netdev - Re: [PATCH v3 10/12] l2tp: Add L2TP ethernet pseudowire support

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Date:	Fri, 02 Apr 2010 17:58:51 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	paulmck@...ux.vnet.ibm.com
Cc:	James Chapman <jchapman@...alix.com>,
	Stephen Hemminger <shemminger@...tta.com>,
	netdev@...r.kernel.org
Subject: Re: [PATCH v3 10/12] l2tp: Add L2TP ethernet pseudowire support

Le vendredi 02 avril 2010 à 08:24 -0700, Paul E. McKenney a écrit :
> On Wed, Mar 31, 2010 at 10:38:36AM -0700, Paul E. McKenney wrote:

> > The list_is_last() is RCU-safe already, since the ->next pointer is
> > only compared, never dereferenced.  I suggest adding a comment to its
> > header stating that it is RCU-safe.
> > 
> > Feel free to create a list_for_each_entry_safe_rcu(), and I will be
> > happy to review it.
> 
> Right.  When using RCU, list_for_each_entry_safe_rcu() is not needed,
> because destruction of the element you removed doesn't happen until the
> end of a subsequent grace period.  By which time you will be done with
> your RCU read-side critical section.
> 
> So you can just use list_for_each_entry_rcu(), you do not need a new
> list_for_each_entry_safe_rcu().
> 
> But you did have me going for a bit!!!  ;-)

Just wondering Paul if you need to rest :)

list_for_each_entry_safe_rcu() is not needed not because of various (and
very intersting) reasons you gave !!!

But because list_for_each_entry_safe() is OK, since if we are in a
destroy phase, we are a writer, and can use regular list primitive.

Remember _rcu() versions of list iterators are for readers, and readers
are NOT supposed to delete an item from the list !

Eric



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