Messages in this thread | | | Date | Fri, 27 Aug 2010 10:55:11 -0700 | Subject | Re: [PATCH] mm: fix hang on anon_vma->root->lock | From | Hugh Dickins <> |
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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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