[Devel] Re: [ckrm-tech] [RFC][PATCH][2/4] Add RSS accounting and control

[Andrew Morton]

On Mon, 19 Feb 2007 16:39:33 +0530 Balbir Singh <balbir at in.ibm.com> wrote:

> Andrew Morton wrote:
> > On Mon, 19 Feb 2007 16:07:44 +0530 Balbir Singh <balbir at in.ibm.com> wrote:
> > 
> >>>> +void memctlr_mm_free(struct mm_struct *mm)
> >>>> +{
> >>>> +	kfree(mm->counter);
> >>>> +}
> >>>> +
> >>>> +static inline void memctlr_mm_assign_container_direct(struct mm_struct *mm,
> >>>> +							struct container *cont)
> >>>> +{
> >>>> +	write_lock(&mm->container_lock);
> >>>> +	mm->container = cont;
> >>>> +	write_unlock(&mm->container_lock);
> >>>> +}
> >>> More weird locking here.
> >>>
> >> The container field of the mm_struct is protected by a read write spin lock.
> > 
> > That doesn't mean anything to me.
> > 
> > What would go wrong if the above locking was simply removed?  And how does
> > the locking prevent that fault?
> > 
> 
> Some pages could charged to the wrong container. Apart from that I do not
> see anything going bad (I'll double check that).

Argh.  Please, think about this.

That locking *doesn't do anything*.  Except for that one situation I
described: some other holder of the lock reads mm->container twice inside
the lock and requires that the value be the same both times (and that sort
of code should be converted to take a local copy, so this locking here can
be removed).

> >>>> +
> >>>> +	read_lock(&mm->container_lock);
> >>>> +	cont = mm->container;
> >>>> +	read_unlock(&mm->container_lock);
> >>>> +
> >>>> +	if (!cont)
> >>>> +		goto done;
> >>> And here.  I mean, if there was a reason for taking the lock around that
> >>> read, then testing `cont' outside the lock just invalidated that reason.
> >>>
> >> We took a consistent snapshot of cont. It cannot change outside the lock,
> >> we check the value outside. I am sure I missed something.
> > 
> > If it cannot change outside the lock then we don't need to take the lock!
> > 
> 
> We took a snapshot that we thought was consistent.

Consistent with what?  That's a single-word read inside that lock.

> We check for the value
> outside. I guess there is no harm, the worst thing that could happen
> is wrong accounting during mm->container changes (when a task changes
> container).

If container->lock is held when a task is removed from the
container then yes, `cont' here can refer to a container to which the task
no longer belongs.

More worrisome is the potential for use-after-free.  What prevents the
pointer at mm->container from referring to freed memory after we're dropped
the lock?