linux-kernel - Re: 3.9-rc1 NULL pointer crash at find_pid

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Message-ID: <1362680324.15793.219.camel@edumazet-glaptop>
Date:	Thu, 07 Mar 2013 10:18:44 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Sasha Levin <sasha.levin@...cle.com>
Cc:	ebiederm@...ssion.com,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Li Zefan <lizefan@...wei.com>, CAI Qian <caiqian@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Containers <containers@...ts.linux-foundation.org>
Subject: Re: 3.9-rc1 NULL pointer crash at find_pid_ns

On Thu, 2013-03-07 at 13:14 -0500, Sasha Levin wrote:

> Okay, I'm even more confused now.
> 
> The expression in question is:
> 
> 	hlist_entry_safe(rcu_dereference_bh(hlist_first_rcu(head)))
> 
> You're saying that "rcu_dereference_bh(hlist_first_rcu(head))" can change between
> the two evaluations we do. That would mean that 'head' has changed in between, right?
> 
> In that case, the list itself has changed - which means that RCU has changed the
> list underneath us.
> 
> hlist_first_rcu() doesn't have any side-effects, it doesn't modify the list whatsoever,
> so the only thing that can change is 'head'. Why is it allowed to change if the list
> is protected by RCU?

Thats the point of RCU ;)

Things can change under us.



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