Re: [PATCH 3/3] rcu: Avoid using xchg() in kfree_call_rcu_add_ptr_to_bulk()

[Joel Fernandes <joel@joelfernandes.org>]

On Wed, Apr 15, 2020 at 06:00:34PM +0200, Sebastian Andrzej Siewior wrote:
> There is no need to use xchg(), the access is serialized by krcp->lock.
> The xchg() function adds some atomic barriers which remain hidden in
> x86's disassembly but are visible on ARM for instance.
> 
> Replace xchg() with a load + store.
> 
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
> ---
>  kernel/rcu/tree.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index b31b61721a9ff..04d11c49ff3b9 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3079,7 +3079,8 @@ kfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp,
>  	/* Check if a new block is required. */
>  	if (!krcp->bhead ||
>  			krcp->bhead->nr_records == KFREE_BULK_MAX_ENTR) {
> -		bnode = xchg(&krcp->bcached, NULL);
> +		bnode = krcp->bcached;

Acked-by: Joel Fernandes (Google) <joel@joelfernandes.org>

I'll resend this with a rewrite of your 1/3 based on the lates tdiscussions.

For your 2/3, let us hold off on that for now as my rewrite of 1/3 will
require rewriting your 2/3.

thanks,

 - Joel


> +		krcp->bcached = NULL;
>  		if (!bnode) {
>  			WARN_ON_ONCE(sizeof(struct kfree_rcu_bulk_data) > PAGE_SIZE);
>  
> -- 
> 2.26.0
>