[Devel] Re: [ckrm-tech] [PATCH 7/7] containers (V7): Container interface to nsproxy subsystem

[Srivatsa Vaddagiri]

On Wed, Apr 04, 2007 at 07:57:40PM -0700, Paul Menage wrote:
> >Firstly, this is not a unique problem introduced by using ->nsproxy.
> >Secondly we have discussed this to some extent before
> >(http://lkml.org/lkml/2007/2/13/122). Essentially if we see zero tasks
> >sharing a resource object pointed to by ->nsproxy, then we can't be
> >racing with a function like bc_file_charge(), which simplifies the
> >problem quite a bit. In other words, seeing zero tasks in xxx_rmdir()
> >after taking manage_mutex is permission to kill nsproxy and associated
> >objects. Correct me if I am wrong here.

Let me clarify first that I wasn't proposing an extra ref count in
nsproxy to account for non-task references to a resource object pointed
to by nsproxy (say nsproxy->ctlr_data[BC_ID]). Refcounts needed
on beancounter because a non-task object is pointing to it (like struct
file) will be put in the beancounter itself.

What I did want to say was this (sorry about the verbose rant):

	mount -t container -obeancounter none /dev/bean
	mkdir /dev/bean/foo
	echo some_pid > /dev/bean/foo

Associated with foo is a beancounter object A1 which contains (among other
things) max files that can be opened by tasks in foo. Also upon
successful file open, file->f_bc will point to A1.

Now lets say that someone is doing 

	rmdir /dev/bean/foo

while will lead us to xxx_rmdir() doing this:
	
	mutex_lock(&manage_mutex);

	count = rcfs_task_count(foo's dentry);

rcfs_task_count will essentially return number of tasks pointing to A1
thr' their nsproxy->ctlr_data[BC_ID].

IF (note that /if/ again) the count returned is zero, then my point was
we can destroy nsproxy behind foo and also B1, not worrying about a
'struct file' still pointing to B1. This stems from the fact that you
cannot have a task's file->f_bc pointing to B1 w/o the task itself
pointing to B1 also (task->nsproxy->ctlr_data[BC_ID] == B1). I also
assume f_bc will get migrated with its owner task across beancounters
(which seems reasonable to me atleast from 'struct file' context).

If there was indeed a file object still pointing to B1, then that can
only be true if rcfs_task_count() returns non-zero value. Correct?

This is what I had in mind when I said this above : "In other words, seeing 
zero tasks in xxx_rmdir() after taking manage_mutex is permission to kill 
nsproxy and associated objects".

OT : In your posting of beancounter patches on top of containers, f_bc
isnt being migrated upon task movements. Is that on intention?

> OK, I've managed to reconstruct my reasoning  remembered why it's
> important to have the refcounts associated with the subsystems, and
> why the simple use of the nsproxy count doesn't work.

I didn't mean to have non-task objects add refcounts to nsproxy. See
above.

> 1) Assume the system has a single task T, and two subsystems, A and B
> 
> 2) Mount hierarchy H1, with subsystem A and root subsystem state A0,
> and hierarchy H2 with subsystem B and root subsystem state B0. Both
> H1/ and H2/ share a single nsproxy N0, with refcount 3 (including the
> reference from T), pointing at A0 and B0.

Why refcount 3? I can only be 1 (from T) ..

> 3) Create directory H1/foo, which creates subsystem state A1 (nsproxy
> N1, refcount 1, pointing at A1 and B0)

right. At this point A1.count should be 1 (because N1 is pointing to it)

> 4) Create directory H2/bar, which creates subsystem state B1 (nsproxy
> N2, refcount 1, pointing at A0 and B1)

right. B1.count = 1 also.

> 5) Move T into H1/foo/tasks and then H2/bar/tasks. It ends up with
> nsproxy N3, refcount 1, pointing at A1 and B1.

right. A1.count = 2 (N1, N3) and B1.count = 2 (N2, N3)

> 6) T creates an object that is charged to A1 and hence needs to take a
> reference on A1 in order to uncharge it later when it's released. So
> N3 now has a refcount of 2

no ..N3 can continue to have 1 while A1.count becomes 3 (N1, N3 and
file->f_bc)

> 7) Move T back to H1/tasks and H2/tasks; assume it picks up nsproxy N0
> again; N3 has a refcount of 1 now. (Assume that the object created in
> step 6 isn't one that's practical/desirable to relocate when the task
> that created it moves to a different container)

The object was created by the task, so I would expect it should get
migrated too to the new task's context (which should be true in case of
f_bc atleast?). Can you give a practical example where you want to
migrate the task and not the object it created?

Anyway, coming down to the impact of all this for a nsproxy based
solution, I would imagine this is what will happen when T moves back to
H1/tasks and H2/tasks:

	- N3.count becomes zero
	- We invoke free_nsproxy(N3), which drops refcounts on
	  all objects it is pointing to i.e
		
	free_nsproxy()
	{
		if (N3->mnt_ns)
			put_mnt_ns(N3->mnt_ns);
		...
		if (N3->ctlr_data[BC_ID])
			put_bc(N3->ctlr_data[BC_ID]);
	}

put/get_bc() manages refcounts on beancounters. It will drop A1.count to 2
(if f_bc wasnt migrated) and not finding it zero will not destroy A1.

Essentially, in the nsproxy based approach, I am having individual
controllers maintain their own refcount mechanism (just like mnt_ns or
uts_ns are doing today).

> In this particular case the extra refcount on N3 is intended to keep
> A1 alive (which prevents H1/foo being deleted), but there's no way to
> tell from the structures in use whether it was taken on A1 or on B1.
> Neither H1/foo nor H2/bar can be deleted, even though nothing is
> intending to have a reference count on H2/bar.
> 
> Putting the extra refcount explicitly either in A1, or else in a
> container object associated with H1/foo makes this more obvious.

Hope the above description resolves these points ..


-- 
Regards,
vatsa