linux-kernel - Re: [PATCH] memcg: restore ss->id_lock to spinlock, using RCU for next

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Date:	Thu, 19 Jan 2012 08:33:21 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Hugh Dickins <hughd@...gle.com>
Cc:	Tejun Heo <tj@...nel.org>, Li Zefan <lizf@...fujitsu.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Manfred Spraul <manfred@...orfullife.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Ying Han <yinghan@...gle.com>,
	Greg Thelen <gthelen@...gle.com>, cgroups@...r.kernel.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] memcg: restore ss->id_lock to spinlock, using RCU for
 next

Le mercredi 18 janvier 2012 à 22:05 -0800, Hugh Dickins a écrit :

> 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(...);
}



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