Re: [PATCH] posix-cpu-timers: rcu_read_lock/unlock protect find_task_by_vpid call

[Oleg Nesterov <oleg@redhat.com>]

> On Tue, 2 Nov 2010, Sergey Senozhatsky wrote:
>
> > On (11/02/10 16:31), Thomas Gleixner wrote:
> > > On Tue, 2 Nov 2010, Sergey Senozhatsky wrote:
> > >
> > > > Commit 4221a9918e38b7494cee341dda7b7b4bb8c04bde "Add RCU check for
> > > > find_task_by_vpid()" introduced rcu_lockdep_assert to find_task_by_pid_ns.
> > > > Add rcu_read_lock/rcu_read_unlock to call find_task_by_vpid.
> > > >
> > > > Tetsuo Handa wrote:
> > > >
> > > > Quoting from one of posts in that thead
> > > > http://kerneltrap.org/mailarchive/linux-kernel/2010/2/8/4536388
> > > >
> > > > | Usually tasklist gives enough protection, but if copy_process() fails
> > > > | it calls free_pid() lockless and does call_rcu(delayed_put_pid().
> > > > | This means, without rcu lock find_pid_ns() can't scan the hash table
> > > > | safely.
> > >
> > > We can remove the tasklist_lock while at it. rcu_read_lock is enough.
> > >

Yes, I believe posix-cpu-timers.c shouldn't use tasklist at all,
but it is not trivial to change this code.

Minor nit,

> > @@ -390,7 +390,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer)
> >
> >  	INIT_LIST_HEAD(&new_timer->it.cpu.entry);
> >
> > -	read_lock(&tasklist_lock);
> > +	rcu_read_lock();
> >  	if (CPUCLOCK_PERTHREAD(new_timer->it_clock)) {
> >  		if (pid == 0) {
> >  			p = current;
> > @@ -414,7 +414,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer)
> >  	} else {
> >  		ret = -EINVAL;
> >  	}
> > -	read_unlock(&tasklist_lock);
> > +	rcu_read_unlock();

I think this change is fine, but please note that thread_group_leader()
check is not relaible without tasklist. If we race with de_thread()
find_task_by_vpid() can find the new leader before it updates its
->group_leader. IOW, posix_cpu_timer_create() can fail when it shouldn't.

Not that I think this really matters, posix_cpu_timer_create() has
other problems with de_thread(). But perhaps it makes sense to
change posix_cpu_timer_create() to use has_group_leader_pid() instead,
just to make this code not look racy and avoid adding new problems.

The real fix, I think, should change cpu_timer_list to use
struct pid* instead of task_struct.

Oleg.