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

[Peter Zijlstra]

On Thu, May 13, 2021 at 02:48:37PM +0800, Huang Ying wrote:
>  mm/swapfile.c | 18 +++++++++---------
>  1 file changed, 9 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 2aad85751991..4c1fb28bbe0e 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -100,10 +100,14 @@ 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. */
> +	/*
> +	 * The data dependency ordering from the READ_ONCE() pairs
> +	 * with smp_wmb() in alloc_swap_info() to guarantee the
> +	 * swap_info_struct fields are read after swap_info[type].
> +	 */
>  	return READ_ONCE(swap_info[type]);
>  }
>  
> @@ -2884,14 +2888,10 @@ 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.)
> -		 */
> +		/* Paired with READ_ONCE() in swap_type_to_swap_info() */
>  		smp_wmb();
> -		WRITE_ONCE(nr_swapfiles, nr_swapfiles + 1);
> +		WRITE_ONCE(swap_info[type], p);
> +		nr_swapfiles++;

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++;
 	} else {
 		defer = p;
 		p = swap_info[type];