Re: [PATCH] mm, swap: Remove unnecessary smp_rmb() in swap_type_to_swap_info()

[Daniel Jordan]

On Thu, May 13, 2021 at 02:46:10PM +0200, Peter Zijlstra wrote:
> Ah, I think I see what you meant to say, it would perhaps help if you
> write it like so:
> 
> 
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 149e77454e3c..94735248dcd2 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -99,11 +99,10 @@ atomic_t nr_rotate_swap = ATOMIC_INIT(0);
>  
>  static struct swap_info_struct *swap_type_to_swap_info(int type)
>  {
> -	if (type >= READ_ONCE(nr_swapfiles))
> +	if (type >= MAX_SWAPFILES)
>  		return NULL;
>  
> -	smp_rmb();	/* Pairs with smp_wmb in alloc_swap_info. */
> -	return READ_ONCE(swap_info[type]);
> +	return READ_ONCE(swap_info[type]); /* rcu_dereference() */
>  }
>  
>  static inline unsigned char swap_count(unsigned char ent)
> @@ -2869,14 +2868,11 @@ static struct swap_info_struct *alloc_swap_info(void)
>  	}
>  	if (type >= nr_swapfiles) {
>  		p->type = type;
> -		WRITE_ONCE(swap_info[type], p);
>  		/*
> -		 * Write swap_info[type] before nr_swapfiles, in case a
> -		 * racing procfs swap_start() or swap_next() is reading them.
> -		 * (We never shrink nr_swapfiles, we never free this entry.)
> +		 * Publish the swap_info_struct.
>  		 */
> -		smp_wmb();
> -		WRITE_ONCE(nr_swapfiles, nr_swapfiles + 1);
> +		smp_store_release(&swap_info[type], p); /* rcu_assign_pointer() */
> +		nr_swapfiles++;

Yes, this does help, I didn't understand why smp_wmb stayed around in
the original post.

I think the only access smp_store_release() orders is p->type.  Wouldn't
it be kinda inconsistent to only initialize that one field before
publishing when many others would be done at the end of
alloc_swap_info() after the fact?  p->type doesn't seem special.  For
instance, get_swap_page_of_type() touches si->lock soon after it calls
swap_type_to_swap_info(), so there could be a small window where there's
a non-NULL si with an uninitialized lock.

It's not as if this is likely to be a problem in practice, it would just
make it harder to understand why smp_store_release is there.  Maybe all
we need is a WRITE_ONCE, or if it's really necessary for certain fields
to be set before publication then move them up and explain?