Messages in this thread | | | Subject | Re: [COUNTERPATCH] mm: avoid overflowing preempt_count() in mmu_take_all_locks() | From | Peter Zijlstra <> | Date | Thu, 01 Apr 2010 18:32:42 +0200 |
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On Thu, 2010-04-01 at 18:07 +0200, Andrea Arcangeli wrote: > 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?
Not directly, but I keep running into that BUG_ON() at the end up exit_mmap() with my conversion patch, and I though that maybe I widened the race window.
But I guess I simply messed something up.
> 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.
Ah, I see so its: down_read(mmap_sem) + page_table_lock that's exclusive against down_write(mmap_sem), nifty, should be a comment somewhere.
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