Re: [PATCH] Introduce ActivePid: in /proc/self/status (v2, was Vpid:)

[Greg Kurz <gkurz@fr.ibm.com>]

On Wed, 2011-06-15 at 20:46 +0200, Oleg Nesterov wrote:
> On 06/15, Greg Kurz wrote:
> >
> > @@ -176,6 +177,17 @@ static inline void task_state(struct seq_file *m, struct pid_namespace *ns,
> >  		if (tracer)
> >  			tpid = task_pid_nr_ns(tracer, ns);
> >  	}
> > +	actpid = 0;
> > +	sighand = rcu_dereference(p->sighand);
> > +	if (sighand) {
> > +		struct pid_namespace *pid_ns;
> > +		unsigned long flags;
> > +		spin_lock_irqsave(&sighand->siglock, flags);
> 
> Well. This is not exactly right. We have lock_task_sighand() for this.
> 

I see... ->sighand could change so we need the for(;;) loop in
__lock_task_sighand() to be sure we have the right pointer, correct ?
By the way, if we use lock_task_sighand() we'll end up with nested
rcu_read_lock(): it will work but I don't know how it may affect
performance...

> But. Why do you need ->siglock? Why rcu_read_lock() is not enough?
> 

Because there's a race with
__exit_signal()->__unhash_process()->detach_pid() that can break
task_active_pid_ns() and rcu won't help here (unless *perhaps* by
modifying __exit_signal() but I don't want to mess with such a critical
path).

> Hmm. You don't even need pid_ns afaics, you could simply look at
> pid->numbers[pid->level].
> 

True but I will have the same problem: detach_pid() nullifies the pid.

Thanks for your comments.

--
Greg