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 17:56:02 +0200 |
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On Thu, 2010-04-01 at 17:50 +0200, Peter Zijlstra wrote: > On Thu, 2010-04-01 at 17:42 +0200, Andrea Arcangeli wrote: > > On Thu, Apr 01, 2010 at 01:43:14PM +0200, Peter Zijlstra wrote: > > > On Thu, 2010-04-01 at 13:27 +0200, Peter Zijlstra wrote: > > > > > > > > I've almost got a patch done that converts those two, still need to look > > > > where that tasklist_lock muck happens. > > > > > > OK, so the below builds and boots, only need to track down that > > > tasklist_lock nesting, but I got to run an errand first. > > > > You should have a look at my old patchset where Christoph already > > implemented this (and not for decreasing latency but to allow > > scheduling in mmu notifier handlers, only needed by XPMEM): > > > > http://www.kernel.org/pub/linux/kernel/people/andrea/patches/v2.6/2.6.26-rc7/mmu-notifier-v18/ > > > > The ugliest part of it (that I think you missed below) is the breakage > > of the RCU locking in the anon-vma which requires adding refcounting > > to it. That was the worst part of the conversion as far as I can tell. > > > > http://www.kernel.org/pub/linux/kernel/people/andrea/patches/v2.6/2.6.26-rc7/mmu-notifier-v18/anon-vma > > > > I personally prefer read-write locks that Christoph used for both of > > them, but I'm not against mutex either. Still the refcounting problem > > should be the same as it's introduced by allowing the critical > > sections under anon_vma->lock to schedule (no matter if it's mutex or > > read-write sem). > > Right, so the problem with the rwsem is that, esp for very short hold > times, they introduce more pain than they're worth. Also the rwsem > doesn't do adaptive spinning nor allows for lock stealing, resulting in > a much much heavier sync. object than the mutex is. > > You also seem to move the tlb_gather stuff around, we have patches in > -rt that make tlb_gather preemptible, once i_mmap_lock is preemptible we > can do in mainline too.
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.
However __pte_alloc() callers do not in fact hold mmap_sem for writing.
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