netdev - Re: [PATCH net-next 5/6] netns: use a spin

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Message-ID: <20150506140557.GA26920@pox.localdomain>
Date:	Wed, 6 May 2015 16:05:57 +0200
From:	Thomas Graf <tgraf@...g.ch>
To:	Nicolas Dichtel <nicolas.dichtel@...nd.com>
Cc:	netdev@...r.kernel.org, davem@...emloft.net, ebiederm@...ssion.com
Subject: Re: [PATCH net-next 5/6] netns: use a spin_lock to protect nsid
 management

On 05/06/15 at 03:40pm, Nicolas Dichtel wrote:
> Le 06/05/2015 14:23, Thomas Graf a ?crit :
> >On 05/06/15 at 11:58am, Nicolas Dichtel wrote:
> >>+/* Should be called with nsid_lock held. If a new id is assigned, the bool alloc
> >>+ * is set to true, thus the caller knows that the new id must be notified via
> >>+ * rtnl.
> >>+ */
> >>  static int __peernet2id(struct net *net, struct net *peer, bool *alloc)
> >>  {
> >>  	int id = idr_for_each(&net->netns_ids, net_eq_idr, peer);
> >>  	bool alloc_it = *alloc;
> >>
> >>-	ASSERT_RTNL();
> >>-
> >>  	*alloc = false;
> >>
> >>  	/* Magic value for id 0. */
> >
> >If split into __peernet2id() and __peernet2id_alloc() then this could
> >live with RCU protection I guess so we only take the lock when we
> >actually allocate.
> >
> The description of idr_for_each says:
> "The caller must serialize idr_for_each() vs idr_get_new() and idr_remove()."
> 
> So, if I understand well, the lock is always needed.

Ah, I looked at idr_alloc which says:

 * The user is responsible for exclusively synchronizing all operations
 * which may modify @idr.  However, read-only accesses such as idr_find()
 * or iteration can be performed under RCU read lock provided the user
 * destroys @ptr in RCU-safe way after removal from idr.
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