Messages in this thread Patch in this message | | | Date | Tue, 10 Dec 2013 10:02:08 -0800 | From | "Paul E. McKenney" <> | Subject | Re: [PATCH 11/18] mm: fix TLB flush race between migration, and change_protection_range |
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On Tue, Dec 10, 2013 at 05:19:36PM +0000, Mel Gorman wrote: > On Tue, Dec 10, 2013 at 09:25:39AM -0500, Rik van Riel wrote: > > On 12/09/2013 02:09 AM, Mel Gorman wrote: > > > > After reading the locking thread that Paul McKenney started, > > I wonder if I got the barriers wrong in these functions... > > If Documentation/memory-barriers.txt could not be used to frighten small > children before, it certainly can now.
Depends on the children. Some might find it quite attractive, sort of like running while carrying a knife.
> > > +#if defined(CONFIG_NUMA_BALANCING) || defined(CONFIG_COMPACTION) > > > +/* > > > + * Memory barriers to keep this state in sync are graciously provided by > > > + * the page table locks, outside of which no page table modifications happen. > > > + * The barriers below prevent the compiler from re-ordering the instructions > > > + * around the memory barriers that are already present in the code. > > > + */ > > > +static inline bool tlb_flush_pending(struct mm_struct *mm) > > > +{ > > > + barrier(); > > > > Should this be smp_mb__after_unlock_lock(); ? > > I think this is still ok. Minimally, it's missing the unlock/lock pair that > would cause smp_mb__after_unlock_lock() to be treated as a full barrier > on architectures that care. The CPU executing this code as already seen > the pmd_numa update if it's in the fault handler so it just needs to be > sure to not reorder the check with respect to the page copy.
You really do need a lock operation somewhere shortly before the smp_mb__after_unlock_lock().
> > > + return mm->tlb_flush_pending; > > > +} > > > +static inline void set_tlb_flush_pending(struct mm_struct *mm) > > > +{ > > > + mm->tlb_flush_pending = true; > > > + barrier(); > > > +} > > That now needs an smp_mb_before_spinlock to guarantee that the store > mm->tlb_flush_pending does not leak into the section updating the page > tables and get re-ordered. The result would pair with tlb_flush_pending > to guarantee that a pagetable update that starts in parallel will be > visible to flush the TLB before the cop
That would be required even if UNLOCK+LOCK continued being a full barrier. A lock acquisition by itself never was guaranteed to be a full barrier.
Thanx, Paul
> > > +/* Clearing is done after a TLB flush, which also provides a barrier. */ > > > +static inline void clear_tlb_flush_pending(struct mm_struct *mm) > > > +{ > > > + barrier(); > > > + mm->tlb_flush_pending = false; > > > +} > > > > This should be ok. Stores updating page tables complete before the ptl > unlock in addition to the TLB flush itself being a barrier that > guarantees the this update takes place afterwards. > > Peter/Paul? > > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h > index c122bb1..33e5519 100644 > --- a/include/linux/mm_types.h > +++ b/include/linux/mm_types.h > @@ -482,7 +482,12 @@ static inline bool tlb_flush_pending(struct mm_struct *mm) > static inline void set_tlb_flush_pending(struct mm_struct *mm) > { > mm->tlb_flush_pending = true; > - barrier(); > + > + /* > + * Guarantee that the tlb_flush_pending store does not leak into the > + * critical section updating the page tables > + */ > + smp_mb_before_spinlock(); > } > /* Clearing is done after a TLB flush, which also provides a barrier. */ > static inline void clear_tlb_flush_pending(struct mm_struct *mm) >
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