Re: [PATCH 11/18] mm: fix TLB flush race between migration, and change_protection_range

[Mel Gorman <mgorman@suse.de>]

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.

> > +#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.

> > +	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

> > +/* 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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