commit 5e85d4abe3f43bb5362f384bab0e20ef082ce0b5
author Eric W. Biederman <ebiederm@xmission.com> Tue Apr 18 22:20:16 2006 -0700
committer Linus Torvalds <torvalds@g5.osdl.org> Wed Apr 19 09:13:49 2006 -0700
tree cd3a29086e5274fd08bc8d22d15568deab144755
parent 181ae4005d0a4010802be534d929b38c42b9ac06

[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

diff --git a/fs/exec.c b/fs/exec.c
index 4121bb5..3a79d97 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -712,7 +712,7 @@
 		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

diff --git a/include/linux/sched.h b/include/linux/sched.h
index b7d31e2..29b7d4f 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1192,8 +1192,7 @@
 #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

diff --git a/kernel/exit.c b/kernel/exit.c
index 1a9787a..f86434d 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -56,7 +56,7 @@
 		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

diff --git a/kernel/fork.c b/kernel/fork.c
index 54b15f8..3451577 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1204,7 +1204,7 @@
 			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);