Re: [PATCH] Update comment on find_task_by_pid_ns

Oleg Nesterov
2010-02-08T13:21:57Z

On 02/08, Tetsuo Handa wrote:
>
> [PATCH] Update comment on find_task_by_pid_ns
>
> Caller of find_task_by_vpid() and find_task_by_pid_ns() needs to call
> rcu_read_lock() rather than read_lock(&tasklist_lock) because find_pid_ns()
> uses RCU primitives but spinlock does not prevent RCU callback if preemptive
> RCU ( CONFIG_TREE_PREEMPT_RCU ) is enabled.

I agree with the patch, but the changelog looks a bit confusing to me.
Perhaps this is just me, though.

tasklist does protect the task and its pid, it can't go away. The problem
is that find_pid_ns() itself is unsafe without rcu lock, it can race with
copy_process()->free_pid(any_pid).

IOW, if we change copy_process()

--- kernel/fork.c
+++ kernel/fork.c
@@ -1304,8 +1304,11 @@ static struct task_struct *copy_process(
return p;

bad_fork_free_pid:
- if (pid != &init_struct_pid)
+ if (pid != &init_struct_pid) {
+ read_lock(&tasklist_lock);
free_pid(pid);
+ read_unlock(&tasklist_lock);
+ }
bad_fork_cleanup_io:
if (p->io_context)
exit_io_context(p);

then find_task_by_pid_ns/etc could be used under tasklist safely even
with PREEMPT_RCU.

In any case, I think the patch is nice.

Oleg.