Re: [PATCH] mm: fix hang on anon_vma->root->lock

[Hugh Dickins <hughd@google.com>]

On Fri, Aug 27, 2010 at 10:13 AM, Christoph Lameter <cl@linux.com> wrote:

> The basic problem with SLAB_DESTROY_BY_RCU is that you get a reference to
> an object that is guaranteed only to have the same type (the instance may
> fluctuate and be replaced from under you unless other measures are taken).

(I wouldn't describe that as a "problem with SLAB_DESTROY_BY_RCU":
it's precisely the nature of SLAB_DESTROY_BY_RCU, what makes it useful
in solving backward-locking problems elsewhere.)

>
> Typically one must take a lock within the memory structure to pin down
> the object (or take a refcount). Only then can you follow pointers and
> such. It is only possible to verify that the right object has been
> reached *after* locking. Following a pointer without having determined
> that we hit the right object should not occur.
>
> A solution here would be to take the anon_vma->lock (prevents the object
> switching under us) and then verify that the mapping is the one we are
> looking for and that the pointer points to the right root. Then take the
> root lock.
>
> Hughs solution takes a global spinlock which will limit scalability.

Eh?  My solution was a second page_mapped(page) test i.e. testing an atomic.

Hugh

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