Re: [PATCH] memcg: restore ss->id_lock to spinlock, using RCU fornext

[Eric Dumazet]

Le mercredi 18 janvier 2012 Ã 22:05 -0800, Hugh Dickins a Ãcrit :

> 2. Make one small adjustment to idr_get_next(): take the height from
> the top layer (stable under RCU) instead of from the root (unprotected
> by RCU), as idr_find() does.
> 

> --- 3.2.0+/lib/idr.c	2012-01-04 15:55:44.000000000 -0800
> +++ linux/lib/idr.c	2012-01-18 21:25:36.947963342 -0800
> @@ -605,11 +605,11 @@ void *idr_get_next(struct idr *idp, int
>  	int n, max;
>  
>  	/* find first ent */
> -	n = idp->layers * IDR_BITS;
> -	max = 1 << n;
>  	p = rcu_dereference_raw(idp->top);
>  	if (!p)
>  		return NULL;
> +	n = (p->layer + 1) * IDR_BITS;
> +	max = 1 << n;
>  
>  	while (id < max) {
>  		while (n > 0 && p) {

Interesting, but should be a patch on its own.

Maybe other idr users can benefit from your idea as well, if patch is
labeled  "idr: allow idr_get_next() from rcu_read_lock" or something...

I suggest introducing idr_get_next_rcu() helper to make the check about
rcu cleaner.

idr_get_next_rcu(...)
{
	WARN_ON_ONCE(!rcu_read_lock_held());
	return idr_get_next(...);
}



--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/