@ -1335,8 +1335,19 @@ static struct task_struct *copy_process(
 	return p;
 
 bad_fork_free_pid:
-	if (pid != &init_struct_pid)
+	if (pid != &init_struct_pid) {
+#ifdef CONFIG_PREEMPT_RCU
+		/*
+		 * read_lock(tasklist_lock) doesn't imply rcu_read_lock(),
+		 * make sure find_pid() is safe under read_lock(tasklist).
+		 */
+		write_lock_irq(&tasklist_lock);
+#endif
 		free_pid(pid);
+#ifdef CONFIG_PREEMPT_RCU
+		write_unlock_irq(&tasklist_lock);
+#endif
+	}
 bad_fork_cleanup_namespaces:
 	exit_task_namespaces(p);
 bad_fork_cleanup_keys: