linux-kernel - Re: kmemcheck caught read from freed memory (cfq_free_io

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Date:	Wed, 2 Apr 2008 15:16:25 +0200
From:	Fabio Checconi <fchecconi@...il.com>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	paulmck@...ux.vnet.ibm.com, Ingo Molnar <mingo@...e.hu>,
	Pekka J Enberg <penberg@...helsinki.fi>,
	Vegard Nossum <vegard.nossum@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: kmemcheck caught read from freed memory (cfq_free_io_context)

> From: Jens Axboe <jens.axboe@...cle.com>
> Date: Wed, Apr 02, 2008 02:58:58PM +0200
>
> On Wed, Apr 02 2008, Jens Axboe wrote:
> > On Wed, Apr 02 2008, Fabio Checconi wrote:
> > > > From: Jens Axboe <jens.axboe@...cle.com>
> > > > Date: Wed, Apr 02, 2008 02:36:39PM +0200
> > > >
> > > > > Looks good and tests fine as well. I've applied it, on top of the
> > > > > hlist_for_each_entry_safe_rcu() fix.
> > > > > 
> > > > > http://git.kernel.dk/?p=linux-2.6-block.git;a=commit;h=436151ad32b4a59e0d36165a7d6312545f126661
> > > > > 
> > > 
> > > ok, thanks.  anyway I don't think the hlist_for_each_entry_safe_rcu()
> > > is needed at this point, since the pos->next pointer is still valid
> > > (at least) until the next rcu_read_unlock().  am I wrong?
> > 
> > it isn't, but it's still clearer. so either that or a nice comment, I
> > just stuck with what I had already committed.
> 

ok I agree on that.  the only remaining concern I have is that when
I first looked at it it seemed to me that hlist_for_each_entry_safe_rcu()
was missing by purpose from the list interface, since hlist_del_rcu()
can be called anyway during the traversal from a concurrent context,
so the semantics of *_safe_* have to be carried out by other means
(i.e., call_rcu()).

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