Re: [PATCH] Update comment on find_task_by_pid_ns

[Thomas Gleixner]

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
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