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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <alpine.LSU.2.00.1201191235330.29542@eggly.anvils>
Date:	Thu, 19 Jan 2012 12:46:56 -0800 (PST)
From:	Hugh Dickins <hughd@...gle.com>
To:	Tejun Heo <tj@...nel.org>
cc:	Eric Dumazet <eric.dumazet@...il.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

On Thu, 19 Jan 2012, Eric Dumazet wrote:
> 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.

Okay, in that case I'd better split into three (idr, revert, remove lock).
I'll send those three in a moment.  I've also slipped an RCU comment from
idr_find into idr_get_next, and put the Acks in all three.

> > 
> > > 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.

idr_get_next() was introduced for memcg, and has only one other user
user in the tree (drivers/mtd/mtdcore.c, which uses a mutex to lock it).
With the RCU fix, idr_get_next() becomes very much like idr_find().
I'll leave any fiddling with their interfaces to you guys.

Hugh