Re: [PATCH] Update comment on find_task_by_pid_ns

Thomas Gleixner
2010-02-08T17:08:28Z

On Mon, 8 Feb 2010, Oleg Nesterov wrote:

> 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.

We try to get rid of the read_lock sites of tasklist_lock, so please
let's not think about adding more :)

Thanks,

tglx