Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933216Ab3CGSOd (ORCPT ); Thu, 7 Mar 2013 13:14:33 -0500 Received: from userp1040.oracle.com ([156.151.31.81]:26244 "EHLO userp1040.oracle.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932541Ab3CGSOc (ORCPT ); Thu, 7 Mar 2013 13:14:32 -0500 Message-ID: <5138D8F2.5020900@oracle.com> Date: Thu, 07 Mar 2013 13:14:10 -0500 From: Sasha Levin User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130227 Thunderbird/17.0.3 MIME-Version: 1.0 To: ebiederm@xmission.com CC: Eric Dumazet , "Paul E. McKenney" , Li Zefan , CAI Qian , linux-kernel , Containers Subject: Re: 3.9-rc1 NULL pointer crash at find_pid_ns References: <611667212.10748821.1362649031475.JavaMail.root@redhat.com> <513860E8.4080807@huawei.com> <876213wmwt.fsf@xmission.com> <5138D001.8000409@oracle.com> <1362678371.15793.218.camel@edumazet-glaptop> <5138D377.6040406@oracle.com> <87boavrspd.fsf@xmission.com> In-Reply-To: <87boavrspd.fsf@xmission.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit X-Source-IP: acsinet21.oracle.com [141.146.126.237] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2437 Lines: 66 On 03/07/2013 01:05 PM, ebiederm@xmission.com wrote: > Sasha Levin writes: > >> On 03/07/2013 12:46 PM, Eric Dumazet wrote: >>> On Thu, 2013-03-07 at 12:36 -0500, Sasha Levin wrote: >>> >>>> Looks like the hlist change is probably the issue, though it specifically >>>> uses: >>>> >>>> #define hlist_entry_safe(ptr, type, member) \ >>>> (ptr) ? hlist_entry(ptr, type, member) : NULL >>>> >>>> I'm still looking at the code in question and it's assembly, but I can't >>>> figure out what's going wrong. I was also trying to see what's so special >>>> about this loop in find_pid_ns as opposed to the rest of the kernel code >>>> that uses hlist_for_each_entry_rcu() but couldn't find out why. >>>> >>>> Is it somehow possible that if we rcu_dereference_raw() the same thing twice >>>> inside the same rcu_read_lock() section we'll get different results? That's >>>> really the only reason for this crash that comes to mind at the moment, very >>>> unlikely - but that's all I have right now. >>>> >>> >>> Yep >>> >>> #define hlist_entry_safe(ptr, type, member) \ >>> (ptr) ? hlist_entry(ptr, type, member) : NULL >>> >>> Is not safe, as ptr can be evaluated twice, and thats not good at all... >> >> ptr is being evaluated twice, but in this case this is an >> rcu_dereference_raw() value within the same rcu_read_lock() section. >> >> Is it still problematic? > > Definitely. > > Head in this instance the expression: &pid_hash[pid_hashfn(nr, ns)] > > And the crash clearly shows that when hilst_entry is being evaluated the > HEAD is NULL. 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? Thanks, Sasha -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/