Re: [PATCH 1/5] sched: Convert thread_group_cputime() to use for_each_thread()

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Apr 09, 2014 at 07:32:34PM +0200, Oleg Nesterov wrote:
> On 04/09, Peter Zijlstra wrote:
> >
> > On Wed, Apr 09, 2014 at 06:11:18PM +0200, Frederic Weisbecker wrote:
> > > do_each_thread/while_each_thread iterators are deprecated by
> > > for_each_thread/for_each_process_thread() APIs.
> > >
> > > Lets convert the callers in the cputime code accounting. The ultimate
> > > goal is to remove the struct task_struct::thread_group field and
> > > the corresponding do_each_thread/while_each_thread iterators that are
> > > RCU unsafe.
> > >
> > > It also makes thread_group_cputime() eventually RCU-safe.
> >
> > this fails to explain how the current code is broken.
> 
> while_each_thread(g, t) will loop forever if g exits and removes itself
> from ->thread_group. This can happen even if it is the group leader,
> de_thread() can do this.
> 
> Another problem is that it is used wrongly very often, even if
> while_each_thread() was fine people forget to check pid_alive() to ensure
> it didn't exit even before we take rcu_read_lock().
> 
> for_each_thread(p, t) is always safe. Unless p's task_struct can't go away,
> of course.
> 
> But there is a difference. Ignoring the bug above
> 
> 	p = g;
> 	do {
> 		printk("pid=%d\n", p->pid);
> 	} while_each_thread(g, p);
> 
> always prints at least one pid.
> 
> 	for_each_thread(g, p)
> 		printk("pid=%d\n", p->pid);
> 
> can print nothing if g has already exited.

Cute,.. and all that lives in sched.h and I wasn't even Cc'ed :-(