Re: Q: cgroup: Questions about possible issues in cgroup locking

[Mandeep Singh Baines <msb@chromium.org>]

Hi Oleg,

Oleg Nesterov (oleg@redhat.com) wrote:
> On 01/06, Mandeep Singh Baines wrote:
> >
> > Oleg Nesterov (oleg@redhat.com) wrote:
> > >
> > > > > in particular, http://marc.info/?l=linux-kernel&m=127714242731448
> > > > > I think this should work, but then we should do something with the
> > > > > users like zap_threads().
> > > > >
> > > >
> > > > With that patch, won't you potentially miss the exec thread if an exec
> > > > occurs while you're iterating over the list? Is that OK?
> > >
> > > Of course it is not OK ;) Note the "we should do something with" above.
> > >
> >
> > So requirements should be something like this:
> 
> (I assume, you mean the lockless case)
> 

Correct.

> > * Any task alive for the duration of the iteration MUST be visited
> > * No task should be visited more than once
> > * Any task born or exiting after starting the iteration MAY be skipped
> > * You can start at any task in the thread group
> 
> Well yes, but it is not easy to exactly define what after/before
> means in this case.
> 
> > Would something like this work:
> >
> > #define while_each_thread(g, t, o) \
> > 	while (t->group_leader == o && (t = next_thread(t)) != g)
> >
> > Where o should have the value of g->group_leader.
> 
> I don't understand how this helps... and how this can work even
> ignoring the barriers.
> 
> OK, we have the main thream M and the sub-thread T, we are doing
> 
> 	do {
> 		do_something(t);
> 	} while_each_thread(M, t, M);
> 
> why we can't miss T if it does exec?
> 

So for:

struct task *M; /* assuming this is passed in to us */
struct task *L = M->group_leader;
struct task *I = M;

do {
	do_something(T);
} while_each_thread(M, T, L);

Here is my thinking.

If some thread K does exec, you won't miss it because:

1) Ignoring the group_leader check, you'll visit K just by following
   next_thread(). That's the case today and is what you except
   when iterating over an rcu_list.
2) (t->group_leader == o) will fail iff t is the exec thread.
   Since we test t->group_leader before re-assigning it (t=next_thread()),
   the test will fail only after visiting the exec thread. So you'll
   visit the exec thread and then terminate the loop.

I realize its a klutzy interface (requires 3 variables) but it seems
correct (ignoring barriers) and meets all the requirements. I'm hoping
it inspires a solution which is less klutzy and meet its all the
requirements.

Regards,
Mandeep

> Oleg.
>