On Thu, 2011-06-16 at 14:35 +0200, Louis Rilling wrote:
> On 16/06/11 13:01 +0200, Greg Kurz wrote:
> > 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...
>
> rcu_read_lock() is very cheap.
>
Fair enough. In this case, lock_task_sighand() would be the right choice
if locking is needed.
> >
> > > 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).
>
> In case of race, the only risk is that task_active_pid_ns() returns NULL.
> Otherwise, RCU guarantees that the pid_ns will stay alive (see below).
>
> >
> > > 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.
>
> But the pid won't be freed until an RCU grace period expires. See free_pid(). So
> the non-determinism here is when /proc/<pid>/status is read at the same as
> threaded execve() or task's exit(), in which case a stale pid (execve()) or
> no pid (exit after __unhash_process()) can be accessed. This does not look like
> a big deal...
>
Ok. You're right, the RCU grace period is just what I need to ensure I
won't dereference a stale pointer. So I don't even have to bother with
->siglock and just check pid_alive() before peeking into pid->numbers.
> Thanks,
>
> Louis
>
Thanks for your help.
--
Greg
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