Re: [PATCH 13/16] ksm: Replace smp_read_barrier_depends() with lockless_dereference()

[Hugh Dickins <hughd@google.com>]

On Thu, 13 Nov 2014, Pranith Kumar wrote:

> Recently lockless_dereference() was added which can be used in place of
> hard-coding smp_read_barrier_depends(). The following PATCH makes the change.
> 
> Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>

Sorry, I don't think your patch is buggy, but I do think it
makes this tricky piece of code harder to follow, not easier.

It is certainly not a standard use of lockless_dereference() (kpfn
is not a pointer), and it both hides and moves where the barrier is.
And then at the end of the function, there's still explicit barriers
and ACCESS_ONCE comparison with kpfn, which this makes more obscure.

Unless you are actually fixing a bug (I don't pretend to have
tested this on Alpha, and I can get barriers wrong as we all do),
or smp_read_barrier_depends() is about to be withdrawn from use,
I'd rather say NAK to this patch.

Hugh

> ---
>  mm/ksm.c | 7 +++----
>  1 file changed, 3 insertions(+), 4 deletions(-)
> 
> diff --git a/mm/ksm.c b/mm/ksm.c
> index d247efa..a67de79 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -542,15 +542,14 @@ static struct page *get_ksm_page(struct stable_node *stable_node, bool lock_it)
>  	expected_mapping = (void *)stable_node +
>  				(PAGE_MAPPING_ANON | PAGE_MAPPING_KSM);
>  again:
> -	kpfn = ACCESS_ONCE(stable_node->kpfn);
> -	page = pfn_to_page(kpfn);
> -
>  	/*
>  	 * page is computed from kpfn, so on most architectures reading
>  	 * page->mapping is naturally ordered after reading node->kpfn,
>  	 * but on Alpha we need to be more careful.
>  	 */
> -	smp_read_barrier_depends();
> +	kpfn = lockless_dereference(stable_node->kpfn);
> +	page = pfn_to_page(kpfn);
> +
>  	if (ACCESS_ONCE(page->mapping) != expected_mapping)
>  		goto stale;
>  
> -- 
> 1.9.1

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