Re: [COUNTERPATCH] mm: avoid overflowing preempt_count() in mmu_take_all_locks()

[Andrea Arcangeli <aarcange@redhat.com>]

On Thu, Apr 01, 2010 at 05:56:02PM +0200, Peter Zijlstra wrote:
> Another thing is mm->nr_ptes, that doens't appear to be properly
> serialized, __pte_alloc() does ++ under mm->page_table_lock, but
> free_pte_range() does -- which afaict isn't always with page_table_lock
> held, it does however always seem to have mmap_sem for writing.

Not saying this is necessarily safe, but how can be that relevant with
spinlock->mutex/rwsem conversion? Only thing that breaks with that
conversion would be RCU (the very anon_vma rcu breaks because it
rcu_read_lock disabling preempt and then takes the anon_vma->lock,
that falls apart because taking the anon_vma->lock will imply a
schedule), but nr_ptes is a write operation so it can't be protected
by RCU.

> However __pte_alloc() callers do not in fact hold mmap_sem for writing.

As long as the mmap_sem readers always also take the page_table_lock
we're safe.