Re: [PATCH] fs, proc: Introduce the /proc/<pid>/children entry v2

[Cyrill Gorcunov]

On Wed, Dec 07, 2011 at 11:03:40PM +0400, Cyrill Gorcunov wrote:
> On Wed, Dec 07, 2011 at 07:53:43PM +0100, Oleg Nesterov wrote:
> > Hi Cyrill,
> > 
> > Sorry, I didn't read this patch yet, but
> > 
> > On 12/06, Cyrill Gorcunov wrote:
> > >
> > > +static void *children_seq_start(struct seq_file *seq, loff_t *pos)
> > > +{
> > > +	struct task_struct *task;
> > > +
> > > +	rcu_read_lock();
> > > +	task = seq->private;
> > > +	if (task)
> > > +		return seq_list_start(&task->children, *pos);
> > 
> > This looks "obviously wrong".
> > 
> > We can not trust ->children->next after rcu_read_unlock(). Another
> > rcu_read_lock() can't help.
> > 
> > Once again, I can be easily wrong, need to read the patch first.
> > 
> 
> Wait, Oleg, I might be wrong as well, but it's now as
> 
> children_seq_open
>  get_proc_task (so ref to task increased)
> 
> the children_seq_start/children_seq_stop works
> in iteration and every new iteration seq_list_next
> walks over the whole children list from the list
> head under rcu lock, so even if task is removed
> or added the link should exsist until rcu is unlocked
> and sync'ed no?
> 

On the other hands some if (task) tests are redundant
and might be dropped since we have a reference to a
task until seq-file is not released. I'll update it
and shrink a patch some more.

	Cyrill
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