linux-kernel - Re: [PATCH v2 2/3] mm/page_ref: Ensure page_ref_unfreeze is ordered against prior accesses

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Date:   Tue, 13 Jun 2017 17:06:01 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Will Deacon <will.deacon@....com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        mark.rutland@....com, akpm@...ux-foundation.org,
        Punit.Agrawal@....com, mgorman@...e.de, steve.capper@....com,
        vbabka@...e.cz
Subject: Re: [PATCH v2 2/3] mm/page_ref: Ensure page_ref_unfreeze is ordered
 against prior accesses

On Tue, Jun 13, 2017 at 11:28:41AM +0100, Will Deacon wrote:
> page_ref_freeze and page_ref_unfreeze are designed to be used as a pair,
> wrapping a critical section where struct pages can be modified without
> having to worry about consistency for a concurrent fast-GUP.
> 
> Whilst page_ref_freeze has full barrier semantics due to its use of
> atomic_cmpxchg, page_ref_unfreeze is implemented using atomic_set, which
> doesn't provide any barrier semantics and allows the operation to be
> reordered with respect to page modifications in the critical section.
> 
> This patch ensures that page_ref_unfreeze is ordered after any critical
> section updates, by invoking smp_mb() prior to the atomic_set.
> 
> Cc: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
> Acked-by: Steve Capper <steve.capper@....com>
> Signed-off-by: Will Deacon <will.deacon@....com>
> ---
>  include/linux/page_ref.h | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/include/linux/page_ref.h b/include/linux/page_ref.h
> index 610e13271918..1fd71733aa68 100644
> --- a/include/linux/page_ref.h
> +++ b/include/linux/page_ref.h
> @@ -174,6 +174,7 @@ static inline void page_ref_unfreeze(struct page *page, int count)
>  	VM_BUG_ON_PAGE(page_count(page) != 0, page);
>  	VM_BUG_ON(count == 0);
>  
> +	smp_mb();

Don't we want some comment here?

Otherwise:

Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

-- 
 Kirill A. Shutemov