[PATCH] task: Make task list manipulations RCU safe

While we can currently walk through thread groups, process groups, and
sessions with just the rcu_read_lock, this opens the door to walking the
entire task list.

We already have all of the other RCU guarantees so there is no cost in
doing this, this should be enough so that proc can stop taking the
tasklist lock during readdir.

prev_task was killed because it has no users, and using it will miss new
tasks when doing an rcu traversal.
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/exec.c

@@ -712,7 +712,7 @@ static int de_thread(struct task_struct *tsk)
 		attach_pid(current, PIDTYPE_PID,  current->pid);
 		attach_pid(current, PIDTYPE_PGID, current->signal->pgrp);
 		attach_pid(current, PIDTYPE_SID,  current->signal->session);
-		list_add_tail(&current->tasks, &init_task.tasks);
+		list_add_tail_rcu(&current->tasks, &init_task.tasks);
 
 		current->group_leader = current;
 		leader->group_leader = current;

include/linux/sched.h

@@ -1192,8 +1192,7 @@ extern void wait_task_inactive(task_t * p);
 #define remove_parent(p)	list_del_init(&(p)->sibling)
 #define add_parent(p)		list_add_tail(&(p)->sibling,&(p)->parent->children)
 
-#define next_task(p)	list_entry((p)->tasks.next, struct task_struct, tasks)
-#define prev_task(p)	list_entry((p)->tasks.prev, struct task_struct, tasks)
+#define next_task(p)	list_entry(rcu_dereference((p)->tasks.next), struct task_struct, tasks)
 
 #define for_each_process(p) \
 	for (p = &init_task ; (p = next_task(p)) != &init_task ; )

kernel/exit.c

@@ -56,7 +56,7 @@ static void __unhash_process(struct task_struct *p)
 		detach_pid(p, PIDTYPE_PGID);
 		detach_pid(p, PIDTYPE_SID);
 
-		list_del_init(&p->tasks);
+		list_del_rcu(&p->tasks);
 		__get_cpu_var(process_counts)--;
 	}
 	list_del_rcu(&p->thread_group);

kernel/fork.c

@@ -1204,7 +1204,7 @@ static task_t *copy_process(unsigned long clone_flags,
 			attach_pid(p, PIDTYPE_PGID, process_group(p));
 			attach_pid(p, PIDTYPE_SID, p->signal->session);
 
-			list_add_tail(&p->tasks, &init_task.tasks);
+			list_add_tail_rcu(&p->tasks, &init_task.tasks);
 			__get_cpu_var(process_counts)++;
 		}
 		attach_pid(p, PIDTYPE_PID, p->pid);