Re: [RFC][PATCH 1/5] mm: Rework {set,clear,mm}_tlb_flush_pending()

[Will Deacon]

On Wed, Jun 07, 2017 at 06:15:02PM +0200, Peter Zijlstra wrote:
> Commit:
> 
>   af2c1401e6f9 ("mm: numa: guarantee that tlb_flush_pending updates are visible before page table updates")
> 
> added smp_mb__before_spinlock() to set_tlb_flush_pending(). I think we
> can solve the same problem without this barrier.
> 
> If instead we mandate that mm_tlb_flush_pending() is used while
> holding the PTL we're guaranteed to observe prior
> set_tlb_flush_pending() instances.
> 
> For this to work we need to rework migrate_misplaced_transhuge_page()
> a little and move the test up into do_huge_pmd_numa_page().
> 
> Cc: Mel Gorman <mgorman@xxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
> --- a/include/linux/mm_types.h
> +++ b/include/linux/mm_types.h
> @@ -527,18 +527,16 @@ static inline cpumask_t *mm_cpumask(stru
>   */
>  static inline bool mm_tlb_flush_pending(struct mm_struct *mm)
>  {
> -	barrier();
> +	/*
> +	 * Must be called with PTL held; such that our PTL acquire will have
> +	 * observed the store from set_tlb_flush_pending().
> +	 */
>  	return mm->tlb_flush_pending;
>  }
>  static inline void set_tlb_flush_pending(struct mm_struct *mm)
>  {
>  	mm->tlb_flush_pending = true;
> -
> -	/*
> -	 * Guarantee that the tlb_flush_pending store does not leak into the
> -	 * critical section updating the page tables
> -	 */
> -	smp_mb__before_spinlock();
> +	barrier();

Why do you need the barrier() here? Isn't the ptl unlock sufficient?

Will