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    SubjectRe: [PATCH] Introduce ActivePid: in /proc/self/status (v2, was Vpid:)
    On 16/06/11 15:00 +0200, Greg Kurz wrote:
    > 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.

    It ends like open-coding an optimized version of task_pid_vnr(). If the
    optimization is really important (I guess this depends on the depth of recursive
    pid namespaces), it would be better to re-write task_pid_vnr(). Otherwise, just
    use task_pid_vnr() as it is.

    Thanks,

    Louis

    >
    > > Thanks,
    > >
    > > Louis
    > >
    >
    > Thanks for your help.
    >
    > --
    > Greg
    >
    >
    > --
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    --
    Dr Louis Rilling Kerlabs
    Skype: louis.rilling Batiment Germanium
    Phone: (+33|0) 6 80 89 08 23 80 avenue des Buttes de Coesmes
    http://www.kerlabs.com/ 35700 Rennes
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