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

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Date:	Thu, 19 Jan 2012 14:30:18 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Hugh Dickins <hughd@...gle.com>, 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 jeudi 19 janvier 2012 à 04:28 -0800, Tejun Heo a écrit :
> Hello,
> 
> On Wed, Jan 18, 2012 at 11:33 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> > Interesting, but should be a patch on its own.
> 
> Yeap, agreed.
> 
> > 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(...);
> > }
> 
> Hmmm... I don't know. Does having a separate set of interface help
> much?  It's easy to avoid/miss the test by using the other one.  If we
> really worry about it, maybe indicating which locking is to be used
> during init is better? We can remember the lockdep map and trigger
> WARN_ON_ONCE() if neither the lock or RCU read lock is held.


There is a rcu_dereference_raw(ptr) in idr_get_next()

This could be changed to rcu_dereference_check(ptr, condition) to get
lockdep support for free :)

[ condition would be the appropriate
lockdep_is_held(&the_lock_protecting_my_idr) or 'I use the rcu variant'
and I hold rcu_read_lock ]

This would need to add a 'condition' parameter to idr_gen_next(), but we
have very few users in kernel at this moment.



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