On Fri, 30 Nov 2012 22:56:27 -0800
Michel Lespinasse [off-list ref] wrote:

expand_stack() runs with a shared mmap_sem lock. Because of this, there
could be multiple concurrent stack expansions in the same mm, which may
cause problems in the vma gap update code.

I propose to solve this by taking the mm->page_table_lock around such vma
expansions, in order to avoid the concurrency issue. We only have to worry
about concurrent expand_stack() calls here, since we hold a shared mmap_sem
lock and all vma modificaitons other than expand_stack() are done under
an exclusive mmap_sem lock.

I previously tried to achieve the same effect by making sure all
growable vmas in a given mm would share the same anon_vma, which we
already lock here. However this turned out to be difficult - all of the
schemes I tried for refcounting the growable anon_vma and clearing
turned out ugly. So, I'm now proposing only the minimal fix.

I think I don't understand the problem fully.  Let me demonstrate:

a) vma_lock_anon_vma() doesn't take a lock which is specific to
   "this" anon_vma.  It takes anon_vma->root->mutex.  That mutex is
   shared with vma->vm_next, yes?  If so, we have no problem here? 
   (which makes me suspect that the races lies other than where I think
   it lies).

b) I can see why a broader lock is needed in expand_upwards(): it
   plays with a different vma: vma->vm_next.  But expand_downwards()
   doesn't do that - it only alters "this" vma.  So I'd have thought
   that vma_lock_anon_vma("this" vma) would be sufficient.


What are the performance costs of this change?

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